Rollup merge of #111074 - WaffleLapkin:🌟unsizes_your_buf_reader🌟, r=Amanieu

Relax implicit `T: Sized` bounds on `BufReader<T>`, `BufWriter<T>` and `LineWriter<T>`

TL;DR:
```diff,rust
-pub struct BufReader<R> { /* ... */ }
+pub struct BufReader<R: ?Sized> { /* ... */ }

-pub struct BufWriter<W: Write> { /* ... */ }
+pub struct BufWriter<W: ?Sized + Write> { /* ... */ }

-pub struct LineWriter<W: Write> { /* ... */ }
+pub struct LineWriter<W: ?Sized + Write> { /* ... */ }
```

This allows using `&mut BufReader<dyn Read>`, for example.

**This is an insta-stable change**.

1 files changed, 1 insertions, 1 deletions

diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
index 1d9d93f5b64..420fc400705 100644
--- a/library/std/src/io/copy.rs
+++ b/library/std/src/io/copy.rs
@@ -86,7 +86,7 @@ impl<W: Write + ?Sized> BufferedCopySpec for W {
     }
 }
 
-impl<I: Write> BufferedCopySpec for BufWriter<I> {
+impl<I: ?Sized + Write> BufferedCopySpec for BufWriter<I> {
     fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
         if writer.capacity() < DEFAULT_BUF_SIZE {
             return stack_buffer_copy(reader, writer);