diff --git a/library/std/src/io/buffered/bufwriter.rs b/library/std/src/io/buffered/bufwriter.rs
index 80f98bbbad366..ef2769d431fbb 100644
--- a/library/std/src/io/buffered/bufwriter.rs
+++ b/library/std/src/io/buffered/bufwriter.rs
@@ -4,6 +4,7 @@ use crate::io::{
     self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
 };
 use crate::mem;
+use crate::ptr;
 
 /// Wraps a writer and buffers its output.
 ///
@@ -68,6 +69,10 @@ use crate::mem;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BufWriter<W: Write> {
     inner: Option<W>,
+    // The buffer. Avoid using this like a normal `Vec` in common code paths.
+    // That is, don't use `buf.push`, `buf.extend_from_slice`, or any other
+    // methods that require bounds checking or the like. This makes an enormous
+    // difference to performance (we may want to stop using a `Vec` entirely).
     buf: Vec<u8>,
     // #30888: If the inner writer panics in a call to write, we don't want to
     // write the buffered data a second time in BufWriter's destructor. This
@@ -181,9 +186,14 @@ impl<W: Write> BufWriter<W> {
     /// data. Writes as much as possible without exceeding capacity. Returns
     /// the number of bytes written.
     pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize {
-        let available = self.buf.capacity() - self.buf.len();
+        let available = self.spare_capacity();
         let amt_to_buffer = available.min(buf.len());
-        self.buf.extend_from_slice(&buf[..amt_to_buffer]);
+
+        // SAFETY: `amt_to_buffer` is <= buffer's spare capacity by construction.
+        unsafe {
+            self.write_to_buffer_unchecked(&buf[..amt_to_buffer]);
+        }
+
         amt_to_buffer
     }
 
@@ -331,6 +341,103 @@ impl<W: Write> BufWriter<W> {
         let buf = if !self.panicked { Ok(buf) } else { Err(WriterPanicked { buf }) };
         (self.inner.take().unwrap(), buf)
     }
+
+    // Ensure this function does not get inlined into `write`, so that it
+    // remains inlineable and its common path remains as short as possible.
+    // If this function ends up being called frequently relative to `write`,
+    // it's likely a sign that the client is using an improperly sized buffer
+    // or their write patterns are somewhat pathological.
+    #[cold]
+    #[inline(never)]
+    fn write_cold(&mut self, buf: &[u8]) -> io::Result<usize> {
+        if buf.len() > self.spare_capacity() {
+            self.flush_buf()?;
+        }
+
+        // Why not len > capacity? To avoid a needless trip through the buffer when the input
+        // exactly fills it. We'd just need to flush it to the underlying writer anyway.
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write(buf);
+            self.panicked = false;
+            r
+        } else {
+            // Write to the buffer. In this case, we write to the buffer even if it fills it
+            // exactly. Doing otherwise would mean flushing the buffer, then writing this
+            // input to the inner writer, which in many cases would be a worse strategy.
+
+            // SAFETY: There was either enough spare capacity already, or there wasn't and we
+            // flushed the buffer to ensure that there is. In the latter case, we know that there
+            // is because flushing ensured that our entire buffer is spare capacity, and we entered
+            // this block because the input buffer length is less than that capacity. In either
+            // case, it's safe to write the input buffer to our buffer.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(buf.len())
+        }
+    }
+
+    // Ensure this function does not get inlined into `write_all`, so that it
+    // remains inlineable and its common path remains as short as possible.
+    // If this function ends up being called frequently relative to `write_all`,
+    // it's likely a sign that the client is using an improperly sized buffer
+    // or their write patterns are somewhat pathological.
+    #[cold]
+    #[inline(never)]
+    fn write_all_cold(&mut self, buf: &[u8]) -> io::Result<()> {
+        // Normally, `write_all` just calls `write` in a loop. We can do better
+        // by calling `self.get_mut().write_all()` directly, which avoids
+        // round trips through the buffer in the event of a series of partial
+        // writes in some circumstances.
+
+        if buf.len() > self.spare_capacity() {
+            self.flush_buf()?;
+        }
+
+        // Why not len > capacity? To avoid a needless trip through the buffer when the input
+        // exactly fills it. We'd just need to flush it to the underlying writer anyway.
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write_all(buf);
+            self.panicked = false;
+            r
+        } else {
+            // Write to the buffer. In this case, we write to the buffer even if it fills it
+            // exactly. Doing otherwise would mean flushing the buffer, then writing this
+            // input to the inner writer, which in many cases would be a worse strategy.
+
+            // SAFETY: There was either enough spare capacity already, or there wasn't and we
+            // flushed the buffer to ensure that there is. In the latter case, we know that there
+            // is because flushing ensured that our entire buffer is spare capacity, and we entered
+            // this block because the input buffer length is less than that capacity. In either
+            // case, it's safe to write the input buffer to our buffer.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(())
+        }
+    }
+
+    // SAFETY: Requires `buf.len() <= self.buf.capacity() - self.buf.len()`,
+    // i.e., that input buffer length is less than or equal to spare capacity.
+    #[inline]
+    unsafe fn write_to_buffer_unchecked(&mut self, buf: &[u8]) {
+        debug_assert!(buf.len() <= self.spare_capacity());
+        let old_len = self.buf.len();
+        let buf_len = buf.len();
+        let src = buf.as_ptr();
+        let dst = self.buf.as_mut_ptr().add(old_len);
+        ptr::copy_nonoverlapping(src, dst, buf_len);
+        self.buf.set_len(old_len + buf_len);
+    }
+
+    #[inline]
+    fn spare_capacity(&self) -> usize {
+        self.buf.capacity() - self.buf.len()
+    }
 }
 
