diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index b6bb11d07ef04..2e792b6053ea3 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -18,7 +18,7 @@
 
 use std::borrow::Cow;
 use std::env;
-use std::fs::{self, File};
+use std::fs;
 use std::io::BufReader;
 use std::io::prelude::*;
 use std::path::{Path, PathBuf};
@@ -707,7 +707,7 @@ impl Step for CodegenBackend {
         }
         let stamp = codegen_backend_stamp(builder, compiler, target, backend);
         let codegen_backend = codegen_backend.to_str().unwrap();
-        t!(t!(File::create(&stamp)).write_all(codegen_backend.as_bytes()));
+        t!(fs::write(&stamp, &codegen_backend));
     }
 }
 
@@ -796,8 +796,7 @@ fn copy_codegen_backends_to_sysroot(builder: &Builder,
 
     for backend in builder.config.rust_codegen_backends.iter() {
         let stamp = codegen_backend_stamp(builder, compiler, target, *backend);
-        let mut dylib = String::new();
-        t!(t!(File::open(&stamp)).read_to_string(&mut dylib));
+        let dylib = t!(fs::read_to_string(&stamp));
         let file = Path::new(&dylib);
         let filename = file.file_name().unwrap().to_str().unwrap();
         // change `librustc_codegen_llvm-xxxxxx.so` to `librustc_codegen_llvm-llvm.so`
@@ -1137,10 +1136,7 @@ pub fn run_cargo(builder: &Builder,
     // contents (the list of files to copy) is different or if any dep's mtime
     // is newer then we rewrite the stamp file.
     deps.sort();
-    let mut stamp_contents = Vec::new();
-    if let Ok(mut f) = File::open(stamp) {
-        t!(f.read_to_end(&mut stamp_contents));
-    }
+    let stamp_contents = fs::read(stamp);
     let stamp_mtime = mtime(&stamp);
     let mut new_contents = Vec::new();
     let mut max = None;
@@ -1156,7 +1152,10 @@ pub fn run_cargo(builder: &Builder,
     }
     let max = max.unwrap();
     let max_path = max_path.unwrap();
-    if stamp_contents == new_contents && max <= stamp_mtime {
+    let contents_equal = stamp_contents
+        .map(|contents| contents == new_contents)
+        .unwrap_or_default();
+    if contents_equal && max <= stamp_mtime {
         builder.verbose(&format!("not updating {:?}; contents equal and {:?} <= {:?}",
                 stamp, max, stamp_mtime));
         return deps
@@ -1166,7 +1165,7 @@ pub fn run_cargo(builder: &Builder,
     } else {
         builder.verbose(&format!("updating {:?} as deps changed", stamp));
     }
-    t!(t!(File::create(stamp)).write_all(&new_contents));
+    t!(fs::write(&stamp, &new_contents));
     deps
 }
 
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 0f249eee00022..dd8cca5999bd3 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -15,8 +15,7 @@
 
 use std::collections::{HashMap, HashSet};
 use std::env;
-use std::fs::{self, File};
-use std::io::prelude::*;
+use std::fs;
 use std::path::{Path, PathBuf};
 use std::process;
 use std::cmp;
@@ -414,9 +413,7 @@ impl Config {
         config.run_host_only = !(flags.host.is_empty() && !flags.target.is_empty());
 
         let toml = file.map(|file| {
-            let mut f = t!(File::open(&file));
-            let mut contents = String::new();
-            t!(f.read_to_string(&mut contents));
+            let contents = t!(fs::read_to_string(&file));
             match toml::from_str(&contents) {
                 Ok(table) => table,
                 Err(err) => {
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index cd8d5642b2570..68aed59074638 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -19,8 +19,8 @@
 //! pieces of `rustup.rs`!
 
 use std::env;
-use std::fs::{self, File};
-use std::io::{Read, Write};
+use std::fs;
+use std::io::Write;
 use std::path::{PathBuf, Path};
 use std::process::{Command, Stdio};
 
@@ -1510,8 +1510,7 @@ impl Step for Extended {
         }
 
         let xform = |p: &Path| {
-            let mut contents = String::new();
-            t!(t!(File::open(p)).read_to_string(&mut contents));
+            let mut contents = t!(fs::read_to_string(p));
             if rls_installer.is_none() {
                 contents = filter(&contents, "rls");
             }
@@ -1522,8 +1521,8 @@ impl Step for Extended {
                 contents = filter(&contents, "rustfmt");
             }
             let ret = tmp.join(p.file_name().unwrap());
-            t!(t!(File::create(&ret)).write_all(contents.as_bytes()));
-            return ret
+            t!(fs::write(&ret, &contents));
+            ret
         };
 
         if target.contains("apple-darwin") {
@@ -1868,8 +1867,7 @@ impl Step for HashSign {
         let file = builder.config.dist_gpg_password_file.as_ref().unwrap_or_else(|| {
             panic!("\n\nfailed to specify `dist.gpg-password-file` in `config.toml`\n\n")
         });
-        let mut pass = String::new();
-        t!(t!(File::open(&file)).read_to_string(&mut pass));
+        let pass = t!(fs::read_to_string(&file));
 
         let today = output(Command::new("date").arg("+%Y-%m-%d"));
 
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index f9b19ffb10d69..4fb17bc51f213 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -18,8 +18,7 @@
 //! `rustdoc`.
 
 use std::collections::HashSet;
-use std::fs::{self, File};
-use std::io::prelude::*;
+use std::fs;
 use std::io;
 use std::path::{PathBuf, Path};
 
@@ -378,12 +377,11 @@ impl Step for Standalone {
         let version_info = out.join("version_info.html");
 
         if !builder.config.dry_run && !up_to_date(&version_input, &version_info) {
-            let mut info = String::new();
-            t!(t!(File::open(&version_input)).read_to_string(&mut info));
-            let info = info.replace("VERSION", &builder.rust_release())
-                           .replace("SHORT_HASH", builder.rust_info.sha_short().unwrap_or(""))
-                           .replace("STAMP", builder.rust_info.sha().unwrap_or(""));
-            t!(t!(File::create(&version_info)).write_all(info.as_bytes()));
+            let info = t!(fs::read_to_string(&version_input))
+                .replace("VERSION", &builder.rust_release())
+                .replace("SHORT_HASH", builder.rust_info.sha_short().unwrap_or(""))
+                .replace("STAMP", builder.rust_info.sha().unwrap_or(""));
+            t!(fs::write(&version_info, &info));
         }
 
         for file in t!(fs::read_dir(builder.src.join("src/doc"))) {
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 2832f5bebddd6..a62830da4bb61 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -1067,9 +1067,8 @@ impl Build {
 
     /// Returns the `a.b.c` version that the given package is at.
     fn release_num(&self, package: &str) -> String {
-        let mut toml = String::new();
         let toml_file_name = self.src.join(&format!("src/tools/{}/Cargo.toml", package));
-        t!(t!(File::open(toml_file_name)).read_to_string(&mut toml));
+        let toml = t!(fs::read_to_string(&toml_file_name));
         for line in toml.lines() {
             let prefix = "version = \"";
             let suffix = "\"";
@@ -1151,8 +1150,7 @@ impl Build {
         }
 
         let mut paths = Vec::new();
-        let mut contents = Vec::new();
-        t!(t!(File::open(stamp)).read_to_end(&mut contents));
+        let contents = t!(fs::read(stamp));
         // This is the method we use for extracting paths from the stamp file passed to us. See
         // run_cargo for more information (in compile.rs).
         for part in contents.split(|b| *b == 0) {
diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index 448967ef0c2e1..150b6bea18064 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -21,7 +21,6 @@
 use std::env;
 use std::ffi::OsString;
 use std::fs::{self, File};
-use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
@@ -75,8 +74,7 @@ impl Step for Llvm {
         }
 
         let rebuild_trigger = builder.src.join("src/rustllvm/llvm-rebuild-trigger");
-        let mut rebuild_trigger_contents = String::new();
-        t!(t!(File::open(&rebuild_trigger)).read_to_string(&mut rebuild_trigger_contents));
+        let rebuild_trigger_contents = t!(fs::read_to_string(&rebuild_trigger));
 
         let (out_dir, llvm_config_ret_dir) = if emscripten {
             let dir = builder.emscripten_llvm_out(target);
@@ -93,8 +91,7 @@ impl Step for Llvm {
         let build_llvm_config = llvm_config_ret_dir
             .join(exe("llvm-config", &*builder.config.build));
         if done_stamp.exists() {
-            let mut done_contents = String::new();
-            t!(t!(File::open(&done_stamp)).read_to_string(&mut done_contents));
+            let done_contents = t!(fs::read_to_string(&done_stamp));
 
             // If LLVM was already built previously and contents of the rebuild-trigger file
             // didn't change from the previous build, then no action is required.
@@ -261,7 +258,7 @@ impl Step for Llvm {
 
         cfg.build();
 
-        t!(t!(File::create(&done_stamp)).write_all(rebuild_trigger_contents.as_bytes()));
+        t!(fs::write(&done_stamp, &rebuild_trigger_contents));
 
         build_llvm_config
     }
diff --git a/src/bootstrap/sanity.rs b/src/bootstrap/sanity.rs
index 15d3bccba09cc..7a9e6d4fd387b 100644
--- a/src/bootstrap/sanity.rs
+++ b/src/bootstrap/sanity.rs
@@ -21,8 +21,7 @@
 use std::collections::HashMap;
 use std::env;
 use std::ffi::{OsString, OsStr};
-use std::fs::{self, File};
-use std::io::Read;
+use std::fs;
 use std::path::PathBuf;
 use std::process::Command;
 
@@ -235,9 +234,7 @@ $ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
     }
 
     if build.config.channel == "stable" {
-        let mut stage0 = String::new();
-        t!(t!(File::open(build.src.join("src/stage0.txt")))
-            .read_to_string(&mut stage0));
+        let stage0 = t!(fs::read_to_string(build.src.join("src/stage0.txt")));
         if stage0.contains("\ndev:") {
             panic!("bootstrapping from a dev compiler in a stable release, but \
                     should only be bootstrapping from a released compiler!");
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index e55773011df8e..056e6ca3403c4 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -16,8 +16,7 @@
 use std::env;
 use std::ffi::OsString;
 use std::fmt;
-use std::fs::{self, File};
-use std::io::Read;
+use std::fs;
 use std::iter;
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -1418,10 +1417,8 @@ impl Step for ErrorIndex {
 }
 
 fn markdown_test(builder: &Builder, compiler: Compiler, markdown: &Path) -> bool {
-    match File::open(markdown) {
-        Ok(mut file) => {
-            let mut contents = String::new();
-            t!(file.read_to_string(&mut contents));
+    match fs::read_to_string(markdown) {
+        Ok(contents) => {
             if !contents.contains("```") {
                 return true;
             }
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index 8d009101ce7da..0b25d911a299c 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -618,6 +618,8 @@ impl String {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
+        // This isn't done via collect::<Result<_, _>>() for performance reasons.
+        // FIXME: the function can be simplified again when #48994 is closed.
         let mut ret = String::with_capacity(v.len());
         for c in decode_utf16(v.iter().cloned()) {
             if let Ok(c) = c {
diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 3d24f8902bd83..26e7a79d35df6 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -148,6 +148,15 @@ macro_rules! array_impls {
                 }
             }
 
