Improve several Read implementations

library/std/src/io/cursor.rs

@@ -364,6 +364,27 @@ where
         self.pos += n as u64;
         Ok(())
     }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        let content = self.remaining_slice();
+        let len = content.len();
+        buf.try_reserve(len)?;
+        buf.extend_from_slice(content);
+        self.pos += len as u64;
+
+        Ok(len)
+    }
+
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        let content =
+            crate::str::from_utf8(self.remaining_slice()).map_err(|_| io::Error::INVALID_UTF8)?;
+        let len = content.len();
+        buf.try_reserve(len)?;
+        buf.push_str(content);
+        self.pos += len as u64;
+
+        Ok(len)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/io/impls.rs

@@ -329,8 +329,9 @@ impl Read for &[u8] {
     #[inline]
     fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
         let content = str::from_utf8(self).map_err(|_| io::Error::INVALID_UTF8)?;
-        buf.push_str(content);
         let len = self.len();
+        buf.try_reserve(len)?;
+        buf.push_str(content);
         *self = &self[len..];
         Ok(len)
     }
@@ -478,6 +479,7 @@ impl<A: Allocator> Read for VecDeque<u8, A> {
         let len = self.len();
         let content = self.make_contiguous();
         let string = str::from_utf8(content).map_err(|_| io::Error::INVALID_UTF8)?;
+        buf.try_reserve(len)?;
         buf.push_str(string);
         self.clear();
         Ok(len)

library/std/src/process.rs

@@ -486,6 +486,10 @@ impl Read for ChildStderr {
     fn is_read_vectored(&self) -> bool {
         self.inner.is_read_vectored()
     }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.inner.read_to_end(buf)
+    }
 }
 
 impl AsInner<AnonPipe> for ChildStderr {