Use is_write_vectored to optimize the write_vectored implementation for BufWriter

library/std/src/io/buffered/bufwriter.rs

@@ -328,24 +328,57 @@ impl<W: Write> Write for BufWriter<W> {
     }
 
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
-        let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
-        if self.buf.len() + total_len > self.buf.capacity() {
-            self.flush_buf()?;
-        }
-        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
-        if total_len >= self.buf.capacity() {
-            self.panicked = true;
-            let r = self.get_mut().write_vectored(bufs);
-            self.panicked = false;
-            r
+        if self.get_ref().is_write_vectored() {
+            let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
+            if self.buf.len() + total_len > self.buf.capacity() {
+                self.flush_buf()?;
+            }
+            if total_len >= self.buf.capacity() {
+                self.panicked = true;
+                let r = self.get_mut().write_vectored(bufs);
+                self.panicked = false;
+                r
+            } else {
+                bufs.iter().for_each(|b| self.buf.extend_from_slice(b));
+                Ok(total_len)
+            }
         } else {
-            bufs.iter().for_each(|b| self.buf.extend_from_slice(b));
-            Ok(total_len)
+            let mut iter = bufs.iter();
+            let mut total_written = if let Some(buf) = iter.by_ref().find(|&buf| !buf.is_empty()) {
+                // This is the first non-empty slice to write, so if it does
+                // not fit in the buffer, we still get to flush and proceed.
+                if self.buf.len() + buf.len() > self.buf.capacity() {
+                    self.flush_buf()?;
+                }
+                if buf.len() >= self.buf.capacity() {
+                    // The slice is at least as large as the buffering capacity,
+                    // so it's better to write it directly, bypassing the buffer.
+                    self.panicked = true;
+                    let r = self.get_mut().write(buf);
+                    self.panicked = false;
+                    return r;
+                } else {
+                    self.buf.extend_from_slice(buf);
+                    buf.len()
+                }
+            } else {
+                return Ok(0);
+            };
+            debug_assert!(total_written != 0);
+            for buf in iter {
+                if self.buf.len() + buf.len() > self.buf.capacity() {
+                    break;
+                } else {
+                    self.buf.extend_from_slice(buf);
+                    total_written += buf.len();
+                }
+            }
+            Ok(total_written)
         }
     }
 
     fn is_write_vectored(&self) -> bool {
-        self.get_ref().is_write_vectored()
+        true
     }
 
     fn flush(&mut self) -> io::Result<()> {

library/std/src/io/buffered/linewritershim.rs

@@ -20,6 +20,12 @@ impl<'a, W: Write> LineWriterShim<'a, W> {
         Self { buffer }
     }
 
+    /// Get a reference to the inner writer (that is, the writer
+    /// wrapped by the BufWriter).
+    fn inner(&self) -> &W {
+        self.buffer.get_ref()
+    }
+
     /// Get a mutable reference to the inner writer (that is, the writer
     /// wrapped by the BufWriter). Be careful with this writer, as writes to
     /// it will bypass the buffer.
@@ -227,7 +233,7 @@ impl<'a, W: Write> Write for LineWriterShim<'a, W> {
     }
 
     fn is_write_vectored(&self) -> bool {
-        self.buffer.is_write_vectored()
+        self.inner().is_write_vectored()
     }
 
     /// Write some data into this BufReader with line buffering. This means