+            #[unstable(feature = "try_from", issue = "33417")]
+            impl<'a, T> TryFrom<&'a [T]> for [T; $N] where T: Copy {
+                type Error = TryFromSliceError;
+
+                fn try_from(slice: &[T]) -> Result<[T; $N], TryFromSliceError> {
+                    <&Self>::try_from(slice).map(|r| *r)
+                }
+            }
+
             #[unstable(feature = "try_from", issue = "33417")]
             impl<'a, T> TryFrom<&'a [T]> for &'a [T; $N] {
                 type Error = TryFromSliceError;
diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index dbc28ef7cf6a9..2d4813718f41a 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -327,7 +327,8 @@ pub trait Into<T>: Sized {
 /// An example usage for error handling:
 ///
 /// ```
-/// use std::io::{self, Read};
+/// use std::fs;
+/// use std::io;
 /// use std::num;
 ///
 /// enum CliError {
@@ -348,9 +349,7 @@ pub trait Into<T>: Sized {
 /// }
 ///
 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
-///     let mut file = std::fs::File::open("test")?;
-///     let mut contents = String::new();
-///     file.read_to_string(&mut contents)?;
+///     let mut contents = fs::read_to_string(&file_name)?;
 ///     let num: i32 = contents.trim().parse()?;
 ///     Ok(num)
 /// }
diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index 57b5903c9d39f..46c098c8236a0 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -1989,6 +1989,19 @@ big endian.
 ```
 let value = ", stringify!($SelfT), "::from_be_bytes(", $be_bytes, ");
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
@@ -2008,6 +2021,19 @@ little endian.
 ```
 let value = ", stringify!($SelfT), "::from_le_bytes(", $le_bytes, ");
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
@@ -2037,6 +2063,19 @@ let value = ", stringify!($SelfT), "::from_ne_bytes(if cfg!(target_endian = \"bi
         ", $le_bytes, "
     });
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
@@ -3719,6 +3758,19 @@ big endian.
 ```
 let value = ", stringify!($SelfT), "::from_be_bytes(", $be_bytes, ");
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
@@ -3738,6 +3790,19 @@ little endian.
 ```
 let value = ", stringify!($SelfT), "::from_le_bytes(", $le_bytes, ");
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
@@ -3767,6 +3832,19 @@ let value = ", stringify!($SelfT), "::from_ne_bytes(if cfg!(target_endian = \"bi
         ", $le_bytes, "
     });
 assert_eq!(value, ", $swap_op, ");
+```
+
+When starting from a slice rather than an array, fallible conversion APIs can be used:
+
+```
+#![feature(try_from)]
+use std::convert::TryInto;
+
+fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT), " {
+    let (int_bytes, rest) = input.split_at(std::mem::size_of::<", stringify!($SelfT), ">());
+    *input = rest;
+    ", stringify!($SelfT), "::from_be_bytes(int_bytes.try_into().unwrap())
+}
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
             #[rustc_const_unstable(feature = "const_int_conversion")]
diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index 947b67e4e9af3..d3a74ed2a6856 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -189,12 +189,22 @@ pub use intrinsics::write_bytes;
 /// i.e., you do not usually have to worry about such issues unless you call `drop_in_place`
 /// manually.
 #[stable(feature = "drop_in_place", since = "1.8.0")]
+#[inline(always)]
+pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+    real_drop_in_place(&mut *to_drop)
+}
+
+// The real `drop_in_place` -- the one that gets called implicitly when variables go
+// out of scope -- should have a safe reference and not a raw pointer as argument
+// type.  When we drop a local variable, we access it with a pointer that behaves
+// like a safe reference; transmuting that to a raw pointer does not mean we can
+// actually access it with raw pointers.
 #[lang = "drop_in_place"]
 #[allow(unconditional_recursion)]
-pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+unsafe fn real_drop_in_place<T: ?Sized>(to_drop: &mut T) {
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
-    drop_in_place(to_drop);
+    real_drop_in_place(to_drop)
 }
 
 /// Creates a null raw pointer.
diff --git a/src/librustc/infer/lexical_region_resolve/graphviz.rs b/src/librustc/infer/lexical_region_resolve/graphviz.rs
index a210d63f129e3..512e57d810aa4 100644
--- a/src/librustc/infer/lexical_region_resolve/graphviz.rs
+++ b/src/librustc/infer/lexical_region_resolve/graphviz.rs
@@ -31,9 +31,8 @@ use std::borrow::Cow;
 use std::collections::hash_map::Entry::Vacant;
 use std::collections::btree_map::BTreeMap;
 use std::env;
-use std::fs::File;
+use std::fs;
 use std::io;
-use std::io::prelude::*;
 use std::sync::atomic::{AtomicBool, Ordering};
 
 fn print_help_message() {
@@ -268,5 +267,5 @@ fn dump_region_data_to<'a, 'gcx, 'tcx>(region_rels: &RegionRelations<'a, 'gcx, '
     debug!("dump_region_data calling render");
     let mut v = Vec::new();
     dot::render(&g, &mut v).unwrap();
-    File::create(path).and_then(|mut f| f.write_all(&v))
+    fs::write(path, &v)
 }
diff --git a/src/librustc/middle/reachable.rs b/src/librustc/middle/reachable.rs
index 0009a517dd1db..ab0094df0e219 100644
--- a/src/librustc/middle/reachable.rs
+++ b/src/librustc/middle/reachable.rs
@@ -117,8 +117,9 @@ impl<'a, 'tcx> Visitor<'tcx> for ReachableContext<'a, 'tcx> {
                 self.reachable_symbols.insert(node_id);
             }
             Some(def) => {
-                let def_id = def.def_id();
-                if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
+                if let Some((node_id, def_id)) = def.opt_def_id().and_then(|def_id| {
+                    self.tcx.hir.as_local_node_id(def_id).map(|node_id| (node_id, def_id))
+                }) {
                     if self.def_id_represents_local_inlined_item(def_id) {
                         self.worklist.push(node_id);
                     } else {
diff --git a/src/librustc/middle/resolve_lifetime.rs b/src/librustc/middle/resolve_lifetime.rs
index 6ff450508d136..07054ee99af76 100644
--- a/src/librustc/middle/resolve_lifetime.rs
+++ b/src/librustc/middle/resolve_lifetime.rs
@@ -1573,7 +1573,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             .collect();
 
         // ensure that we issue lints in a repeatable order
-        def_ids.sort_by_key(|&def_id| self.tcx.def_path_hash(def_id));
+        def_ids.sort_by_cached_key(|&def_id| self.tcx.def_path_hash(def_id));
 
         for def_id in def_ids {
             debug!(
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 3fd22793a08a0..480d4a8e48f0e 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1286,8 +1286,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "print some performance-related statistics"),
     hir_stats: bool = (false, parse_bool, [UNTRACKED],
         "print some statistics about AST and HIR"),
-    mir_stats: bool = (false, parse_bool, [UNTRACKED],
-        "print some statistics about MIR"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],
         "encode MIR of all functions into the crate metadata"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
diff --git a/src/librustc/traits/object_safety.rs b/src/librustc/traits/object_safety.rs
index c79fa3861234f..2909daf22b3ba 100644
--- a/src/librustc/traits/object_safety.rs
+++ b/src/librustc/traits/object_safety.rs
@@ -409,7 +409,7 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
             .collect::<Vec<_>>();
 
         // existential predicates need to be in a specific order
-        associated_types.sort_by_key(|item| self.def_path_hash(item.def_id));
+        associated_types.sort_by_cached_key(|item| self.def_path_hash(item.def_id));
 
         let projection_predicates = associated_types.into_iter().map(|item| {
             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index 7945d381760a1..4eb920324bd00 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -23,7 +23,7 @@ use llvm_util;
 use ModuleLlvm;
 use rustc_codegen_ssa::{ModuleCodegen, CompiledModule};
 use rustc::util::common::time_ext;
-use rustc_fs_util::{path2cstr, link_or_copy};
+use rustc_fs_util::{path_to_c_string, link_or_copy};
 use rustc_data_structures::small_c_str::SmallCStr;
 use errors::{self, Handler, FatalError};
 use type_::Type;
@@ -80,7 +80,7 @@ pub fn write_output_file(
         output: &Path,
         file_type: llvm::FileType) -> Result<(), FatalError> {
     unsafe {
-        let output_c = path2cstr(output);
+        let output_c = path_to_c_string(output);
         let result = llvm::LLVMRustWriteOutputFile(target, pm, m, output_c.as_ptr(), file_type);
         if result.into_result().is_err() {
             let msg = format!("could not write output to {}", output.display());
@@ -211,7 +211,7 @@ pub(crate) fn save_temp_bitcode(
         let ext = format!("{}.bc", name);
         let cgu = Some(&module.name[..]);
         let path = cgcx.output_filenames.temp_path_ext(&ext, cgu);
-        let cstr = path2cstr(&path);
+        let cstr = path_to_c_string(&path);
         let llmod = module.module_llvm.llmod();
         llvm::LLVMWriteBitcodeToFile(llmod, cstr.as_ptr());
     }
@@ -324,7 +324,7 @@ pub(crate) unsafe fn optimize(cgcx: &CodegenContext<LlvmCodegenBackend>,
 
     if config.emit_no_opt_bc {
         let out = cgcx.output_filenames.temp_path_ext("no-opt.bc", module_name);
-        let out = path2cstr(&out);
+        let out = path_to_c_string(&out);
         llvm::LLVMWriteBitcodeToFile(llmod, out.as_ptr());
     }
 
@@ -530,7 +530,7 @@ pub(crate) unsafe fn codegen(cgcx: &CodegenContext<LlvmCodegenBackend>,
             || -> Result<(), FatalError> {
             if config.emit_ir {
                 let out = cgcx.output_filenames.temp_path(OutputType::LlvmAssembly, module_name);
-                let out = path2cstr(&out);
+                let out = path_to_c_string(&out);
 
                 extern "C" fn demangle_callback(input_ptr: *const c_char,
                                                 input_len: size_t,
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index 81f2769800d2f..47e92fbe87d31 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -39,7 +39,7 @@ use rustc::ty::layout::{self, Align, HasDataLayout, Integer, IntegerExt, LayoutO
                         PrimitiveExt, Size, TyLayout};
 use rustc::session::config;
 use rustc::util::nodemap::FxHashMap;
-use rustc_fs_util::path2cstr;
+use rustc_fs_util::path_to_c_string;
 use rustc_data_structures::small_c_str::SmallCStr;
 
 use libc::{c_uint, c_longlong};
@@ -892,7 +892,7 @@ pub fn compile_unit_metadata(tcx: TyCtxt,
     };
 
     fn path_to_mdstring(llcx: &'ll llvm::Context, path: &Path) -> &'ll Value {
-        let path_str = path2cstr(path);
+        let path_str = path_to_c_string(path);
         unsafe {
             llvm::LLVMMDStringInContext(llcx,
                                         path_str.as_ptr(),
diff --git a/src/librustc_codegen_llvm/llvm/archive_ro.rs b/src/librustc_codegen_llvm/llvm/archive_ro.rs
index 2a77f256e3a0c..d5c73fecf814a 100644
--- a/src/librustc_codegen_llvm/llvm/archive_ro.rs
+++ b/src/librustc_codegen_llvm/llvm/archive_ro.rs
@@ -10,10 +10,10 @@
 
