Rollup merge of #86202 - a1phyr:spec_io_bytes_size_hint, r=m-ou-se

Specialize `io::Bytes::size_hint` for more types

Improve the result of `<io::Bytes as Iterator>::size_hint` for some readers. I did not manage to specialize `SizeHint` for `io::Cursor`

Side question: would it be interesting for `io::Read` to have an optional `size_hint` method ?

library/std/src/io/tests.rs

@@ -224,6 +224,24 @@ fn empty_size_hint() {
     assert_eq!(size_hint, (0, Some(0)));
 }
 
+#[test]
+fn slice_size_hint() {
+    let size_hint = (&[1, 2, 3]).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+}
+
+#[test]
+fn take_size_hint() {
+    let size_hint = (&[1, 2, 3]).take(2).bytes().size_hint();
+    assert_eq!(size_hint, (2, Some(2)));
+
+    let size_hint = (&[1, 2, 3]).take(4).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+
+    let size_hint = io::repeat(0).take(3).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+}
+
 #[test]
 fn chain_empty_size_hint() {
     let chain = io::empty().chain(io::empty());
@@ -242,7 +260,7 @@ fn chain_size_hint() {
 
     let chain = buf_reader_1.chain(buf_reader_2);
     let size_hint = chain.bytes().size_hint();
-    assert_eq!(size_hint, (testdata.len(), None));
+    assert_eq!(size_hint, (testdata.len(), Some(testdata.len())));
 }
 
 #[test]