Auto merge of #85815 - YuhanLiin:buf-read-data-left, r=m-ou-se

Add has_data_left() to BufRead

This is a continuation of #40747 and also addresses #40745. The problem with the previous PR was that it had "eof" in its method name. This PR uses a more descriptive method name, but I'm open to changing it.

library/std/src/io/tests.rs

@@ -71,6 +71,16 @@ fn lines() {
     assert!(s.next().is_none());
 }
 
+#[test]
+fn buf_read_has_data_left() {
+    let mut buf = Cursor::new(&b"abcd"[..]);
+    assert!(buf.has_data_left().unwrap());
+    buf.read_exact(&mut [0; 2]).unwrap();
+    assert!(buf.has_data_left().unwrap());
+    buf.read_exact(&mut [0; 2]).unwrap();
+    assert!(!buf.has_data_left().unwrap());
+}
+
 #[test]
 fn read_to_end() {
     let mut c = Cursor::new(&b""[..]);