 //! A wrapper around LLVM's archive (.a) code
 
-use std::ffi::CString;
 use std::path::Path;
 use std::slice;
 use std::str;
+use rustc_fs_util::path_to_c_string;
 
 pub struct ArchiveRO {
     pub raw: &'static mut super::Archive,
@@ -38,24 +38,12 @@ impl ArchiveRO {
     /// raised.
     pub fn open(dst: &Path) -> Result<ArchiveRO, String> {
         return unsafe {
-            let s = path2cstr(dst);
+            let s = path_to_c_string(dst);
             let ar = super::LLVMRustOpenArchive(s.as_ptr()).ok_or_else(|| {
                 super::last_error().unwrap_or_else(|| "failed to open archive".to_owned())
             })?;
             Ok(ArchiveRO { raw: ar })
         };
-
-        #[cfg(unix)]
-        fn path2cstr(p: &Path) -> CString {
-            use std::os::unix::prelude::*;
-            use std::ffi::OsStr;
-            let p: &OsStr = p.as_ref();
-            CString::new(p.as_bytes()).unwrap()
-        }
-        #[cfg(windows)]
-        fn path2cstr(p: &Path) -> CString {
-            CString::new(p.to_str().unwrap()).unwrap()
-        }
     }
 
     pub fn iter(&self) -> Iter {
diff --git a/src/librustc_codegen_llvm/metadata.rs b/src/librustc_codegen_llvm/metadata.rs
index 7752465d885bb..5605f64c2e72c 100644
--- a/src/librustc_codegen_llvm/metadata.rs
+++ b/src/librustc_codegen_llvm/metadata.rs
@@ -18,7 +18,7 @@ use rustc_data_structures::owning_ref::OwningRef;
 use std::path::Path;
 use std::ptr;
 use std::slice;
-use rustc_fs_util::path2cstr;
+use rustc_fs_util::path_to_c_string;
 
 pub use rustc_data_structures::sync::MetadataRef;
 
@@ -57,7 +57,7 @@ impl MetadataLoader for LlvmMetadataLoader {
                           filename: &Path)
                           -> Result<MetadataRef, String> {
         unsafe {
-            let buf = path2cstr(filename);
+            let buf = path_to_c_string(filename);
             let mb = llvm::LLVMRustCreateMemoryBufferWithContentsOfFile(buf.as_ptr())
                 .ok_or_else(|| format!("error reading library: '{}'", filename.display()))?;
             let of = ObjectFile::new(mb)
diff --git a/src/librustc_codegen_ssa/back/link.rs b/src/librustc_codegen_ssa/back/link.rs
index b0575b841d5d5..24a70dc797771 100644
--- a/src/librustc_codegen_ssa/back/link.rs
+++ b/src/librustc_codegen_ssa/back/link.rs
@@ -161,7 +161,11 @@ pub fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
                 LinkerFlavor::Lld(_) => "lld",
             }), flavor)),
             (Some(linker), None) => {
-                let stem = linker.file_stem().and_then(|stem| stem.to_str()).unwrap_or_else(|| {
+                let stem = if linker.extension().and_then(|ext| ext.to_str()) == Some("exe") {
+                    linker.file_stem().and_then(|stem| stem.to_str())
+                } else {
+                    linker.to_str()
+                }.unwrap_or_else(|| {
                     sess.fatal("couldn't extract file stem from specified linker");
                 }).to_owned();
 
diff --git a/src/librustc_codegen_utils/codegen_backend.rs b/src/librustc_codegen_utils/codegen_backend.rs
index 046f0cccfee05..f1f798e770089 100644
--- a/src/librustc_codegen_utils/codegen_backend.rs
+++ b/src/librustc_codegen_utils/codegen_backend.rs
@@ -22,8 +22,8 @@
 #![feature(box_syntax)]
 
 use std::any::Any;
-use std::io::{self, Write};
-use std::fs::File;
+use std::io::Write;
+use std::fs;
 use std::path::Path;
 use std::sync::{mpsc, Arc};
 
@@ -81,12 +81,8 @@ pub struct NoLlvmMetadataLoader;
 
 impl MetadataLoader for NoLlvmMetadataLoader {
     fn get_rlib_metadata(&self, _: &Target, filename: &Path) -> Result<MetadataRef, String> {
-        let mut file = File::open(filename)
-            .map_err(|e| format!("metadata file open err: {:?}", e))?;
-
-        let mut buf = Vec::new();
-        io::copy(&mut file, &mut buf).unwrap();
-        let buf: OwningRef<Vec<u8>, [u8]> = OwningRef::new(buf).into();
+        let buf = fs::read(filename).map_err(|e| format!("metadata file open err: {:?}", e))?;
+        let buf: OwningRef<Vec<u8>, [u8]> = OwningRef::new(buf);
         return Ok(rustc_erase_owner!(buf.map_owner_box()));
     }
 
@@ -212,8 +208,7 @@ impl CodegenBackend for MetadataOnlyCodegenBackend {
             } else {
                 &ongoing_codegen.metadata.raw_data
             };
-            let mut file = File::create(&output_name).unwrap();
-            file.write_all(metadata).unwrap();
+            fs::write(&output_name, metadata).unwrap();
         }
 
         sess.abort_if_errors();
diff --git a/src/librustc_fs_util/lib.rs b/src/librustc_fs_util/lib.rs
index ffe420b109d3e..1b0ff4f861c7b 100644
--- a/src/librustc_fs_util/lib.rs
+++ b/src/librustc_fs_util/lib.rs
@@ -116,13 +116,13 @@ pub fn rename_or_copy_remove<P: AsRef<Path>, Q: AsRef<Path>>(p: P,
 }
 
 #[cfg(unix)]
-pub fn path2cstr(p: &Path) -> CString {
-    use std::os::unix::prelude::*;
+pub fn path_to_c_string(p: &Path) -> CString {
+    use std::os::unix::ffi::OsStrExt;
     use std::ffi::OsStr;
     let p: &OsStr = p.as_ref();
     CString::new(p.as_bytes()).unwrap()
 }
 #[cfg(windows)]
-pub fn path2cstr(p: &Path) -> CString {
+pub fn path_to_c_string(p: &Path) -> CString {
     CString::new(p.to_str().unwrap()).unwrap()
 }
diff --git a/src/librustc_mir/interpret/terminator.rs b/src/librustc_mir/interpret/terminator.rs
index 617d204fe1042..300f3d639b588 100644
--- a/src/librustc_mir/interpret/terminator.rs
+++ b/src/librustc_mir/interpret/terminator.rs
@@ -182,6 +182,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     }
 
     fn check_argument_compat(
+        rust_abi: bool,
         caller: TyLayout<'tcx>,
         callee: TyLayout<'tcx>,
     ) -> bool {
@@ -189,13 +190,20 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             // No question
             return true;
         }
+        if !rust_abi {
+            // Don't risk anything
+            return false;
+        }
         // Compare layout
         match (&caller.abi, &callee.abi) {
+            // Different valid ranges are okay (once we enforce validity,
+            // that will take care to make it UB to leave the range, just
+            // like for transmute).
             (layout::Abi::Scalar(ref caller), layout::Abi::Scalar(ref callee)) =>
-                // Different valid ranges are okay (once we enforce validity,
-                // that will take care to make it UB to leave the range, just
-                // like for transmute).
                 caller.value == callee.value,
+            (layout::Abi::ScalarPair(ref caller1, ref caller2),
+             layout::Abi::ScalarPair(ref callee1, ref callee2)) =>
+                caller1.value == callee1.value && caller2.value == callee2.value,
             // Be conservative
             _ => false
         }
@@ -204,22 +212,22 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     /// Pass a single argument, checking the types for compatibility.
     fn pass_argument(
         &mut self,
-        skip_zst: bool,
+        rust_abi: bool,
         caller_arg: &mut impl Iterator<Item=OpTy<'tcx, M::PointerTag>>,
         callee_arg: PlaceTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx> {
-        if skip_zst && callee_arg.layout.is_zst() {
+        if rust_abi && callee_arg.layout.is_zst() {
             // Nothing to do.
             trace!("Skipping callee ZST");
             return Ok(());
         }
         let caller_arg = caller_arg.next()
             .ok_or_else(|| EvalErrorKind::FunctionArgCountMismatch)?;
-        if skip_zst {
+        if rust_abi {
             debug_assert!(!caller_arg.layout.is_zst(), "ZSTs must have been already filtered out");
         }
         // Now, check
-        if !Self::check_argument_compat(caller_arg.layout, callee_arg.layout) {
+        if !Self::check_argument_compat(rust_abi, caller_arg.layout, callee_arg.layout) {
             return err!(FunctionArgMismatch(caller_arg.layout.ty, callee_arg.layout.ty));
         }
         // We allow some transmutes here
@@ -319,7 +327,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                     // Figure out how to pass which arguments.
                     // We have two iterators: Where the arguments come from,
                     // and where they go to.
-                    let skip_zst = match caller_abi {
+                    let rust_abi = match caller_abi {
                         Abi::Rust | Abi::RustCall => true,
                         _ => false
                     };
@@ -344,7 +352,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                         };
                     // Skip ZSTs
                     let mut caller_iter = caller_args.iter()
-                        .filter(|op| !skip_zst || !op.layout.is_zst())
+                        .filter(|op| !rust_abi || !op.layout.is_zst())
                         .map(|op| *op);
 
