rustc: Don't use relative paths for extended errors

These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

src/bootstrap/builder.rs

@@ -484,8 +484,8 @@ impl<'a> Builder<'a> {
              } else {
                  PathBuf::from("/path/to/nowhere/rustdoc/not/required")
              })
-             .env("TEST_MIRI", self.config.test_miri.to_string());
-
+             .env("TEST_MIRI", self.config.test_miri.to_string())
+             .env("RUSTC_ERROR_METADATA_DST", self.extended_error_dir());
         if let Some(n) = self.config.rust_codegen_units {
             cargo.env("RUSTC_CODEGEN_UNITS", n.to_string());
         }

src/bootstrap/check.rs

@@ -980,7 +980,8 @@ impl Step for ErrorIndex {
         build.run(builder.tool_cmd(Tool::ErrorIndex)
                     .arg("markdown")
                     .arg(&output)
-                    .env("CFG_BUILD", &build.build));
+                    .env("CFG_BUILD", &build.build)
+                    .env("RUSTC_ERROR_METADATA_DST", build.extended_error_dir()));
 
         markdown_test(builder, compiler, &output);
     }

src/bootstrap/doc.rs

@@ -671,7 +671,8 @@ impl Step for ErrorIndex {
         index.arg(out.join("error-index.html"));
 
         // FIXME: shouldn't have to pass this env var
-        index.env("CFG_BUILD", &build.build);
+        index.env("CFG_BUILD", &build.build)
+             .env("RUSTC_ERROR_METADATA_DST", build.extended_error_dir());
 
         build.run(&mut index);
     }

src/bootstrap/lib.rs

@@ -721,6 +721,11 @@ impl Build {
         self.config.python.as_ref().unwrap()
     }
 
+    /// Temporary directory that extended error information is emitted to.
+    fn extended_error_dir(&self) -> PathBuf {
+        self.out.join("tmp/extended-error-metadata")
+    }
+
     /// Tests whether the `compiler` compiling for `target` should be forced to
     /// use a stage1 compiler instead.
     ///

src/libsyntax/diagnostics/metadata.rs

@@ -14,19 +14,17 @@
 //! currently always a crate name.
 
 use std::collections::BTreeMap;
-use std::path::PathBuf;
+use std::env;
 use std::fs::{remove_file, create_dir_all, File};
 use std::io::Write;
+use std::path::PathBuf;
 use std::error::Error;
 use rustc_serialize::json::as_json;
 
 use syntax_pos::{Span, FileName};
 use ext::base::ExtCtxt;
 use diagnostics::plugin::{ErrorMap, ErrorInfo};
 
-// Default metadata directory to use for extended error JSON.
-const ERROR_METADATA_PREFIX: &'static str = "tmp/extended-errors";
-
 /// JSON encodable/decodable version of `ErrorInfo`.
 #[derive(PartialEq, RustcDecodable, RustcEncodable)]
 pub struct ErrorMetadata {
@@ -59,7 +57,10 @@ impl ErrorLocation {
 ///
 /// See `output_metadata`.
 pub fn get_metadata_dir(prefix: &str) -> PathBuf {
-    PathBuf::from(ERROR_METADATA_PREFIX).join(prefix)
+    env::var_os("RUSTC_ERROR_METADATA_DST")
+        .map(PathBuf::from)
+        .expect("env var `RUSTC_ERROR_METADATA_DST` isn't set")
+        .join(prefix)
 }
 
 /// Map `name` to a path in the given directory: <directory>/<name>.json