Implement SizeHint trait for BufReader, Emtpy, and Chain

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

@@ -1,6 +1,6 @@
 use crate::cmp;
 use crate::fmt;
-use crate::io::{self, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, DEFAULT_BUF_SIZE};
+use crate::io::{self, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, SizeHint, DEFAULT_BUF_SIZE};
 
 /// The `BufReader<R>` struct adds buffering to any reader.
 ///
@@ -435,3 +435,10 @@ impl<R: Seek> Seek for BufReader<R> {
         })
     }
 }
+
+impl<T> SizeHint for BufReader<T> {
+    fn lower_bound(&self) -> usize {
+        self.buffer().len()
+    }
+}
+

library/std/src/io/mod.rs

@@ -2239,6 +2239,20 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     }
 }
 
+impl<T, U> SizeHint for Chain<T, U> {
+    fn lower_bound(&self) -> usize {
+        SizeHint::lower_bound(&self.first) + SizeHint::lower_bound(&self.second)
+    }
+
+    fn upper_bound(&self) -> Option<usize > {
+        match (SizeHint::upper_bound(&self.first), SizeHint::upper_bound(&self.second)) {
+            (Some(first), Some(second)) => Some(first + second),
+            _ => None,
+        }
+    }
+}
+
+
 /// Reader adaptor which limits the bytes read from an underlying reader.
 ///
 /// This struct is generally created by calling [`take`] on a reader.
@@ -2491,12 +2505,6 @@ impl<T> SizeHint for T {
     }
 }
 
-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
 /// particular byte.
 ///

library/std/src/io/util.rs

@@ -4,7 +4,7 @@
 mod tests;
 
 use crate::fmt;
-use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
+use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write};
 
 /// A reader which is always at EOF.
 ///
@@ -80,6 +80,12 @@ impl fmt::Debug for Empty {
     }
 }
 
+impl SizeHint for Empty {
+    fn upper_bound(&self) -> Option<usize> {
+       Some(0)
+    }
+}
+
 /// A reader which yields one byte over and over and over and over and over and...
 ///
 /// This struct is generally created by calling [`repeat()`]. Please