                     // Now we have to spread them out across the callee's locals,
@@ -359,11 +367,11 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                             // Must be a tuple
                             for i in 0..dest.layout.fields.count() {
                                 let dest = self.place_field(dest, i as u64)?;
-                                self.pass_argument(skip_zst, &mut caller_iter, dest)?;
+                                self.pass_argument(rust_abi, &mut caller_iter, dest)?;
                             }
                         } else {
                             // Normal argument
-                            self.pass_argument(skip_zst, &mut caller_iter, dest)?;
+                            self.pass_argument(rust_abi, &mut caller_iter, dest)?;
                         }
                     }
                     // Now we should have no more caller args
@@ -374,7 +382,11 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                     // Don't forget to check the return type!
                     if let Some(caller_ret) = dest {
                         let callee_ret = self.eval_place(&mir::Place::Local(mir::RETURN_PLACE))?;
-                        if !Self::check_argument_compat(caller_ret.layout, callee_ret.layout) {
+                        if !Self::check_argument_compat(
+                            rust_abi,
+                            caller_ret.layout,
+                            callee_ret.layout,
+                        ) {
                             return err!(FunctionRetMismatch(
                                 caller_ret.layout.ty, callee_ret.layout.ty
                             ));
diff --git a/src/librustc_mir/monomorphize/partitioning.rs b/src/librustc_mir/monomorphize/partitioning.rs
index 6dba020120f84..3a6ee6da42215 100644
--- a/src/librustc_mir/monomorphize/partitioning.rs
+++ b/src/librustc_mir/monomorphize/partitioning.rs
@@ -985,7 +985,7 @@ fn collect_and_partition_mono_items<'a, 'tcx>(
                 output.push_str(" @@");
                 let mut empty = Vec::new();
                 let cgus = item_to_cgus.get_mut(i).unwrap_or(&mut empty);
-                cgus.as_mut_slice().sort_by_key(|&(ref name, _)| name.clone());
+                cgus.as_mut_slice().sort_by_cached_key(|&(ref name, _)| name.clone());
                 cgus.dedup();
                 for &(ref cgu_name, (linkage, _)) in cgus.iter() {
                     output.push_str(" ");
diff --git a/src/librustc_mir/shim.rs b/src/librustc_mir/shim.rs
index 93bf1b3e36e38..04079319a7877 100644
--- a/src/librustc_mir/shim.rs
+++ b/src/librustc_mir/shim.rs
@@ -226,8 +226,13 @@ fn build_drop_shim<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         // The first argument (index 0), but add 1 for the return value.
         let dropee_ptr = Place::Local(Local::new(1+0));
         if tcx.sess.opts.debugging_opts.mir_emit_retag {
-            // We use raw ptr operations, better prepare the alias tracking for that
+            // Function arguments should be retagged
             mir.basic_blocks_mut()[START_BLOCK].statements.insert(0, Statement {
+                source_info,
+                kind: StatementKind::Retag { fn_entry: true, place: dropee_ptr.clone() },
+            });
+            // We use raw ptr operations, better prepare the alias tracking for that
+            mir.basic_blocks_mut()[START_BLOCK].statements.insert(1, Statement {
                 source_info,
                 kind: StatementKind::EscapeToRaw(Operand::Copy(dropee_ptr.clone())),
             })
diff --git a/src/librustdoc/html/render.rs b/src/librustdoc/html/render.rs
index b46efb20d8f6b..6a7b351540fdd 100644
--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -778,10 +778,7 @@ fn write_shared(
     let mut themes: FxHashSet<String> = FxHashSet::default();
 
     for entry in &cx.shared.themes {
-        let mut content = Vec::with_capacity(100000);
-
-        let mut f = try_err!(File::open(&entry), &entry);
-        try_err!(f.read_to_end(&mut content), &entry);
+        let content = try_err!(fs::read(&entry), &entry);
         let theme = try_none!(try_none!(entry.file_stem(), &entry).to_str(), &entry);
         let extension = try_none!(try_none!(entry.extension(), &entry).to_str(), &entry);
         write(cx.dst.join(format!("{}{}.{}", theme, cx.shared.resource_suffix, extension)),
@@ -874,10 +871,7 @@ themePicker.onblur = handleThemeButtonsBlur;
         if !options.enable_minification {
             try_err!(fs::copy(css, out), css);
         } else {
-            let mut f = try_err!(File::open(css), css);
-            let mut buffer = String::with_capacity(1000);
-
-            try_err!(f.read_to_string(&mut buffer), css);
+            let buffer = try_err!(fs::read_to_string(css), css);
             write_minify(out, &buffer, options.enable_minification)?;
         }
     }
@@ -2016,8 +2010,7 @@ impl Context {
                 if !buf.is_empty() {
                     try_err!(this.shared.ensure_dir(&this.dst), &this.dst);
                     let joint_dst = this.dst.join("index.html");
-                    let mut dst = try_err!(File::create(&joint_dst), &joint_dst);
-                    try_err!(dst.write_all(&buf), &joint_dst);
+                    try_err!(fs::write(&joint_dst, buf), &joint_dst);
                 }
 
                 let m = match item.inner {
@@ -2051,8 +2044,7 @@ impl Context {
                 let file_name = &item_path(item_type, name);
                 try_err!(self.shared.ensure_dir(&self.dst), &self.dst);
                 let joint_dst = self.dst.join(file_name);
-                let mut dst = try_err!(File::create(&joint_dst), &joint_dst);
-                try_err!(dst.write_all(&buf), &joint_dst);
+                try_err!(fs::write(&joint_dst, buf), &joint_dst);
 
                 if !self.render_redirect_pages {
                     all.append(full_path(self, &item), &item_type);
diff --git a/src/librustdoc/theme.rs b/src/librustdoc/theme.rs
index 55a11d1976599..206a72cebfef8 100644
--- a/src/librustdoc/theme.rs
+++ b/src/librustdoc/theme.rs
@@ -9,9 +9,8 @@
 // except according to those terms.
 
 use rustc_data_structures::fx::FxHashSet;
-use std::fs::File;
+use std::fs;
 use std::hash::{Hash, Hasher};
-use std::io::Read;
 use std::path::Path;
 
 use errors::Handler;
@@ -278,12 +277,9 @@ pub fn get_differences(against: &CssPath, other: &CssPath, v: &mut Vec<String>)
 pub fn test_theme_against<P: AsRef<Path>>(f: &P, against: &CssPath, diag: &Handler)
     -> (bool, Vec<String>)
 {
-    let mut file = try_something!(File::open(f), diag, (false, Vec::new()));
-    let mut data = Vec::with_capacity(1000);
-
-    try_something!(file.read_to_end(&mut data), diag, (false, Vec::new()));
+    let data = try_something!(fs::read(f), diag, (false, vec![]));
     let paths = load_css_paths(&data);
-    let mut ret = Vec::new();
+    let mut ret = vec![];
     get_differences(against, &paths, &mut ret);
     (true, ret)
 }
diff --git a/src/libserialize/collection_impls.rs b/src/libserialize/collection_impls.rs
index 3e028d755c6d4..cbd642dd6ad91 100644
--- a/src/libserialize/collection_impls.rs
+++ b/src/libserialize/collection_impls.rs
@@ -86,7 +86,7 @@ impl<T: Encodable> Encodable for VecDeque<T> {
 impl<T:Decodable> Decodable for VecDeque<T> {
     fn decode<D: Decoder>(d: &mut D) -> Result<VecDeque<T>, D::Error> {
         d.read_seq(|d, len| {
-            let mut deque: VecDeque<T> = VecDeque::new();
+            let mut deque: VecDeque<T> = VecDeque::with_capacity(len);
             for i in 0..len {
                 deque.push_back(d.read_seq_elt(i, |d| Decodable::decode(d))?);
             }
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index cc8af70a050c9..e2913f399fdb9 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -34,8 +34,8 @@ use tokenstream::{TokenStream, TokenTree};
 use visit::{self, Visitor};
 
 use rustc_data_structures::fx::FxHashMap;
-use std::fs::File;
-use std::io::Read;
+use std::fs;
+use std::io::ErrorKind;
 use std::{iter, mem};
 use std::rc::Rc;
 use std::path::PathBuf;
@@ -1201,50 +1201,62 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
 
 impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
     fn fold_expr(&mut self, expr: P<ast::Expr>) -> P<ast::Expr> {
-        let mut expr = self.cfg.configure_expr(expr).into_inner();
-        expr.node = self.cfg.configure_expr_kind(expr.node);
-
-        // ignore derives so they remain unused
-        let (attr, expr, after_derive) = self.classify_nonitem(expr);
-
-        if attr.is_some() {
-            // collect the invoc regardless of whether or not attributes are permitted here
-            // expansion will eat the attribute so it won't error later
-            attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
-
-            // AstFragmentKind::Expr requires the macro to emit an expression
-            return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
-                                     AstFragmentKind::Expr, after_derive).make_expr();
-        }
+        let expr = self.cfg.configure_expr(expr);
+        expr.map(|mut expr| {
+            expr.node = self.cfg.configure_expr_kind(expr.node);
+
+            // ignore derives so they remain unused
+            let (attr, expr, after_derive) = self.classify_nonitem(expr);
+
+            if attr.is_some() {
+                // Collect the invoc regardless of whether or not attributes are permitted here
+                // expansion will eat the attribute so it won't error later.
+                attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
+
+                // AstFragmentKind::Expr requires the macro to emit an expression.
+                return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
+                                         AstFragmentKind::Expr, after_derive)
+                    .make_expr()
+                    .into_inner()
+            }
 
-        if let ast::ExprKind::Mac(mac) = expr.node {
-            self.check_attributes(&expr.attrs);
-            self.collect_bang(mac, expr.span, AstFragmentKind::Expr).make_expr()
-        } else {
-            P(noop_fold_expr(expr, self))
-        }
+            if let ast::ExprKind::Mac(mac) = expr.node {
+                self.check_attributes(&expr.attrs);
+                self.collect_bang(mac, expr.span, AstFragmentKind::Expr)
+                    .make_expr()
+                    .into_inner()
+            } else {
+                noop_fold_expr(expr, self)
+            }
+        })
     }
 
     fn fold_opt_expr(&mut self, expr: P<ast::Expr>) -> Option<P<ast::Expr>> {
-        let mut expr = configure!(self, expr).into_inner();
-        expr.node = self.cfg.configure_expr_kind(expr.node);
+        let expr = configure!(self, expr);
+        expr.filter_map(|mut expr| {
+            expr.node = self.cfg.configure_expr_kind(expr.node);
 
-        // ignore derives so they remain unused
-        let (attr, expr, after_derive) = self.classify_nonitem(expr);
+            // Ignore derives so they remain unused.
+            let (attr, expr, after_derive) = self.classify_nonitem(expr);
 
-        if attr.is_some() {
-            attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
+            if attr.is_some() {
+                attr.as_ref().map(|a| self.cfg.maybe_emit_expr_attr_err(a));
 
-            return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
-                                     AstFragmentKind::OptExpr, after_derive).make_opt_expr();
-        }
+                return self.collect_attr(attr, vec![], Annotatable::Expr(P(expr)),
+                                         AstFragmentKind::OptExpr, after_derive)
+                    .make_opt_expr()
+                    .map(|expr| expr.into_inner())
+            }
 
