Tidy up maybe_uninit_write_slice API

- Move methods to extensions

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

Author: SUPERCILEX

library/core/src/mem/maybe_uninit.rs

@@ -1014,154 +1014,6 @@ impl<T> MaybeUninit<T> {
         this.as_mut_ptr() as *mut T
     }
 
-    /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
-    ///
-    /// If `T` does not implement `Copy`, use [`write_slice_cloned`]
-    ///
-    /// This is similar to [`slice::copy_from_slice`].
-    ///
-    /// # Panics
-    ///
-    /// This function will panic if the two slices have different lengths.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut dst = [MaybeUninit::uninit(); 32];
-    /// let src = [0; 32];
-    ///
-    /// let init = MaybeUninit::write_slice(&mut dst, &src);
-    ///
-    /// assert_eq!(init, src);
-    /// ```
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut vec = Vec::with_capacity(32);
-    /// let src = [0; 16];
-    ///
-    /// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
-    ///
-    /// // SAFETY: we have just copied all the elements of len into the spare capacity
-    /// // the first src.len() elements of the vec are valid now.
-    /// unsafe {
-    ///     vec.set_len(src.len());
-    /// }
-    ///
-    /// assert_eq!(vec, src);
-    /// ```
-    ///
-    /// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned
-    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
-    pub fn write_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
-    where
-        T: Copy,
-    {
-        // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
-        let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
-
-        this.copy_from_slice(uninit_src);
-
-        // SAFETY: Valid elements have just been copied into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
-    }
-
-    /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
-    /// Any already initialized elements will not be dropped.
-    ///
-    /// If `T` implements `Copy`, use [`write_slice`]
-    ///
-    /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
-    ///
-    /// # Panics
-    ///
-    /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
-    ///
-    /// If there is a panic, the already cloned elements will be dropped.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
-    /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
-    ///
-    /// let init = MaybeUninit::write_slice_cloned(&mut dst, &src);
-    ///
-    /// assert_eq!(init, src);
-    /// ```
-    ///
-    /// ```
-    /// #![feature(maybe_uninit_write_slice)]
-    /// use std::mem::MaybeUninit;
-    ///
-    /// let mut vec = Vec::with_capacity(32);
-    /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
-    ///
-    /// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src);
-    ///
-    /// // SAFETY: we have just cloned all the elements of len into the spare capacity
-    /// // the first src.len() elements of the vec are valid now.
-    /// unsafe {
-    ///     vec.set_len(src.len());
-    /// }
-    ///
-    /// assert_eq!(vec, src);
-    /// ```
-    ///
-    /// [`write_slice`]: MaybeUninit::write_slice
-    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
-    pub fn write_slice_cloned<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
-    where
-        T: Clone,
-    {
-        // unlike copy_from_slice this does not call clone_from_slice on the slice
-        // this is because `MaybeUninit<T: Clone>` does not implement Clone.
-
-        struct Guard<'a, T> {
-            slice: &'a mut [MaybeUninit<T>],
-            initialized: usize,
-        }
-
-        impl<'a, T> Drop for Guard<'a, T> {
-            fn drop(&mut self) {
-                let initialized_part = &mut self.slice[..self.initialized];
-                // SAFETY: this raw slice will contain only initialized objects
-                // that's why, it is allowed to drop it.
-                unsafe {
-                    crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part));
-                }
-            }
-        }
-
-        assert_eq!(this.len(), src.len(), "destination and source slices have different lengths");
-        // NOTE: We need to explicitly slice them to the same length
-        // for bounds checking to be elided, and the optimizer will
-        // generate memcpy for simple cases (for example T = u8).
-        let len = this.len();
-        let src = &src[..len];
-
-        // guard is needed b/c panic might happen during a clone
-        let mut guard = Guard { slice: this, initialized: 0 };
-
-        for i in 0..len {
-            guard.slice[i].write(src[i].clone());
-            guard.initialized += 1;
-        }
-
-        super::forget(guard);
-
-        // SAFETY: Valid elements have just been written into `this` so it is initialized
-        unsafe { MaybeUninit::slice_assume_init_mut(this) }
-    }
-
     /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
     ///
     /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
@@ -1265,7 +1117,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
     /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
-    /// MaybeUninit::write_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
+    /// uninit_bytes.write_slice(&[0x12, 0x34, 0x56, 0x78]);
     /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
     /// if cfg!(target_endian = "little") {
     ///     assert_eq!(vals, &[0x3412u16, 0x7856u16]);
@@ -1321,3 +1173,145 @@ impl<T, const N: usize> [MaybeUninit<T>; N] {
         unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
     }
 }
+
+impl<T> [MaybeUninit<T>] {
+    /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
+    ///
+    /// If `T` does not implement `Copy`, use [`write_slice_cloned`](slice::write_slice_cloned)
+    ///
+    /// This is similar to [`slice::copy_from_slice`].
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the two slices have different lengths.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    /// use std::mem::MaybeUninit;
+    ///
+    /// let mut dst = [MaybeUninit::uninit(); 32];
+    /// let src = [0; 32];
+    ///
+    /// let init = dst.write_slice(&src);
+    ///
+    /// assert_eq!(init, src);
+    /// ```
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    /// let mut vec = Vec::with_capacity(32);
+    /// let src = [0; 16];
+    ///
+    /// vec.spare_capacity_mut()[..src.len()].write_slice(&src);
+    ///
+    /// // SAFETY: we have just copied all the elements of len into the spare capacity
+    /// // the first src.len() elements of the vec are valid now.
+    /// unsafe {
+    ///     vec.set_len(src.len());
+    /// }
+    ///
+    /// assert_eq!(vec, src);
+    /// ```
+    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    pub fn write_slice(&mut self, src: &[T]) -> &mut [T]
+    where
+        T: Copy,
+    {
+        // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
+        let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
+
+        self.copy_from_slice(uninit_src);
+
+        // SAFETY: Valid elements have just been copied into `this` so it is initialized
+        unsafe { MaybeUninit::slice_assume_init_mut(self) }
+    }
+
+    /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
+    /// Any already initialized elements will not be dropped.
+    ///
+    /// If `T` implements `Copy`, use [`write_slice`](slice::write_slice)
+    ///
+    /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
+    ///
+    /// If there is a panic, the already cloned elements will be dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    /// use std::mem::MaybeUninit;
+    ///
+    /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
+    /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
+    ///
+    /// let init = dst.write_slice_cloned(&src);
+    ///
+    /// assert_eq!(init, src);
+    /// ```
+    ///
+    /// ```
+    /// #![feature(maybe_uninit_write_slice)]
+    /// let mut vec = Vec::with_capacity(32);
+    /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
+    ///
+    /// vec.spare_capacity_mut()[..src.len()].write_slice_cloned(&src);
+    ///
+    /// // SAFETY: we have just cloned all the elements of len into the spare capacity
+    /// // the first src.len() elements of the vec are valid now.
+    /// unsafe {
+    ///     vec.set_len(src.len());
+    /// }
+    ///
+    /// assert_eq!(vec, src);
+    /// ```
+    #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
+    pub fn write_slice_cloned(&mut self, src: &[T]) -> &mut [T]
+    where
+        T: Clone,
+    {
+        // unlike copy_from_slice this does not call clone_from_slice on the slice
+        // this is because `MaybeUninit<T: Clone>` does not implement Clone.
+
+        struct Guard<'a, T> {
+            slice: &'a mut [MaybeUninit<T>],
+            initialized: usize,
+        }
+
+        impl<'a, T> Drop for Guard<'a, T> {
+            fn drop(&mut self) {
+                let initialized_part = &mut self.slice[..self.initialized];
+                // SAFETY: this raw slice will contain only initialized objects
+                // that's why, it is allowed to drop it.
+                unsafe {
+                    ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part));
+                }
+            }
+        }
+
+        assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
+        // NOTE: We need to explicitly slice them to the same length
+        // for bounds checking to be elided, and the optimizer will
+        // generate memcpy for simple cases (for example T = u8).
+        let len = self.len();
+        let src = &src[..len];
+
+        // guard is needed b/c panic might happen during a clone
+        let mut guard = Guard { slice: self, initialized: 0 };
+
+        for i in 0..len {
+            guard.slice[i].write(src[i].clone());
+            guard.initialized += 1;
+        }
+
+        super::forget(guard);
+
+        // SAFETY: Valid elements have just been written into `self` so it is initialized
+        unsafe { MaybeUninit::slice_assume_init_mut(self) }
+    }
+}