 #[unstable(feature = "bufwriter_into_raw_parts", issue = "80690")]
@@ -402,63 +509,82 @@ impl fmt::Debug for WriterPanicked {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<W: Write> Write for BufWriter<W> {
+    #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        if self.buf.len() + buf.len() > self.buf.capacity() {
-            self.flush_buf()?;
-        }
-        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
-        if buf.len() >= self.buf.capacity() {
-            self.panicked = true;
-            let r = self.get_mut().write(buf);
-            self.panicked = false;
-            r
-        } else {
-            self.buf.extend_from_slice(buf);
+        // Use < instead of <= to avoid a needless trip through the buffer in some cases.
+        // See `write_cold` for details.
+        if buf.len() < self.spare_capacity() {
+            // SAFETY: safe by above conditional.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
             Ok(buf.len())
+        } else {
+            self.write_cold(buf)
         }
     }
 
+    #[inline]
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
-        // Normally, `write_all` just calls `write` in a loop. We can do better
-        // by calling `self.get_mut().write_all()` directly, which avoids
-        // round trips through the buffer in the event of a series of partial
-        // writes in some circumstances.
-        if self.buf.len() + buf.len() > self.buf.capacity() {
-            self.flush_buf()?;
-        }
-        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
-        if buf.len() >= self.buf.capacity() {
-            self.panicked = true;
-            let r = self.get_mut().write_all(buf);
-            self.panicked = false;
-            r
-        } else {
-            self.buf.extend_from_slice(buf);
+        // Use < instead of <= to avoid a needless trip through the buffer in some cases.
+        // See `write_all_cold` for details.
+        if buf.len() < self.spare_capacity() {
+            // SAFETY: safe by above conditional.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
             Ok(())
+        } else {
+            self.write_all_cold(buf)
         }
     }
 
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        // FIXME: Consider applying `#[inline]` / `#[inline(never)]` optimizations already applied
+        // to `write` and `write_all`. The performance benefits can be significant. See #79930.
         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() {
+            // We have to handle the possibility that the total length of the buffers overflows
+            // `usize` (even though this can only happen if multiple `IoSlice`s reference the
+            // same underlying buffer, as otherwise the buffers wouldn't fit in memory). If the
+            // computation overflows, then surely the input cannot fit in our buffer, so we forward
+            // to the inner writer's `write_vectored` method to let it handle it appropriately.
+            let saturated_total_len =
+                bufs.iter().fold(0usize, |acc, b| acc.saturating_add(b.len()));
+
+            if saturated_total_len > self.spare_capacity() {
+                // Flush if the total length of the input exceeds our buffer's spare capacity.
+                // If we would have overflowed, this condition also holds, and we need to flush.
                 self.flush_buf()?;
             }
-            if total_len >= self.buf.capacity() {
+
+            if saturated_total_len >= self.buf.capacity() {
+                // Forward to our inner writer if the total length of the input is greater than or
+                // equal to our buffer capacity. If we would have overflowed, this condition also
+                // holds, and we punt to the inner writer.
                 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)
+                // `saturated_total_len < self.buf.capacity()` implies that we did not saturate.
+
+                // SAFETY: We checked whether or not the spare capacity was large enough above. If
+                // it was, then we're safe already. If it wasn't, we flushed, making sufficient
+                // room for any input <= the buffer size, which includes this input.
+                unsafe {
+                    bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
+                };
+
+                Ok(saturated_total_len)
             }
         } else {
             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() {
+                if buf.len() > self.spare_capacity() {
                     self.flush_buf()?;
                 }
                 if buf.len() >= self.buf.capacity() {
@@ -469,7 +595,13 @@ impl<W: Write> Write for BufWriter<W> {
                     self.panicked = false;
                     return r;
                 } else {
-                    self.buf.extend_from_slice(buf);
+                    // SAFETY: We checked whether or not the spare capacity was large enough above.
+                    // If it was, then we're safe already. If it wasn't, we flushed, making
+                    // sufficient room for any input <= the buffer size, which includes this input.
+                    unsafe {
+                        self.write_to_buffer_unchecked(buf);
+                    }
+
                     buf.len()
                 }
             } else {
@@ -477,11 +609,18 @@ impl<W: Write> Write for BufWriter<W> {
             };
             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);
+                if buf.len() <= self.spare_capacity() {
+                    // SAFETY: safe by above conditional.
+                    unsafe {
+                        self.write_to_buffer_unchecked(buf);
+                    }
+
+                    // This cannot overflow `usize`. If we are here, we've written all of the bytes
+                    // so far to our buffer, and we've ensured that we never exceed the buffer's
+                    // capacity. Therefore, `total_written` <= `self.buf.capacity()` <= `usize::MAX`.
                     total_written += buf.len();
+                } else {
+                    break;
                 }
             }
             Ok(total_written)