-        if let ast::ExprKind::Mac(mac) = expr.node {
-            self.check_attributes(&expr.attrs);
-            self.collect_bang(mac, expr.span, AstFragmentKind::OptExpr).make_opt_expr()
-        } else {
-            Some(P(noop_fold_expr(expr, self)))
-        }
+            if let ast::ExprKind::Mac(mac) = expr.node {
+                self.check_attributes(&expr.attrs);
+                self.collect_bang(mac, expr.span, AstFragmentKind::OptExpr)
+                    .make_opt_expr()
+                    .map(|expr| expr.into_inner())
+            } else {
+                Some(noop_fold_expr(expr, self))
+            }
+        })
     }
 
     fn fold_pat(&mut self, pat: P<ast::Pat>) -> P<ast::Pat> {
@@ -1507,20 +1519,8 @@ impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
                         return noop_fold_attribute(at, self);
                     }
 
-                    let mut buf = vec![];
                     let filename = self.cx.root_path.join(file.to_string());
-
-                    match File::open(&filename).and_then(|mut f| f.read_to_end(&mut buf)) {
-                        Ok(..) => {}
-                        Err(e) => {
-                            self.cx.span_err(at.span,
-                                             &format!("couldn't read {}: {}",
-                                                      filename.display(),
-                                                      e));
-                        }
-                    }
-
-                    match String::from_utf8(buf) {
+                    match fs::read_to_string(&filename) {
                         Ok(src) => {
                             let src_interned = Symbol::intern(&src);
 
@@ -1530,21 +1530,34 @@ impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
 
                             let include_info = vec![
                                 dummy_spanned(ast::NestedMetaItemKind::MetaItem(
-                                        attr::mk_name_value_item_str(Ident::from_str("file"),
-                                                                     dummy_spanned(file)))),
+                                    attr::mk_name_value_item_str(
+                                        Ident::from_str("file"),
+                                        dummy_spanned(file),
+                                    ),
+                                )),
                                 dummy_spanned(ast::NestedMetaItemKind::MetaItem(
-                                        attr::mk_name_value_item_str(Ident::from_str("contents"),
-                                                            dummy_spanned(src_interned)))),
+                                    attr::mk_name_value_item_str(
+                                        Ident::from_str("contents"),
+                                        dummy_spanned(src_interned),
+                                    ),
+                                )),
                             ];
 
                             let include_ident = Ident::from_str("include");
                             let item = attr::mk_list_item(DUMMY_SP, include_ident, include_info);
                             items.push(dummy_spanned(ast::NestedMetaItemKind::MetaItem(item)));
                         }
-                        Err(_) => {
-                            self.cx.span_err(at.span,
-                                             &format!("{} wasn't a utf-8 file",
-                                                      filename.display()));
+                        Err(ref e) if e.kind() == ErrorKind::InvalidData => {
+                            self.cx.span_err(
+                                at.span,
+                                &format!("{} wasn't a utf-8 file", filename.display()),
+                            );
+                        }
+                        Err(e) => {
+                            self.cx.span_err(
+                                at.span,
+                                &format!("couldn't read {}: {}", filename.display(), e),
+                            );
                         }
                     }
                 } else {
diff --git a/src/libsyntax/ext/source_util.rs b/src/libsyntax/ext/source_util.rs
index e1ba8897a47f4..a7875b0ba0506 100644
--- a/src/libsyntax/ext/source_util.rs
+++ b/src/libsyntax/ext/source_util.rs
@@ -21,8 +21,8 @@ use smallvec::SmallVec;
 use symbol::Symbol;
 use tokenstream;
 
-use std::fs::File;
-use std::io::prelude::*;
+use std::fs;
+use std::io::ErrorKind;
 use std::path::PathBuf;
 use rustc_data_structures::sync::Lrc;
 
@@ -137,18 +137,7 @@ pub fn expand_include_str(cx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::TokenT
         None => return DummyResult::expr(sp)
     };
     let file = res_rel_file(cx, sp, file);
-    let mut bytes = Vec::new();
-    match File::open(&file).and_then(|mut f| f.read_to_end(&mut bytes)) {
-        Ok(..) => {}
-        Err(e) => {
-            cx.span_err(sp,
-                        &format!("couldn't read {}: {}",
-                                file.display(),
-                                e));
-            return DummyResult::expr(sp);
-        }
-    };
-    match String::from_utf8(bytes) {
+    match fs::read_to_string(&file) {
         Ok(src) => {
             let interned_src = Symbol::intern(&src);
 
@@ -157,11 +146,13 @@ pub fn expand_include_str(cx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::TokenT
             cx.source_map().new_source_file(file.into(), src);
 
             base::MacEager::expr(cx.expr_str(sp, interned_src))
+        },
+        Err(ref e) if e.kind() == ErrorKind::InvalidData => {
+            cx.span_err(sp, &format!("{} wasn't a utf-8 file", file.display()));
+            DummyResult::expr(sp)
         }
-        Err(_) => {
-            cx.span_err(sp,
-                        &format!("{} wasn't a utf-8 file",
-                                file.display()));
+        Err(e) => {
+            cx.span_err(sp, &format!("couldn't read {}: {}", file.display(), e));
             DummyResult::expr(sp)
         }
     }
@@ -174,20 +165,18 @@ pub fn expand_include_bytes(cx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::Toke
         None => return DummyResult::expr(sp)
     };
     let file = res_rel_file(cx, sp, file);
-    let mut bytes = Vec::new();
-    match File::open(&file).and_then(|mut f| f.read_to_end(&mut bytes)) {
-        Err(e) => {
-            cx.span_err(sp,
-                        &format!("couldn't read {}: {}", file.display(), e));
-            DummyResult::expr(sp)
-        }
-        Ok(..) => {
+    match fs::read(&file) {
+        Ok(bytes) => {
             // Add this input file to the code map to make it available as
             // dependency information, but don't enter it's contents
             cx.source_map().new_source_file(file.into(), String::new());
 
             base::MacEager::expr(cx.expr_lit(sp, ast::LitKind::ByteStr(Lrc::new(bytes))))
         }
+        Err(e) => {
+            cx.span_err(sp, &format!("couldn't read {}: {}", file.display(), e));
+            DummyResult::expr(sp)
+        }
     }
 }
 
diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index bb47d9b535bef..9fbc64758da4d 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -72,7 +72,7 @@ impl<T: 'static> P<T> {
         *self.ptr
     }
 
-    /// Transform the inner value, consuming `self` and producing a new `P<T>`.
+    /// Produce a new `P<T>` from `self` without reallocating.
     pub fn map<F>(mut self, f: F) -> P<T> where
         F: FnOnce(T) -> T,
     {
@@ -88,8 +88,30 @@ impl<T: 'static> P<T> {
             ptr::write(p, f(ptr::read(p)));
 
             // Recreate self from the raw pointer.
-            P {
-                ptr: Box::from_raw(p)
+            P { ptr: Box::from_raw(p) }
+        }
+    }
+
+    /// Optionally produce a new `P<T>` from `self` without reallocating.
+    pub fn filter_map<F>(mut self, f: F) -> Option<P<T>> where
+        F: FnOnce(T) -> Option<T>,
+    {
+        let p: *mut T = &mut *self.ptr;
+
+        // Leak self in case of panic.
+        // FIXME(eddyb) Use some sort of "free guard" that
+        // only deallocates, without dropping the pointee,
+        // in case the call the `f` below ends in a panic.
+        mem::forget(self);
+
+        unsafe {
+            if let Some(v) = f(ptr::read(p)) {
+                ptr::write(p, v);
+
+                // Recreate self from the raw pointer.
+                Some(P { ptr: Box::from_raw(p) })
+            } else {
+                None
             }
         }
     }
diff --git a/src/libsyntax/source_map.rs b/src/libsyntax/source_map.rs
index e8cacc3b5aff3..ff158b8369cb7 100644
--- a/src/libsyntax/source_map.rs
+++ b/src/libsyntax/source_map.rs
@@ -31,7 +31,7 @@ use std::path::{Path, PathBuf};
 
 use std::env;
 use std::fs;
-use std::io::{self, Read};
+use std::io;
 use errors::SourceMapper;
 
 /// Return the span itself if it doesn't come from a macro expansion,
@@ -96,9 +96,7 @@ impl FileLoader for RealFileLoader {
     }
 
     fn read_file(&self, path: &Path) -> io::Result<String> {
-        let mut src = String::new();
-        fs::File::open(path)?.read_to_string(&mut src)?;
-        Ok(src)
+        fs::read_to_string(path)
     }
 }
 
diff --git a/src/test/codegen-units/item-collection/drop_in_place_intrinsic.rs b/src/test/codegen-units/item-collection/drop_in_place_intrinsic.rs
index cec88f1c6a245..1793a311f968a 100644
--- a/src/test/codegen-units/item-collection/drop_in_place_intrinsic.rs
+++ b/src/test/codegen-units/item-collection/drop_in_place_intrinsic.rs
@@ -14,7 +14,7 @@
 
 #![feature(start)]
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<drop_in_place_intrinsic::StructWithDtor[0]> @@ drop_in_place_intrinsic-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<drop_in_place_intrinsic::StructWithDtor[0]> @@ drop_in_place_intrinsic-cgu.0[Internal]
 struct StructWithDtor(u32);
 
 impl Drop for StructWithDtor {
@@ -26,7 +26,7 @@ impl Drop for StructWithDtor {
 #[start]
 fn start(_: isize, _: *const *const u8) -> isize {
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<[drop_in_place_intrinsic::StructWithDtor[0]; 2]> @@ drop_in_place_intrinsic-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<[drop_in_place_intrinsic::StructWithDtor[0]; 2]> @@ drop_in_place_intrinsic-cgu.0[Internal]
     let x = [StructWithDtor(0), StructWithDtor(1)];
 
     drop_slice_in_place(&x);
@@ -41,6 +41,7 @@ fn drop_slice_in_place(x: &[StructWithDtor]) {
         // not have drop-glue for the unsized [StructWithDtor]. This has to be
         // generated though when the drop_in_place() intrinsic is used.
         //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<[drop_in_place_intrinsic::StructWithDtor[0]]> @@ drop_in_place_intrinsic-cgu.0[Internal]
+        //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<[drop_in_place_intrinsic::StructWithDtor[0]]> @@ drop_in_place_intrinsic-cgu.0[Internal]
         ::std::ptr::drop_in_place(x as *const _ as *mut [StructWithDtor]);
     }
 }
diff --git a/src/test/codegen-units/item-collection/generic-drop-glue.rs b/src/test/codegen-units/item-collection/generic-drop-glue.rs
index 5afa519bc5902..4ae5832ec4ed4 100644
--- a/src/test/codegen-units/item-collection/generic-drop-glue.rs
+++ b/src/test/codegen-units/item-collection/generic-drop-glue.rs
@@ -47,7 +47,7 @@ enum EnumNoDrop<T1, T2> {
 struct NonGenericNoDrop(i32);
 
 struct NonGenericWithDrop(i32);
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::NonGenericWithDrop[0]> @@ generic_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::NonGenericWithDrop[0]> @@ generic_drop_glue-cgu.0[Internal]
 
 impl Drop for NonGenericWithDrop {
     //~ MONO_ITEM fn generic_drop_glue::{{impl}}[2]::drop[0]
@@ -57,11 +57,11 @@ impl Drop for NonGenericWithDrop {
 //~ MONO_ITEM fn generic_drop_glue::start[0]
 #[start]
 fn start(_: isize, _: *const *const u8) -> isize {
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::StructWithDrop[0]<i8, char>> @@ generic_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::StructWithDrop[0]<i8, char>> @@ generic_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn generic_drop_glue::{{impl}}[0]::drop[0]<i8, char>
     let _ = StructWithDrop { x: 0i8, y: 'a' }.x;
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::StructWithDrop[0]<&str, generic_drop_glue::NonGenericNoDrop[0]>> @@ generic_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::StructWithDrop[0]<&str, generic_drop_glue::NonGenericNoDrop[0]>> @@ generic_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn generic_drop_glue::{{impl}}[0]::drop[0]<&str, generic_drop_glue::NonGenericNoDrop[0]>
     let _ = StructWithDrop { x: "&str", y: NonGenericNoDrop(0) }.y;
 
