Unrolled build for rust-lang#127661

Rollup merge of rust-lang#127661 - eduardosm:stabilize-io_slice_advance, r=cuviper

Stabilize io_slice_advance

Closes rust-lang#62726 (FCP completed)

Stabilized API:

```rust
impl<'a> IoSlice<'a> {
    pub fn advance(&mut self, n: usize);
    pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize);
}

impl<'a> IoSliceMut<'a> {
    pub fn advance(&mut self, n: usize);
    pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize);
}
```

library/std/src/io/mod.rs

@@ -1256,8 +1256,6 @@ impl<'a> IoSliceMut<'a> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(io_slice_advance)]
-    ///
     /// use std::io::IoSliceMut;
     /// use std::ops::Deref;
     ///
@@ -1268,7 +1266,7 @@ impl<'a> IoSliceMut<'a> {
     /// buf.advance(3);
     /// assert_eq!(buf.deref(), [1; 5].as_ref());
     /// ```
-    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[stable(feature = "io_slice_advance", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub fn advance(&mut self, n: usize) {
         self.0.advance(n)
@@ -1290,8 +1288,6 @@ impl<'a> IoSliceMut<'a> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(io_slice_advance)]
-    ///
     /// use std::io::IoSliceMut;
     /// use std::ops::Deref;
     ///
@@ -1309,7 +1305,7 @@ impl<'a> IoSliceMut<'a> {
     /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
     /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
     /// ```
-    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[stable(feature = "io_slice_advance", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize) {
         // Number of buffers to remove.
@@ -1400,8 +1396,6 @@ impl<'a> IoSlice<'a> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(io_slice_advance)]
-    ///
     /// use std::io::IoSlice;
     /// use std::ops::Deref;
     ///
@@ -1412,7 +1406,7 @@ impl<'a> IoSlice<'a> {
     /// buf.advance(3);
     /// assert_eq!(buf.deref(), [1; 5].as_ref());
     /// ```
-    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[stable(feature = "io_slice_advance", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub fn advance(&mut self, n: usize) {
         self.0.advance(n)
@@ -1434,8 +1428,6 @@ impl<'a> IoSlice<'a> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(io_slice_advance)]
-    ///
     /// use std::io::IoSlice;
     /// use std::ops::Deref;
     ///
@@ -1452,7 +1444,7 @@ impl<'a> IoSlice<'a> {
     /// IoSlice::advance_slices(&mut bufs, 10);
     /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
     /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
-    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[stable(feature = "io_slice_advance", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize) {
         // Number of buffers to remove.