Rollup merge of #135394 - clarfonthey:uninit-slices-part-2, r=tgross35

`MaybeUninit` inherent slice methods part 2

These were moved out of #129259 since they require additional libs-api approval. Tracking issue: #117428.

New API surface:

```rust
impl<T> [MaybeUninit<T>] {
    // replacing fill; renamed to avoid conflict
    pub fn write_filled(&mut self, value: T) -> &mut [T] where T: Clone;

    // replacing fill_with; renamed to avoid conflict
    pub fn write_with<F>(&mut self, value: F) -> &mut [T] where F: FnMut() -> T;

    // renamed to remove "fill" terminology, since this is closer to the write_*_of_slice methods
    pub fn write_iter<I>(&mut self, iter: I) -> (&mut [T], &mut Self) where I: Iterator<Item = T>;
}
```

Relevant motivation for these methods; see #129259 for earlier methods' motiviations.

* I chose `write_filled` since `filled` is being used as an object here, whereas it's being used as an action in `fill`.
* I chose `write_with` instead of `write_filled_with` since it's shorter and still matches well.
* I chose `write_iter` because it feels completely different from the fill methods, and still has the intent clear.

In all of the methods, it felt appropriate to ensure that they contained `write` to clarify that they are effectively just special ways of doing `MaybeUninit::write` for each element of a slice.

Tracking issue: https://github.com/rust-lang/rust/issues/117428

r? libs-api

1 files changed, 1 insertions, 2 deletions

diff --git a/library/std/src/io/util.rs b/library/std/src/io/util.rs
index cb3f864fd4e..3b66dbf40b6 100644
--- a/library/std/src/io/util.rs
+++ b/library/std/src/io/util.rs
@@ -7,7 +7,6 @@ use crate::fmt;
 use crate::io::{
     self, BorrowedCursor, BufRead, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write,
 };
-use crate::mem::MaybeUninit;
 
 /// `Empty` ignores any data written via [`Write`], and will always be empty
 /// (returning zero bytes) when read via [`Read`].
@@ -196,7 +195,7 @@ impl Read for Repeat {
     #[inline]
     fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> io::Result<()> {
         // SAFETY: No uninit bytes are being written.
-        MaybeUninit::fill(unsafe { buf.as_mut() }, self.byte);
+        unsafe { buf.as_mut() }.write_filled(self.byte);
         // SAFETY: the entire unfilled portion of buf has been initialized.
         unsafe { buf.advance_unchecked(buf.capacity()) };
         Ok(())