library/core/tests/mem.rs

@@ -182,7 +182,7 @@ fn uninit_write_slice() {
     let mut dst = [MaybeUninit::new(255); 64];
     let src = [0; 64];
 
-    assert_eq!(MaybeUninit::write_slice(&mut dst, &src), &src);
+    assert_eq!(dst.write_slice(&src), &src);
 }
 
 #[test]
@@ -191,7 +191,7 @@ fn uninit_write_slice_panic_lt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 32];
 
-    MaybeUninit::write_slice(&mut dst, &src);
+    dst.write_slice(&src);
 }
 
 #[test]
@@ -200,15 +200,15 @@ fn uninit_write_slice_panic_gt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 128];
 
-    MaybeUninit::write_slice(&mut dst, &src);
+    dst.write_slice(&src);
 }
 
 #[test]
-fn uninit_clone_from_slice() {
+fn uninit_write_slice_cloned() {
     let mut dst = [MaybeUninit::new(255); 64];
     let src = [0; 64];
 
-    assert_eq!(MaybeUninit::write_slice_cloned(&mut dst, &src), &src);
+    assert_eq!(dst.write_slice_cloned(&src), &src);
 }
 
 #[test]
@@ -217,7 +217,7 @@ fn uninit_write_slice_cloned_panic_lt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 32];
 
-    MaybeUninit::write_slice_cloned(&mut dst, &src);
+    dst.write_slice_cloned(&src);
 }
 
 #[test]
@@ -226,7 +226,7 @@ fn uninit_write_slice_cloned_panic_gt() {
     let mut dst = [MaybeUninit::uninit(); 64];
     let src = [0; 128];
 
-    MaybeUninit::write_slice_cloned(&mut dst, &src);
+    dst.write_slice_cloned(&src);
 }
 
 #[test]
@@ -267,7 +267,7 @@ fn uninit_write_slice_cloned_mid_panic() {
     ];
 
     let err = panic::catch_unwind(panic::AssertUnwindSafe(|| {
-        MaybeUninit::write_slice_cloned(&mut dst, &src);
+        dst.write_slice_cloned(&src);
     }));
 
     drop(src);
@@ -299,7 +299,7 @@ fn uninit_write_slice_cloned_no_drop() {
     let mut dst = [MaybeUninit::uninit()];
     let src = [Bomb];
 
-    MaybeUninit::write_slice_cloned(&mut dst, &src);
+    dst.write_slice_cloned(&src);
 
     forget(src);
 }

library/proc_macro/src/bridge/arena.rs

@@ -104,7 +104,7 @@ impl Arena {
 
     pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
         let alloc = self.alloc_raw(string.len());
-        let bytes = MaybeUninit::write_slice(alloc, string.as_bytes());
+        let bytes = alloc.write_slice(string.as_bytes());
 
         // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
         // and immediately convert the clone back to `&str`.