@@ -70,17 +70,17 @@ fn start(_: isize, _: *const *const u8) -> isize {
 
     // This is supposed to generate drop-glue because it contains a field that
     // needs to be dropped.
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::StructNoDrop[0]<generic_drop_glue::NonGenericWithDrop[0], f64>> @@ generic_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::StructNoDrop[0]<generic_drop_glue::NonGenericWithDrop[0], f64>> @@ generic_drop_glue-cgu.0[Internal]
     let _ = StructNoDrop { x: NonGenericWithDrop(0), y: 0f64 }.y;
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::EnumWithDrop[0]<i32, i64>> @@ generic_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::EnumWithDrop[0]<i32, i64>> @@ generic_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn generic_drop_glue::{{impl}}[1]::drop[0]<i32, i64>
     let _ = match EnumWithDrop::A::<i32, i64>(0) {
         EnumWithDrop::A(x) => x,
         EnumWithDrop::B(x) => x as i32
     };
 
-    //~MONO_ITEM fn core::ptr[0]::drop_in_place[0]<generic_drop_glue::EnumWithDrop[0]<f64, f32>> @@ generic_drop_glue-cgu.0[Internal]
+    //~MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<generic_drop_glue::EnumWithDrop[0]<f64, f32>> @@ generic_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn generic_drop_glue::{{impl}}[1]::drop[0]<f64, f32>
     let _ = match EnumWithDrop::B::<f64, f32>(1.0) {
         EnumWithDrop::A(x) => x,
diff --git a/src/test/codegen-units/item-collection/instantiation-through-vtable.rs b/src/test/codegen-units/item-collection/instantiation-through-vtable.rs
index d09d343a8458e..bfa67b16d0e63 100644
--- a/src/test/codegen-units/item-collection/instantiation-through-vtable.rs
+++ b/src/test/codegen-units/item-collection/instantiation-through-vtable.rs
@@ -34,13 +34,13 @@ impl<T> Trait for Struct<T> {
 fn start(_: isize, _: *const *const u8) -> isize {
     let s1 = Struct { _a: 0u32 };
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<instantiation_through_vtable::Struct[0]<u32>> @@ instantiation_through_vtable-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<instantiation_through_vtable::Struct[0]<u32>> @@ instantiation_through_vtable-cgu.0[Internal]
     //~ MONO_ITEM fn instantiation_through_vtable::{{impl}}[0]::foo[0]<u32>
     //~ MONO_ITEM fn instantiation_through_vtable::{{impl}}[0]::bar[0]<u32>
     let _ = &s1 as &Trait;
 
     let s1 = Struct { _a: 0u64 };
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<instantiation_through_vtable::Struct[0]<u64>> @@ instantiation_through_vtable-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<instantiation_through_vtable::Struct[0]<u64>> @@ instantiation_through_vtable-cgu.0[Internal]
     //~ MONO_ITEM fn instantiation_through_vtable::{{impl}}[0]::foo[0]<u64>
     //~ MONO_ITEM fn instantiation_through_vtable::{{impl}}[0]::bar[0]<u64>
     let _ = &s1 as &Trait;
diff --git a/src/test/codegen-units/item-collection/non-generic-drop-glue.rs b/src/test/codegen-units/item-collection/non-generic-drop-glue.rs
index a939dd56cda5b..2d72383391daa 100644
--- a/src/test/codegen-units/item-collection/non-generic-drop-glue.rs
+++ b/src/test/codegen-units/item-collection/non-generic-drop-glue.rs
@@ -15,7 +15,7 @@
 #![deny(dead_code)]
 #![feature(start)]
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<non_generic_drop_glue::StructWithDrop[0]> @@ non_generic_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<non_generic_drop_glue::StructWithDrop[0]> @@ non_generic_drop_glue-cgu.0[Internal]
 struct StructWithDrop {
     x: i32
 }
@@ -29,7 +29,7 @@ struct StructNoDrop {
     x: i32
 }
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<non_generic_drop_glue::EnumWithDrop[0]> @@ non_generic_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<non_generic_drop_glue::EnumWithDrop[0]> @@ non_generic_drop_glue-cgu.0[Internal]
 enum EnumWithDrop {
     A(i32)
 }
diff --git a/src/test/codegen-units/item-collection/transitive-drop-glue.rs b/src/test/codegen-units/item-collection/transitive-drop-glue.rs
index 7bbc9b6d0fbc9..d90171726b2c6 100644
--- a/src/test/codegen-units/item-collection/transitive-drop-glue.rs
+++ b/src/test/codegen-units/item-collection/transitive-drop-glue.rs
@@ -15,11 +15,11 @@
 #![deny(dead_code)]
 #![feature(start)]
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::Root[0]> @@ transitive_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::Root[0]> @@ transitive_drop_glue-cgu.0[Internal]
 struct Root(Intermediate);
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::Intermediate[0]> @@ transitive_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::Intermediate[0]> @@ transitive_drop_glue-cgu.0[Internal]
 struct Intermediate(Leaf);
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::Leaf[0]> @@ transitive_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::Leaf[0]> @@ transitive_drop_glue-cgu.0[Internal]
 struct Leaf;
 
 impl Drop for Leaf {
@@ -40,15 +40,15 @@ impl<T> Drop for LeafGen<T> {
 fn start(_: isize, _: *const *const u8) -> isize {
     let _ = Root(Intermediate(Leaf));
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::RootGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::IntermediateGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::LeafGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::RootGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::IntermediateGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::LeafGen[0]<u32>> @@ transitive_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn transitive_drop_glue::{{impl}}[1]::drop[0]<u32>
     let _ = RootGen(IntermediateGen(LeafGen(0u32)));
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::RootGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::IntermediateGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<transitive_drop_glue::LeafGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::RootGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::IntermediateGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<transitive_drop_glue::LeafGen[0]<i16>> @@ transitive_drop_glue-cgu.0[Internal]
     //~ MONO_ITEM fn transitive_drop_glue::{{impl}}[1]::drop[0]<i16>
     let _ = RootGen(IntermediateGen(LeafGen(0i16)));
 
diff --git a/src/test/codegen-units/item-collection/tuple-drop-glue.rs b/src/test/codegen-units/item-collection/tuple-drop-glue.rs
index 865570ccfa55a..e94af0c2989b2 100644
--- a/src/test/codegen-units/item-collection/tuple-drop-glue.rs
+++ b/src/test/codegen-units/item-collection/tuple-drop-glue.rs
@@ -15,7 +15,7 @@
 #![deny(dead_code)]
 #![feature(start)]
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<tuple_drop_glue::Dropped[0]> @@ tuple_drop_glue-cgu.0[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<tuple_drop_glue::Dropped[0]> @@ tuple_drop_glue-cgu.0[Internal]
 struct Dropped;
 
 impl Drop for Dropped {
@@ -26,11 +26,11 @@ impl Drop for Dropped {
 //~ MONO_ITEM fn tuple_drop_glue::start[0]
 #[start]
 fn start(_: isize, _: *const *const u8) -> isize {
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<(u32, tuple_drop_glue::Dropped[0])> @@ tuple_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<(u32, tuple_drop_glue::Dropped[0])> @@ tuple_drop_glue-cgu.0[Internal]
     let x = (0u32, Dropped);
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<(i16, (tuple_drop_glue::Dropped[0], bool))> @@ tuple_drop_glue-cgu.0[Internal]
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<(tuple_drop_glue::Dropped[0], bool)> @@ tuple_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<(i16, (tuple_drop_glue::Dropped[0], bool))> @@ tuple_drop_glue-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<(tuple_drop_glue::Dropped[0], bool)> @@ tuple_drop_glue-cgu.0[Internal]
     let x = (0i16, (Dropped, true));
 
     0
diff --git a/src/test/codegen-units/item-collection/unsizing.rs b/src/test/codegen-units/item-collection/unsizing.rs
index 5e9a3258c7adf..c85f7fc1bdd8d 100644
--- a/src/test/codegen-units/item-collection/unsizing.rs
+++ b/src/test/codegen-units/item-collection/unsizing.rs
@@ -59,13 +59,13 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Wrapper<U>> for Wrapper<T>
 fn start(_: isize, _: *const *const u8) -> isize {
     // simple case
     let bool_sized = &true;
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<bool> @@ unsizing-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<bool> @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn unsizing::{{impl}}[0]::foo[0]
     let _bool_unsized = bool_sized as &Trait;
 
     let char_sized = &'a';
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<char> @@ unsizing-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<char> @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn unsizing::{{impl}}[1]::foo[0]
     let _char_unsized = char_sized as &Trait;
 
@@ -75,13 +75,13 @@ fn start(_: isize, _: *const *const u8) -> isize {
         _b: 2,
         _c: 3.0f64
     };
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<f64> @@ unsizing-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<f64> @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn unsizing::{{impl}}[2]::foo[0]
     let _struct_unsized = struct_sized as &Struct<Trait>;
 
     // custom coercion
     let wrapper_sized = Wrapper(&0u32);
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<u32> @@ unsizing-cgu.0[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<u32> @@ unsizing-cgu.0[Internal]
     //~ MONO_ITEM fn unsizing::{{impl}}[3]::foo[0]
     let _wrapper_sized = wrapper_sized as Wrapper<Trait>;
 
diff --git a/src/test/codegen-units/partitioning/extern-drop-glue.rs b/src/test/codegen-units/partitioning/extern-drop-glue.rs
index cbad3a638840c..87b4430b4cab6 100644
--- a/src/test/codegen-units/partitioning/extern-drop-glue.rs
+++ b/src/test/codegen-units/partitioning/extern-drop-glue.rs
@@ -21,14 +21,14 @@
 // aux-build:cgu_extern_drop_glue.rs
 extern crate cgu_extern_drop_glue;
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<cgu_extern_drop_glue::Struct[0]> @@ extern_drop_glue[Internal] extern_drop_glue-mod1[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<cgu_extern_drop_glue::Struct[0]> @@ extern_drop_glue[Internal] extern_drop_glue-mod1[Internal]
 
 struct LocalStruct(cgu_extern_drop_glue::Struct);
 
