Improve several Read implementations

Author: a1phyr

library/std/src/io/cursor.rs

@@ -357,6 +357,28 @@ 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::const_io_error!(ErrorKind::InvalidData, "stream did not contain valid UTF-8")
+        })?;
+        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

@@ -315,8 +315,9 @@ impl Read for &[u8] {
         let content = str::from_utf8(self).map_err(|_| {
             io::const_io_error!(ErrorKind::InvalidData, "stream did not contain valid UTF-8")
         })?;
-        buf.push_str(content);
         let len = self.len();
+        buf.try_reserve(len)?;
+        buf.push_str(content);
         *self = &self[len..];
         Ok(len)
     }
@@ -470,6 +471,7 @@ impl<A: Allocator> Read for VecDeque<u8, A> {
         let string = str::from_utf8(content).map_err(|_| {
             io::const_io_error!(ErrorKind::InvalidData, "stream did not contain valid UTF-8")
         })?;
+        buf.try_reserve(len)?;
         buf.push_str(string);
         self.clear();
         Ok(len)

library/std/src/process.rs

@@ -439,6 +439,10 @@ 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 {