Unrolled build for #158541

Rollup merge of #158541 - bushrat011899:core_io_write, r=clarfonthey

Move `std::io::Write` to `core::io`

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Split From: #156527
~~Blocked On: #158537
~~Blocked On: #158540

## Description

Moves `std::io::Write` to `core::io`. This is effectively a direct cut and paste, but with a large diff due to how many implementations need to be moved into `alloc` and `core`. Blocked on #158537 and #158540.

---

## Notes

* No AI tooling of any kind was used during the creation of this PR.
* Please see #154046 (comment) for a review order and broader context for this PR.

Author: rust-timer

library/alloc/src/io/cursor.rs

@@ -0,0 +1,260 @@
+use crate::alloc::Allocator;
+use crate::boxed::Box;
+use crate::io::{
+    self, Cursor, ErrorKind, IoSlice, WriteThroughCursor, slice_write, slice_write_all,
+    slice_write_all_vectored, slice_write_vectored,
+};
+use crate::vec::Vec;
+
+/// Reserves the required space, and pads the vec with 0s if necessary.
+fn reserve_and_pad<A: Allocator>(
+    pos_mut: &mut u64,
+    vec: &mut Vec<u8, A>,
+    buf_len: usize,
+) -> io::Result<usize> {
+    let pos: usize = (*pos_mut).try_into().map_err(|_| {
+        io::const_error!(
+            ErrorKind::InvalidInput,
+            "cursor position exceeds maximum possible vector length",
+        )
+    })?;
+
+    // For safety reasons, we don't want these numbers to overflow
+    // otherwise our allocation won't be enough
+    let desired_cap = pos.saturating_add(buf_len);
+    if desired_cap > vec.capacity() {
+        // We want our vec's total capacity
+        // to have room for (pos+buf_len) bytes. Reserve allocates
+        // based on additional elements from the length, so we need to
+        // reserve the difference
+        cfg_select! {
+            no_global_oom_handling => {
+                vec.try_reserve(desired_cap - vec.len())?;
+            }
+            _ => {
+                vec.reserve(desired_cap - vec.len());
+            }
+        }
+    }
+    // Pad if pos is above the current len.
+    if pos > vec.len() {
+        let diff = pos - vec.len();
+        // Unfortunately, `resize()` would suffice but the optimiser does not
+        // realise the `reserve` it does can be eliminated. So we do it manually
+        // to eliminate that extra branch
+        let spare = vec.spare_capacity_mut();
+        debug_assert!(spare.len() >= diff);
+        // Safety: we have allocated enough capacity for this.
+        // And we are only writing, not reading
+        unsafe {
+            spare.get_unchecked_mut(..diff).fill(core::mem::MaybeUninit::new(0));
+            vec.set_len(pos);
+        }
+    }
+
+    Ok(pos)
+}
+
+/// Writes the slice to the vec without allocating.
+///
+/// # Safety
+///
+/// `vec` must have `buf.len()` spare capacity.
+unsafe fn vec_write_all_unchecked<A>(pos: usize, vec: &mut Vec<u8, A>, buf: &[u8]) -> usize
+where
+    A: Allocator,
+{
+    debug_assert!(vec.capacity() >= pos + buf.len());
+    unsafe { vec.as_mut_ptr().add(pos).copy_from(buf.as_ptr(), buf.len()) };
+    pos + buf.len()
+}
+
+/// Resizing `write_all` implementation for [`Cursor`].
+///
+/// Cursor is allowed to have a pre-allocated and initialised
+/// vector body, but with a position of 0. This means the [`Write`]
+/// will overwrite the contents of the vec.
+///
+/// This also allows for the vec body to be empty, but with a position of N.
+/// This means that [`Write`] will pad the vec with 0 initially,
+/// before writing anything from that point
+///
+/// [`Write`]: crate::io::Write
+fn vec_write_all<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, buf: &[u8]) -> io::Result<usize>
+where
+    A: Allocator,
+{
+    let buf_len = buf.len();
+    let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
+
+    // Write the buf then progress the vec forward if necessary
+    // Safety: we have ensured that the capacity is available
+    // and that all bytes get written up to pos
+    unsafe {
+        pos = vec_write_all_unchecked(pos, vec, buf);
+        if pos > vec.len() {
+            vec.set_len(pos);
+        }
+    };
+
