Use the inner stream_len impl instead of using the Seek default

This is useful if stream_len is overridden by the Seek implementation
of the inner member.

Author: est31

library/std/src/io/buffered/bufreader.rs

@@ -489,6 +489,10 @@ impl<R: Seek> Seek for BufReader<R> {
             )
         })
     }
+
+    fn stream_len(&mut self) -> io::Result<u64> {
+        self.inner.stream_len()
+    }
 }
 
 impl<T> SizeHint for BufReader<T> {

library/std/src/io/buffered/bufwriter.rs

@@ -661,6 +661,10 @@ impl<W: Write + Seek> Seek for BufWriter<W> {
         self.flush_buf()?;
         self.get_mut().seek(pos)
     }
+
+    fn stream_len(&mut self) -> io::Result<u64> {
+        self.inner.stream_len()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/io/impls.rs

@@ -93,6 +93,11 @@ impl<S: Seek + ?Sized> Seek for &mut S {
     fn stream_position(&mut self) -> io::Result<u64> {
         (**self).stream_position()
     }
+
+    #[inline]
+    fn stream_len(&mut self) -> io::Result<u64> {
+        (**self).stream_len()
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<B: BufRead + ?Sized> BufRead for &mut B {
@@ -197,6 +202,11 @@ impl<S: Seek + ?Sized> Seek for Box<S> {
     fn stream_position(&mut self) -> io::Result<u64> {
         (**self).stream_position()
     }
+
+    #[inline]
+    fn stream_len(&mut self) -> io::Result<u64> {
+        (**self).stream_len()
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<B: BufRead + ?Sized> BufRead for Box<B> {