 //~ MONO_ITEM fn extern_drop_glue::user[0] @@ extern_drop_glue[External]
 pub fn user()
 {
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<extern_drop_glue::LocalStruct[0]> @@ extern_drop_glue[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<extern_drop_glue::LocalStruct[0]> @@ extern_drop_glue[Internal]
     let _ = LocalStruct(cgu_extern_drop_glue::Struct(0));
 }
 
@@ -40,7 +40,7 @@ pub mod mod1 {
     //~ MONO_ITEM fn extern_drop_glue::mod1[0]::user[0] @@ extern_drop_glue-mod1[External]
     pub fn user()
     {
-        //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<extern_drop_glue::mod1[0]::LocalStruct[0]> @@ extern_drop_glue-mod1[Internal]
+        //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<extern_drop_glue::mod1[0]::LocalStruct[0]> @@ extern_drop_glue-mod1[Internal]
         let _ = LocalStruct(cgu_extern_drop_glue::Struct(0));
     }
 }
diff --git a/src/test/codegen-units/partitioning/local-drop-glue.rs b/src/test/codegen-units/partitioning/local-drop-glue.rs
index 98729d99ea950..e09eb318708e1 100644
--- a/src/test/codegen-units/partitioning/local-drop-glue.rs
+++ b/src/test/codegen-units/partitioning/local-drop-glue.rs
@@ -17,7 +17,7 @@
 #![allow(dead_code)]
 #![crate_type="rlib"]
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<local_drop_glue::Struct[0]> @@ local_drop_glue[Internal] local_drop_glue-mod1[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<local_drop_glue::Struct[0]> @@ local_drop_glue[Internal] local_drop_glue-mod1[Internal]
 struct Struct {
     _a: u32
 }
@@ -27,7 +27,7 @@ impl Drop for Struct {
     fn drop(&mut self) {}
 }
 
-//~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<local_drop_glue::Outer[0]> @@ local_drop_glue[Internal]
+//~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<local_drop_glue::Outer[0]> @@ local_drop_glue[Internal]
 struct Outer {
     _a: Struct
 }
@@ -46,10 +46,10 @@ pub mod mod1
 {
     use super::Struct;
 
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<local_drop_glue::mod1[0]::Struct2[0]> @@ local_drop_glue-mod1[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<local_drop_glue::mod1[0]::Struct2[0]> @@ local_drop_glue-mod1[Internal]
     struct Struct2 {
         _a: Struct,
-        //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<(u32, local_drop_glue::Struct[0])> @@ local_drop_glue-mod1[Internal]
+        //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<(u32, local_drop_glue::Struct[0])> @@ local_drop_glue-mod1[Internal]
         _b: (u32, Struct),
     }
 
diff --git a/src/test/codegen-units/partitioning/vtable-through-const.rs b/src/test/codegen-units/partitioning/vtable-through-const.rs
index ebda08a29f66d..459c6b6f15410 100644
--- a/src/test/codegen-units/partitioning/vtable-through-const.rs
+++ b/src/test/codegen-units/partitioning/vtable-through-const.rs
@@ -76,7 +76,7 @@ mod mod1 {
 //~ MONO_ITEM fn vtable_through_const::start[0]
 #[start]
 fn start(_: isize, _: *const *const u8) -> isize {
-    //~ MONO_ITEM fn core::ptr[0]::drop_in_place[0]<u32> @@ vtable_through_const[Internal]
+    //~ MONO_ITEM fn core::ptr[0]::real_drop_in_place[0]<u32> @@ vtable_through_const[Internal]
 
     // Since Trait1::do_something() is instantiated via its default implementation,
     // it is considered a generic and is instantiated here only because it is
diff --git a/src/test/run-make/llvm-outputs/Makefile b/src/test/run-make/llvm-outputs/Makefile
new file mode 100644
index 0000000000000..d7f67577b04ce
--- /dev/null
+++ b/src/test/run-make/llvm-outputs/Makefile
@@ -0,0 +1,5 @@
+-include ../../run-make-fulldeps/tools.mk
+
+all:
+	echo 'fn main() {}' | $(BARE_RUSTC) - --out-dir=$(TMPDIR)/random_directory_that_does_not_exist_ir/ --emit=llvm-ir
+	echo 'fn main() {}' | $(BARE_RUSTC) - --out-dir=$(TMPDIR)/random_directory_that_does_not_exist_bc/ --emit=llvm-bc
diff --git a/src/test/ui/issues/issue-56202.rs b/src/test/ui/issues/issue-56202.rs
new file mode 100644
index 0000000000000..c80c372c9e2ea
--- /dev/null
+++ b/src/test/ui/issues/issue-56202.rs
@@ -0,0 +1,15 @@
+#![feature(self_struct_ctor)]
+
+trait FooTrait {}
+
+trait BarTrait {
+    fn foo<T: FooTrait>(_: T) -> Self;
+}
+
+struct FooStruct(u32);
+
+impl BarTrait for FooStruct {
+    fn foo<T: FooTrait>(_: T) -> Self {
+        Self(u32::default())
+    }
+}
diff --git a/src/test/ui/issues/issue-56202.stderr b/src/test/ui/issues/issue-56202.stderr
new file mode 100644
index 0000000000000..3007b08450298
--- /dev/null
+++ b/src/test/ui/issues/issue-56202.stderr
@@ -0,0 +1,7 @@
+error[E0601]: `main` function not found in crate `issue_56202`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-56202.rs`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0601`.
diff --git a/src/tools/build-manifest/src/main.rs b/src/tools/build-manifest/src/main.rs
index bf95b31ae3ca4..d9834f9f0a004 100644
--- a/src/tools/build-manifest/src/main.rs
+++ b/src/tools/build-manifest/src/main.rs
@@ -606,7 +606,7 @@ impl Builder {
 
         let filename = path.file_name().unwrap().to_str().unwrap();
         let sha256 = self.output.join(format!("{}.sha256", filename));
-        t!(t!(File::create(&sha256)).write_all(&sha.stdout));
+        t!(fs::write(&sha256, &sha.stdout));
 
         let stdout = String::from_utf8_lossy(&sha.stdout);
         stdout.split_whitespace().next().unwrap().to_string()
@@ -643,7 +643,7 @@ impl Builder {
 
     fn write(&self, contents: &str, channel_name: &str, suffix: &str) {
         let dst = self.output.join(format!("channel-rust-{}{}", channel_name, suffix));
-        t!(t!(File::create(&dst)).write_all(contents.as_bytes()));
+        t!(fs::write(&dst, contents));
         self.hash(&dst);
         self.sign(&dst);
     }
diff --git a/src/tools/cargotest/main.rs b/src/tools/cargotest/main.rs
index 5b717c45254cc..ab74e7b7a7fd8 100644
--- a/src/tools/cargotest/main.rs
+++ b/src/tools/cargotest/main.rs
@@ -11,8 +11,7 @@
 use std::env;
 use std::process::Command;
 use std::path::{Path, PathBuf};
-use std::fs::File;
-use std::io::Write;
+use std::fs;
 
 struct Test {
     repo: &'static str,
@@ -91,10 +90,7 @@ fn test_repo(cargo: &Path, out_dir: &Path, test: &Test) {
     println!("testing {}", test.repo);
     let dir = clone_repo(test, out_dir);
     if let Some(lockfile) = test.lock {
-        File::create(&dir.join("Cargo.lock"))
-            .expect("")
-            .write_all(lockfile.as_bytes())
-            .expect("");
+        fs::write(&dir.join("Cargo.lock"), lockfile).unwrap();
     }
     if !run_cargo_test(cargo, &dir, test.packages) {
         panic!("tests failed for {}", test.repo);
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index f63950b448a14..65f6bff7eaf55 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -38,7 +38,7 @@ use getopts::Options;
 use std::env;
 use std::ffi::OsString;
 use std::fs;
-use std::io::{self, Read};
+use std::io::{self, ErrorKind};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 use test::ColorConfig;
@@ -686,13 +686,11 @@ fn up_to_date(
 ) -> bool {
     let stamp_name = stamp(config, testpaths, revision);
     // Check hash.
-    let mut f = match fs::File::open(&stamp_name) {
+    let contents = match fs::read_to_string(&stamp_name) {
         Ok(f) => f,
+        Err(ref e) if e.kind() == ErrorKind::InvalidData => panic!("Can't read stamp contents"),
         Err(_) => return true,
     };
-    let mut contents = String::new();
-    f.read_to_string(&mut contents)
-        .expect("Can't read stamp contents");
     let expected_hash = runtest::compute_stamp_hash(config);
     if contents != expected_hash {
         return true;
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index 399f9f577edf5..f7a3348b771d1 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -458,11 +458,7 @@ impl<'test> TestCx<'test> {
             None => 2,
         };
 
-        let mut src = String::new();
-        File::open(&self.testpaths.file)
-            .unwrap()
-            .read_to_string(&mut src)
-            .unwrap();
+        let src = fs::read_to_string(&self.testpaths.file).unwrap();
         let mut srcs = vec![src];
 
         let mut round = 0;
@@ -500,12 +496,7 @@ impl<'test> TestCx<'test> {
         let mut expected = match self.props.pp_exact {
             Some(ref file) => {
                 let filepath = self.testpaths.file.parent().unwrap().join(file);
-                let mut s = String::new();
-                File::open(&filepath)
-                    .unwrap()
-                    .read_to_string(&mut s)
-                    .unwrap();
-                s
+                fs::read_to_string(&filepath).unwrap()
             }
             None => srcs[srcs.len() - 2].clone(),
         };
@@ -1949,10 +1940,7 @@ impl<'test> TestCx<'test> {
 
     fn dump_output_file(&self, out: &str, extension: &str) {
         let outfile = self.make_out_name(extension);
-        File::create(&outfile)
-            .unwrap()
-            .write_all(out.as_bytes())
-            .unwrap();
+        fs::write(&outfile, out).unwrap();
     }
 
     /// Create a filename for output with the given extension.  Example:
@@ -2149,11 +2137,7 @@ impl<'test> TestCx<'test> {
         path: &P,
         mut other_files: Option<&mut Vec<String>>,
     ) -> Vec<usize> {
-        let mut file =
-            fs::File::open(path).expect("markdown_test_output_check_entry File::open failed");
-        let mut content = String::new();
-        file.read_to_string(&mut content)
-            .expect("markdown_test_output_check_entry read_to_string failed");
+        let content = fs::read_to_string(&path).unwrap();
         let mut ignore = false;
         content
             .lines()
@@ -2824,11 +2808,7 @@ impl<'test> TestCx<'test> {
     }
 
     fn check_mir_dump(&self) {
-        let mut test_file_contents = String::new();
-        fs::File::open(self.testpaths.file.clone())
-            .unwrap()
-            .read_to_string(&mut test_file_contents)
-            .unwrap();
+        let test_file_contents = fs::read_to_string(&self.testpaths.file).unwrap();
         if let Some(idx) = test_file_contents.find("// END RUST SOURCE") {
             let (_, tests_text) = test_file_contents.split_at(idx + "// END_RUST SOURCE".len());
             let tests_text_str = String::from(tests_text);
@@ -2892,9 +2872,7 @@ impl<'test> TestCx<'test> {
         }
         self.check_mir_test_timestamp(test_name, &output_file);
 
