[beta] remove specialization that caused perf regression

the fix on nightly is recent and should get time to bake, removal
of the optimization is the less risky approach for a backport

library/std/src/io/copy.rs

@@ -1,6 +1,5 @@
 use super::{BorrowedBuf, BufReader, BufWriter, Read, Result, Write, DEFAULT_BUF_SIZE};
 use crate::alloc::Allocator;
-use crate::cmp;
 use crate::collections::VecDeque;
 use crate::io::IoSlice;
 use crate::mem::MaybeUninit;
@@ -255,47 +254,6 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
     }
 }
 
-impl<A: Allocator> BufferedWriterSpec for Vec<u8, A> {
-    fn buffer_size(&self) -> usize {
-        cmp::max(DEFAULT_BUF_SIZE, self.capacity() - self.len())
-    }
-
-    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
-        let mut bytes = 0;
-
-        // avoid allocating before we have determined that there's anything to read
-        if self.capacity() == 0 {
-            bytes = stack_buffer_copy(&mut reader.take(DEFAULT_BUF_SIZE as u64), self)?;
-            if bytes == 0 {
-                return Ok(0);
-            }
-        }
-
-        loop {
-            self.reserve(DEFAULT_BUF_SIZE);
-            let mut buf: BorrowedBuf<'_> = self.spare_capacity_mut().into();
-            match reader.read_buf(buf.unfilled()) {
-                Ok(()) => {}
-                Err(e) if e.is_interrupted() => continue,
-                Err(e) => return Err(e),
-            };
-
-            let read = buf.filled().len();
-            if read == 0 {
-                break;
-            }
-
-            // SAFETY: BorrowedBuf guarantees all of its filled bytes are init
-            // and the number of read bytes can't exceed the spare capacity since
-            // that's what the buffer is borrowing from.
-            unsafe { self.set_len(self.len() + read) };
-            bytes += read as u64;
-        }
-
-        Ok(bytes)
-    }
-}
-
 fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
     reader: &mut R,
     writer: &mut W,

library/std/src/io/copy/tests.rs

@@ -80,18 +80,6 @@ fn copy_specializes_bufreader() {
     );
 }
 
-#[test]
-fn copy_specializes_to_vec() {
-    let cap = 123456;
-    let mut source = ShortReader { cap, observed_buffer: 0, read_size: 1337 };
-    let mut sink = Vec::new();
-    assert_eq!(cap as u64, io::copy(&mut source, &mut sink).unwrap());
-    assert!(
-        source.observed_buffer > DEFAULT_BUF_SIZE,
-        "expected a large buffer to be provided to the reader"
-    );
-}
-
 #[test]
 fn copy_specializes_from_vecdeque() {
     let mut source = VecDeque::with_capacity(100 * 1024);