Change Read::size_hint return io::Result

src/libstd/fs.rs

@@ -449,11 +449,8 @@ impl Read for File {
         self.inner.read(buf)
     }
 
-    fn size_hint(&self) -> usize {
-        match self.metadata() {
-            Ok(meta) => meta.len() as usize,
-            Err(_) => 0,
-        }
+    fn size_hint(&self) -> io::Result<usize> {
+        Ok(self.metadata()?.len() as usize)
     }
 
     #[inline]
@@ -480,7 +477,7 @@ impl<'a> Read for &'a File {
         self.inner.read(buf)
     }
 
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> io::Result<usize> {
         (**self).size_hint()
     }
 

src/libstd/io/buffered.rs

@@ -212,9 +212,9 @@ impl<R: Read> Read for BufReader<R> {
     }
 
     #[inline]
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> io::Result<usize> {
         let buffered_len = self.cap - self.pos;
-        buffered_len.saturating_add(self.inner.size_hint())
+        Ok(buffered_len.saturating_add(self.inner.size_hint()?))
     }
 
     // we can't skip unconditionally because of the large buffer case in read.

src/libstd/io/cursor.rs

@@ -230,8 +230,8 @@ impl<T> Read for Cursor<T> where T: AsRef<[u8]> {
         Ok(n)
     }
 
-    fn size_hint(&self) -> usize {
-        (self.inner.as_ref().len() as u64).saturating_sub(self.pos) as usize
+    fn size_hint(&self) -> io::Result<usize> {
+        Ok((self.inner.as_ref().len() as u64).saturating_sub(self.pos) as usize)
     }
 
     #[inline]

src/libstd/io/impls.rs

@@ -24,7 +24,7 @@ impl<'a, R: Read + ?Sized> Read for &'a mut R {
     }
 
     #[inline]
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> io::Result<usize> {
         (**self).size_hint()
     }
 
@@ -98,7 +98,7 @@ impl<R: Read + ?Sized> Read for Box<R> {
     }
 
     #[inline]
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> io::Result<usize> {
         (**self).size_hint()
     }
 
@@ -192,8 +192,8 @@ impl<'a> Read for &'a [u8] {
     }
 
     #[inline]
-    fn size_hint(&self) -> usize {
-        self.len()
+    fn size_hint(&self) -> io::Result<usize> {
+        Ok(self.len())
     }
 
     #[inline]

src/libstd/io/mod.rs

@@ -367,7 +367,7 @@ fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize>
     let start_len = buf.len();
     let mut g = Guard { len: buf.len(), buf: buf };
 
-    let size_hint = r.size_hint();
+    let size_hint = r.size_hint()?;
     if size_hint > 0 {
         unsafe {
             g.buf.reserve(size_hint.saturating_add(1));
@@ -564,8 +564,8 @@ pub trait Read {
     /// [`read_to_string`]: #method.read_to_string
     #[unstable(feature = "read_size_hint", issue = /* FIXME */ "0")]
     #[inline]
-    fn size_hint(&self) -> usize {
-        0
+    fn size_hint(&self) -> Result<usize> {
+        Ok(0)
     }
 
     /// Read all bytes until EOF in this source, placing them into `buf`.
@@ -1744,11 +1744,11 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
         self.second.read(buf)
     }
 
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> Result<usize> {
         if self.done_first {
             self.second.size_hint()
         } else {
-            self.first.size_hint().saturating_add(self.second.size_hint())
+            Ok(self.first.size_hint()?.saturating_add(self.second.size_hint()?))
         }
     }
 
@@ -1950,11 +1950,11 @@ impl<T: Read> Read for Take<T> {
         Ok(n)
     }
 
-    fn size_hint(&self) -> usize {
+    fn size_hint(&self) -> Result<usize> {
         if self.limit == 0 {
-            0
+            Ok(0)
         } else {
-            cmp::min(self.limit, self.inner.size_hint() as u64) as usize
+            Ok(cmp::min(self.limit, self.inner.size_hint()? as u64) as usize)
         }
     }