Implement size_hint for BufReader

library/std/src/io/mod.rs

@@ -2213,7 +2213,11 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
 
     unsafe fn initializer(&self) -> Initializer {
         let initializer = self.first.initializer();
-        if initializer.should_initialize() { initializer } else { self.second.initializer() }
+        if initializer.should_initialize() {
+            initializer
+        } else {
+            self.second.initializer()
+        }
     }
 }
 
@@ -2232,7 +2236,11 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     }
 
     fn consume(&mut self, amt: usize) {
-        if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
+        if !self.done_first {
+            self.first.consume(amt)
+        } else {
+            self.second.consume(amt)
+        }
     }
 }
 
@@ -2448,7 +2456,7 @@ pub struct Bytes<R> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<R: Read> Iterator for Bytes<R> {
+impl<R: Read + SizeHint> Iterator for Bytes<R> {
     type Item = Result<u8>;
 
     fn next(&mut self) -> Option<Result<u8>> {
@@ -2462,6 +2470,35 @@ impl<R: Read> Iterator for Bytes<R> {
             };
         }
     }
+
+    default fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+#[stable(feature = "bufreader_size_hint", since = "1.51.0")]
+trait SizeHint {
+    fn upper_bound(&self) -> Option<usize> {
+        None
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.lower_bound(), self.upper_bound())
+    }
+}
+
+#[stable(feature = "bufreader_size_hint", since = "1.51.0")]
+impl<T> SizeHint for T {
+    default fn lower_bound(&self) -> usize {
+        0
+    }
+}
+
+#[stable(feature = "bufreader_size_hint", since = "1.51.0")]
+impl<T> SizeHint for BufReader<T> {
+    fn lower_bound(&self) -> usize {
+        self.buffer().len()
+    }
 }
 
 /// An iterator over the contents of an instance of `BufRead` split on a