Add size hint for byte iterator over file

library/std/src/io/mod.rs

@@ -253,6 +253,7 @@ mod tests;
 
 use crate::cmp;
 use crate::fmt;
+use crate::fs;
 use crate::memchr;
 use crate::ops::{Deref, DerefMut};
 use crate::ptr;
@@ -2462,6 +2463,23 @@ impl<R: Read> Iterator for Bytes<R> {
             };
         }
     }
+
+    default fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, None)
+    }
+}
+
+#[unstable]
+impl Iterator for Bytes<fs::File> {
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self.inner.metadata() {
+            Ok(metadata) => {
+                let file_length = metadata.len() as usize;
+                (0, Some(file_length))
+            }
+            Err(_) => (0, None),
+        }
+    }
 }
 
 /// An iterator over the contents of an instance of `BufRead` split on a