Drop unstable maybe_uninit_slice and vec_into_raw_parts features

The functionality we need from those features can be provided with a few pretty
simple one-liner functions. Add those functions to the uefi crate in a new
polyfill module.

Author: nicholasbishop

uefi/src/data_types/owned_strs.rs

@@ -3,6 +3,7 @@ use super::strs::{CStr16, FromSliceWithNulError};
 use crate::alloc::vec::Vec;
 use crate::data_types::strs::EqStrUntilNul;
 use crate::data_types::UnalignedSlice;
+use crate::polyfill::vec_into_raw_parts;
 use core::fmt;
 use core::ops;
 
@@ -92,7 +93,7 @@ impl TryFrom<Vec<u16>> for CString16 {
         // Safety: `Char16` is a transparent struct wrapping `u16`, so
         // the types are compatible. The pattern used here matches the
         // example in the docs for `into_raw_parts`.
-        let (ptr, len, cap) = input.into_raw_parts();
+        let (ptr, len, cap) = vec_into_raw_parts(input);
         let rebuilt = unsafe {
             let ptr = ptr.cast::<Char16>();
             Vec::from_raw_parts(ptr, len, cap)

uefi/src/data_types/strs.rs

@@ -1,5 +1,6 @@
 use super::chars::{Char16, Char8, NUL_16, NUL_8};
 use super::UnalignedSlice;
+use crate::polyfill::maybe_uninit_slice_assume_init_ref;
 use core::ffi::CStr;
 use core::fmt;
 use core::iter::Iterator;
@@ -300,7 +301,7 @@ impl CStr16 {
         src.copy_to_maybe_uninit(buf);
         let buf = unsafe {
             // Safety: `copy_buf` fully initializes the slice.
-            MaybeUninit::slice_assume_init_ref(buf)
+            maybe_uninit_slice_assume_init_ref(buf)
         };
         CStr16::from_u16_with_nul(buf).map_err(|e| match e {
             FromSliceWithNulError::InvalidChar(v) => UnalignedCStr16Error::InvalidChar(v),

uefi/src/lib.rs

@@ -59,8 +59,6 @@
 //! [unstable features]: https://doc.rust-lang.org/unstable-book/
 
 #![feature(abi_efiapi)]
-#![feature(maybe_uninit_slice)]
-#![cfg_attr(feature = "alloc", feature(vec_into_raw_parts))]
 #![cfg_attr(feature = "unstable", feature(error_in_core))]
 #![cfg_attr(all(feature = "unstable", feature = "alloc"), feature(allocator_api))]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
@@ -105,3 +103,5 @@ pub mod logger;
 // As long as this is behind "alloc", we can simplify cfg-feature attributes in this module.
 #[cfg(feature = "alloc")]
 pub(crate) mod mem;
+
+pub(crate) mod polyfill;

uefi/src/polyfill.rs

@@ -0,0 +1,29 @@
+//! Polyfills for functions in the standard library that are currently gated
+//! behind unstable features.
+
+use core::mem::MaybeUninit;
+#[cfg(feature = "alloc")]
+use {alloc::vec::Vec, core::mem::ManuallyDrop};
+
+/// Polyfill for the unstable `MaybeUninit::slice_assume_init_ref` function.
+///
+/// See <https://github.com/rust-lang/rust/issues/63569>.
+pub const unsafe fn maybe_uninit_slice_assume_init_ref<T>(s: &[MaybeUninit<T>]) -> &[T] {
+    unsafe { &*(s as *const [MaybeUninit<T>] as *const [T]) }
+}
+
+/// Polyfill for the unstable `MaybeUninit::slice_as_mut_ptr` function.
+///
+/// See <https://github.com/rust-lang/rust/issues/63569>.
+pub fn maybe_uninit_slice_as_mut_ptr<T>(s: &mut [MaybeUninit<T>]) -> *mut T {
+    s.as_mut_ptr().cast::<T>()
+}
+
+/// Polyfill for the unstable `Vec::into_raw_parts` function.
+///
+/// See <https://github.com/rust-lang/rust/issues/65816>.
+#[cfg(feature = "alloc")]
+pub fn vec_into_raw_parts<T>(v: Vec<T>) -> (*mut T, usize, usize) {
+    let mut v = ManuallyDrop::new(v);
+    (v.as_mut_ptr(), v.len(), v.capacity())
+}

uefi/src/proto/device_path/build.rs

@@ -7,6 +7,7 @@
 
 pub use uefi::proto::device_path::device_path_gen::build::*;
 
+use crate::polyfill::{maybe_uninit_slice_as_mut_ptr, maybe_uninit_slice_assume_init_ref};
 use crate::proto::device_path::{DevicePath, DevicePathNode};
 use core::mem::MaybeUninit;
 
@@ -131,7 +132,7 @@ impl<'a> DevicePathBuilder<'a> {
 
         let data: &[u8] = match &this.storage {
             BuilderStorage::Buf { buf, offset } => unsafe {
-                MaybeUninit::slice_assume_init_ref(&buf[..*offset])
+                maybe_uninit_slice_assume_init_ref(&buf[..*offset])
             },
             #[cfg(feature = "alloc")]
             BuilderStorage::Vec(vec) => vec,
@@ -206,7 +207,7 @@ unsafe impl BuildNode for &DevicePathNode {
     fn write_data(&self, out: &mut [MaybeUninit<u8>]) {
         let src: *const u8 = self.as_ffi_ptr().cast();
 
-        let dst: *mut u8 = MaybeUninit::slice_as_mut_ptr(out);
+        let dst: *mut u8 = maybe_uninit_slice_as_mut_ptr(out);
         unsafe {
             dst.copy_from_nonoverlapping(src, out.len());
         }