avoid excessive initialization when copying to a Vec

It now keeps track of initialized bytes to avoid reinitialization.
It also keeps track of read sizes to avoid initializing more bytes
than the reader needs. This is important when passing a huge vector to a
Read that only has a few bytes to offer and doesn't implement read_buf().

library/std/src/io/copy.rs

@@ -1,6 +1,7 @@
 use super::{BorrowedBuf, BufReader, BufWriter, Read, Result, Write, DEFAULT_BUF_SIZE};
 use crate::alloc::Allocator;
 use crate::cmp;
+use crate::cmp::min;
 use crate::collections::VecDeque;
 use crate::io::IoSlice;
 use crate::mem::MaybeUninit;
@@ -271,28 +272,57 @@ impl<A: Allocator> BufferedWriterSpec for Vec<u8, A> {
             }
         }
 
+        // don't immediately offer the vec's whole spare capacity, otherwise
+        // we might have to fully initialize it if the reader doesn't have a custom read_buf() impl
+        let mut max_read_size = DEFAULT_BUF_SIZE;
+
         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 mut initialized_spare_capacity = 0;
 
-            let read = buf.filled().len();
-            if read == 0 {
-                break;
+            loop {
+                let buf = self.spare_capacity_mut();
+                let read_size = min(max_read_size, buf.len());
+                let mut buf = BorrowedBuf::from(&mut buf[..read_size]);
+                // SAFETY: init is either 0 or the init_len from the previous iteration.
+                unsafe {
+                    buf.set_init(initialized_spare_capacity);
+                }
+                match reader.read_buf(buf.unfilled()) {
+                    Ok(()) => {
+                        let bytes_read = buf.len();
+
+                        // EOF
+                        if bytes_read == 0 {
+                            return Ok(bytes);
+                        }
+
+                        // the reader is returning short reads but it doesn't call ensure_init()
+                        if buf.init_len() < buf.capacity() {
+                            max_read_size = usize::MAX;
+                        }
+                        // the reader hasn't returned short reads so far
+                        if bytes_read == buf.capacity() {
+                            max_read_size *= 2;
+                        }
+
+                        initialized_spare_capacity = buf.init_len() - bytes_read;
+                        bytes += bytes_read as u64;
+                        // 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() + bytes_read) };
+
+                        // spare capacity full, reserve more
+                        if self.len() == self.capacity() {
+                            break;
+                        }
+                    }
+                    Err(e) if e.is_interrupted() => continue,
+                    Err(e) => return Err(e),
+                }
             }
-
-            // 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)
     }
 }