+    // Bump us forward
+    *pos_mut += buf_len as u64;
+    Ok(buf_len)
+}
+
+/// Resizing `write_all_vectored` implementation for [`Cursor`].
+///
+/// Cursor is allowed to have a pre-allocated and initialised
+/// vector body, but with a position of 0. This means the [`Write`]
+/// will overwrite the contents of the vec.
+///
+/// This also allows for the vec body to be empty, but with a position of N.
+/// This means that [`Write`] will pad the vec with 0 initially,
+/// before writing anything from that point
+///
+/// [`Write`]: crate::io::Write
+fn vec_write_all_vectored<A>(
+    pos_mut: &mut u64,
+    vec: &mut Vec<u8, A>,
+    bufs: &[IoSlice<'_>],
+) -> io::Result<usize>
+where
+    A: Allocator,
+{
+    // For safety reasons, we don't want this sum to overflow ever.
+    // If this saturates, the reserve should panic to avoid any unsound writing.
+    let buf_len = bufs.iter().fold(0usize, |a, b| a.saturating_add(b.len()));
+    let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
+
+    // Write the buf then progress the vec forward if necessary
+    // Safety: we have ensured that the capacity is available
+    // and that all bytes get written up to the last pos
+    unsafe {
+        for buf in bufs {
+            pos = vec_write_all_unchecked(pos, vec, buf);
+        }
+        if pos > vec.len() {
+            vec.set_len(pos);
+        }
+    }
+
+    // Bump us forward
+    *pos_mut += buf_len as u64;
+    Ok(buf_len)
+}
+
+#[stable(feature = "cursor_mut_vec", since = "1.25.0")]
+impl<A> WriteThroughCursor for &mut Vec<u8, A>
+where
+    A: Allocator,
+{
+    fn write(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all(pos, inner, buf)
+    }
+
+    fn write_vectored(this: &mut Cursor<Self>, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all_vectored(pos, inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(_this: &Cursor<Self>) -> bool {
+        true
+    }
+
+    fn write_all(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all(pos, inner, buf)?;
+        Ok(())
+    }
+
+    fn write_all_vectored(this: &mut Cursor<Self>, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all_vectored(pos, inner, bufs)?;
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(_this: &mut Cursor<Self>) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> WriteThroughCursor for Vec<u8, A>
+where
+    A: Allocator,
+{
+    fn write(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all(pos, inner, buf)
+    }
+
+    fn write_vectored(this: &mut Cursor<Self>, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all_vectored(pos, inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(_this: &Cursor<Self>) -> bool {
+        true
+    }
+
+    fn write_all(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all(pos, inner, buf)?;
+        Ok(())
+    }
+
+    fn write_all_vectored(this: &mut Cursor<Self>, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        vec_write_all_vectored(pos, inner, bufs)?;
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(_this: &mut Cursor<Self>) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[stable(feature = "cursor_box_slice", since = "1.5.0")]
+impl<A> WriteThroughCursor for Box<[u8], A>
+where
+    A: Allocator,
+{
+    #[inline]
+    fn write(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        slice_write(pos, inner, buf)
+    }
+
+    #[inline]
+    fn write_vectored(this: &mut Cursor<Self>, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let (pos, inner) = this.into_parts_mut();
+        slice_write_vectored(pos, inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(_this: &Cursor<Self>) -> bool {
+        true
+    }
+
+    #[inline]
+    fn write_all(this: &mut Cursor<Self>, buf: &[u8]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        slice_write_all(pos, inner, buf)
+    }
+
+    #[inline]
+    fn write_all_vectored(this: &mut Cursor<Self>, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        let (pos, inner) = this.into_parts_mut();
+        slice_write_all_vectored(pos, inner, bufs)
+    }
+
+    #[inline]
+    fn flush(_this: &mut Cursor<Self>) -> io::Result<()> {
+        Ok(())
+    }
+}