file: use a nightly API for DST initialization

The `FileInfo`, `FileSystemInfo`, and `FileSystemVolumeLabel` structs
are all DSTs because they end with a `Char16` slice containing a name. A
wide pointer (or reference) to such a struct contains two parts: the
base memory address, and the number of elements in the slice.

Previously there was no officially-documented way to create such a wide
pointer, so uefi-rs used a trick that is used [elsewhere in the Rust
ecosystem][1]: create a slice matching the type and length of the slice
at the end of the struct, but passing in the desired base pointer
instead. Then the slice type is cast to match the desired struct type.

There is now a standard (albeit currently unstable) API for performing
this construction added by [RFC 2580][2]. This requires enabling the
`ptr_metadata` feature. The runtime result should be the same, but
without relying on undocumented Rust internals.

[1]: rust-lang/unsafe-code-guidelines#288 (comment)
[2]: https://rust-lang.github.io/rfcs/2580-ptr-meta.html

src/lib.rs

@@ -29,6 +29,7 @@
 #![feature(try_trait_v2)]
 #![feature(abi_efiapi)]
 #![feature(negative_impls)]
+#![feature(ptr_metadata)]
 #![no_std]
 // Enable some additional warnings and lints.
 #![warn(missing_docs, unused)]

src/proto/media/file/info.rs

@@ -2,11 +2,9 @@ use super::FileAttribute;
 use crate::data_types::{chars::NUL_16, Align};
 use crate::table::runtime::Time;
 use crate::{unsafe_guid, CStr16, Char16, Identify};
-use core::cmp;
 use core::convert::TryInto;
 use core::ffi::c_void;
-use core::mem;
-use core::slice;
+use core::{cmp, mem, ptr};
 
 /// Common trait for data structures that can be used with
 /// `File::set_info()` or `File::get_info()`.
@@ -89,11 +87,17 @@ impl<Header> NamedFileProtocolInfo<Header> {
         // Drop implementation. Thus, we are now ready to build a correctly
         // sized &mut Self and go back to the realm of safe code.
         debug_assert!(!mem::needs_drop::<Char16>());
-        let info_ptr = unsafe {
-            slice::from_raw_parts_mut(storage.as_mut_ptr() as *mut Char16, name_length_ucs2)
-                as *mut [Char16] as *mut Self
+        let info = unsafe {
+            // A wide pointer to a dynamically-sized struct ending with
+            // a slice contains a pointer to the start of the memory,
+            // followed by the number of elements in the slice.
+            //
+            // For more details of wide pointers, see
+            // https://doc.rust-lang.org/nightly/core/ptr/trait.Pointee.html
+            let fat_ptr =
+                ptr::from_raw_parts_mut::<Self>(storage.as_mut_ptr() as *mut (), name_length_ucs2);
+            &mut *fat_ptr
         };
-        let info = unsafe { &mut *info_ptr };
         debug_assert_eq!(info.name.len(), name_length_ucs2);
 
         // Write down the UCS-2 name before returning the storage reference
@@ -119,10 +123,9 @@ impl<Header> FromUefi for NamedFileProtocolInfo<Header> {
         let byte_ptr = ptr as *mut u8;
         let name_ptr = byte_ptr.add(mem::size_of::<Header>()) as *mut Char16;
         let name = CStr16::from_ptr(name_ptr);
-        let name_len = name.to_u16_slice_with_nul().len();
-        let fat_ptr = slice::from_raw_parts_mut(ptr as *mut Char16, name_len);
-        let self_ptr = fat_ptr as *mut [Char16] as *mut Self;
-        &mut *self_ptr
+        let name_len_chars = name.as_slice().len();
+        let fat_ptr = ptr::from_raw_parts_mut::<Self>(ptr as *mut (), name_len_chars);
+        &mut *fat_ptr
     }
 }