-        let mut dumped_file = fs::File::open(output_file.clone()).unwrap();
-        let mut dumped_string = String::new();
-        dumped_file.read_to_string(&mut dumped_string).unwrap();
+        let dumped_string = fs::read_to_string(&output_file).unwrap();
         let mut dumped_lines = dumped_string
             .lines()
             .map(|l| nocomment_mir_line(l))
@@ -3106,19 +3084,13 @@ impl<'test> TestCx<'test> {
     }
 
     fn load_expected_output_from_path(&self, path: &Path) -> Result<String, String> {
-        let mut result = String::new();
-        match File::open(path).and_then(|mut f| f.read_to_string(&mut result)) {
-            Ok(_) => Ok(result),
-            Err(e) => Err(format!(
-                "failed to load expected output from `{}`: {}",
-                path.display(),
-                e
-            )),
-        }
+        fs::read_to_string(path).map_err(|err| {
+            format!("failed to load expected output from `{}`: {}", path.display(), err)
+        })
     }
 
     fn delete_file(&self, file: &PathBuf) {
-        if let Err(e) = ::std::fs::remove_file(file) {
+        if let Err(e) = fs::remove_file(file) {
             self.fatal(&format!(
                 "failed to delete `{}`: {}",
                 file.display(),
@@ -3180,16 +3152,13 @@ impl<'test> TestCx<'test> {
         for output_file in &files {
             if actual.is_empty() {
                 self.delete_file(output_file);
-            } else {
-                match File::create(&output_file).and_then(|mut f| f.write_all(actual.as_bytes())) {
-                    Ok(()) => {}
-                    Err(e) => self.fatal(&format!(
-                        "failed to write {} to `{}`: {}",
-                        kind,
-                        output_file.display(),
-                        e
-                    )),
-                }
+            } else if let Err(err) = fs::write(&output_file, &actual) {
+                self.fatal(&format!(
+                    "failed to write {} to `{}`: {}",
+                    kind,
+                    output_file.display(),
+                    err,
+                ));
             }
         }
 
@@ -3241,9 +3210,8 @@ impl<'test> TestCx<'test> {
     }
 
     fn create_stamp(&self) {
-        let mut f = File::create(::stamp(&self.config, self.testpaths, self.revision)).unwrap();
-        f.write_all(compute_stamp_hash(&self.config).as_bytes())
-            .unwrap();
+        let stamp = ::stamp(&self.config, self.testpaths, self.revision);
+        fs::write(&stamp, compute_stamp_hash(&self.config)).unwrap();
     }
 }
 
diff --git a/src/tools/error_index_generator/main.rs b/src/tools/error_index_generator/main.rs
index a5556e1d570eb..03add412b6451 100644
--- a/src/tools/error_index_generator/main.rs
+++ b/src/tools/error_index_generator/main.rs
@@ -18,8 +18,8 @@ extern crate serialize as rustc_serialize;
 use std::collections::BTreeMap;
 use std::env;
 use std::error::Error;
-use std::fs::{read_dir, File};
-use std::io::{Read, Write};
+use std::fs::{self, read_dir, File};
+use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
 use std::cell::RefCell;
@@ -210,8 +210,7 @@ fn load_all_errors(metadata_dir: &Path) -> Result<ErrorMetadataMap, Box<dyn Erro
     for entry in read_dir(metadata_dir)? {
         let path = entry?.path();
 
-        let mut metadata_str = String::new();
-        File::open(&path).and_then(|mut f| f.read_to_string(&mut metadata_str))?;
+        let metadata_str = fs::read_to_string(&path)?;
 
         let some_errors: ErrorMetadataMap = json::decode(&metadata_str)?;
 
diff --git a/src/tools/linkchecker/main.rs b/src/tools/linkchecker/main.rs
index ca7e0224dc328..9fe24074ac90b 100644
--- a/src/tools/linkchecker/main.rs
+++ b/src/tools/linkchecker/main.rs
@@ -25,8 +25,7 @@
 //! but this should catch the majority of "broken link" cases.
 
 use std::env;
-use std::fs::File;
-use std::io::prelude::*;
+use std::fs;
 use std::path::{Path, PathBuf, Component};
 use std::collections::{HashMap, HashSet};
 use std::collections::hash_map::Entry;
@@ -288,7 +287,7 @@ fn load_file(cache: &mut Cache,
              file: &Path,
              redirect: Redirect)
              -> Result<(PathBuf, String), LoadError> {
-    let mut contents = String::new();
+    let contents;
     let pretty_file = PathBuf::from(file.strip_prefix(root).unwrap_or(&file));
 
     let maybe_redirect = match cache.entry(pretty_file.clone()) {
@@ -297,14 +296,13 @@ fn load_file(cache: &mut Cache,
             None
         }
         Entry::Vacant(entry) => {
-            let mut fp = File::open(file).map_err(|err| {
+            contents = fs::read_to_string(&file).map_err(|err| {
                 if let FromRedirect(true) = redirect {
                     LoadError::BrokenRedirect(file.to_path_buf(), err)
                 } else {
                     LoadError::IOError(err)
                 }
             })?;
-            fp.read_to_string(&mut contents).map_err(|err| LoadError::IOError(err))?;
 
             let maybe = maybe_redirect(&contents);
             if maybe.is_some() {
diff --git a/src/tools/tidy/src/bins.rs b/src/tools/tidy/src/bins.rs
index f6e42c8dc17b1..ae930555d0912 100644
--- a/src/tools/tidy/src/bins.rs
+++ b/src/tools/tidy/src/bins.rs
@@ -24,13 +24,10 @@ pub fn check(_path: &Path, _bad: &mut bool) {}
 #[cfg(unix)]
 pub fn check(path: &Path, bad: &mut bool) {
     use std::fs;
-    use std::io::Read;
     use std::process::{Command, Stdio};
     use std::os::unix::prelude::*;
 
-    if let Ok(mut file) = fs::File::open("/proc/version") {
-        let mut contents = String::new();
-        file.read_to_string(&mut contents).unwrap();
+    if let Ok(contents) = fs::read_to_string("/proc/version") {
         // Probably on Windows Linux Subsystem or Docker via VirtualBox,
         // all files will be marked as executable, so skip checking.
         if contents.contains("Microsoft") || contents.contains("boot2docker") {
diff --git a/src/tools/tidy/src/cargo.rs b/src/tools/tidy/src/cargo.rs
index 466d2fa0d2bd6..ac7e968dff76c 100644
--- a/src/tools/tidy/src/cargo.rs
+++ b/src/tools/tidy/src/cargo.rs
@@ -15,8 +15,7 @@
 //! `extern crate` declarations. This should help us keep the DAG correctly
 //! structured through various refactorings to prune out unnecessary edges.
 
-use std::io::prelude::*;
-use std::fs::File;
+use std::fs;
 use std::path::Path;
 
 pub fn check(path: &Path, bad: &mut bool) {
@@ -41,10 +40,8 @@ pub fn check(path: &Path, bad: &mut bool) {
 // Verify that the dependencies in Cargo.toml at `tomlfile` are sync'd with the
 // `extern crate` annotations in the lib.rs at `libfile`.
 fn verify(tomlfile: &Path, libfile: &Path, bad: &mut bool) {
-    let mut toml = String::new();
-    let mut librs = String::new();
-    t!(t!(File::open(tomlfile)).read_to_string(&mut toml));
-    t!(t!(File::open(libfile)).read_to_string(&mut librs));
+    let toml = t!(fs::read_to_string(&tomlfile));
+    let librs = t!(fs::read_to_string(&libfile));
 
     if toml.contains("name = \"bootstrap\"") {
         return
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index a40ae8894d5e7..7aad6bfd15e7c 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -11,8 +11,7 @@
 //! Check license of third-party deps by inspecting vendor
 
 use std::collections::{BTreeSet, HashSet, HashMap};
-use std::fs::File;
-use std::io::Read;
+use std::fs;
 use std::path::Path;
 use std::process::Command;
 
@@ -261,8 +260,7 @@ fn check_license(path: &Path) -> bool {
     if !path.exists() {
         panic!("{} does not exist", path.display());
     }
-    let mut contents = String::new();
-    t!(t!(File::open(path)).read_to_string(&mut contents));
+    let contents = t!(fs::read_to_string(&path));
 
     let mut found_license = false;
     for line in contents.lines() {
diff --git a/src/tools/tidy/src/extdeps.rs b/src/tools/tidy/src/extdeps.rs
index a78d2d4ee4e15..377a7c33d75ae 100644
--- a/src/tools/tidy/src/extdeps.rs
+++ b/src/tools/tidy/src/extdeps.rs
@@ -10,8 +10,7 @@
 
 // ! Check for external package sources. Allow only vendorable packages.
 
-use std::fs::File;
-use std::io::Read;
+use std::fs;
 use std::path::Path;
 
 /// List of whitelisted sources for packages
@@ -25,8 +24,7 @@ pub fn check(path: &Path, bad: &mut bool) {
     let path = path.join("../Cargo.lock");
 
     // open and read the whole file
-    let mut cargo_lock = String::new();
-    t!(t!(File::open(path)).read_to_string(&mut cargo_lock));
+    let cargo_lock = t!(fs::read_to_string(&path));
 
     // process each line
     for line in cargo_lock.lines() {
diff --git a/src/tools/tidy/src/features.rs b/src/tools/tidy/src/features.rs
index 85b123e4af51f..b0bd5ba8dbdbd 100644
--- a/src/tools/tidy/src/features.rs
+++ b/src/tools/tidy/src/features.rs
@@ -20,7 +20,7 @@
 
 use std::collections::HashMap;
 use std::fmt;
-use std::fs::File;
+use std::fs::{self, File};
 use std::io::prelude::*;
 use std::path::Path;
 
@@ -183,9 +183,7 @@ fn test_filen_gate(filen_underscore: &str, features: &mut Features) -> bool {
 }
 
 pub fn collect_lang_features(base_src_path: &Path, bad: &mut bool) -> Features {
-    let mut contents = String::new();
-    let path = base_src_path.join("libsyntax/feature_gate.rs");
-    t!(t!(File::open(path)).read_to_string(&mut contents));
+    let contents = t!(fs::read_to_string(base_src_path.join("libsyntax/feature_gate.rs")));
 
     // we allow rustc-internal features to omit a tracking issue.
     // these features must be marked with `// rustc internal` in its own group.