rollup merge of #21258: aturon/stab-3-index

Conflicts:
	src/libcore/ops.rs
	src/librustc_typeck/astconv.rs
	src/libstd/io/mem.rs
	src/libsyntax/parse/lexer/mod.rs

src/libstd/io/buffered.rs

@@ -221,7 +221,7 @@ impl<W: Writer> Writer for BufferedWriter<W> {
         if buf.len() > self.buf.len() {
             self.inner.as_mut().unwrap().write(buf)
         } else {
-            let dst = self.buf.slice_from_mut(self.pos);
+            let dst = &mut self.buf[self.pos..];
             slice::bytes::copy_memory(dst, buf);
             self.pos += buf.len();
             Ok(())