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

Add has_data_left() to BufRead

This is a continuation of rust-lang#40747 and also addresses rust-lang#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/mod.rs

@@ -2005,6 +2005,37 @@ pub trait BufRead: Read {
  #[stable(feature = "rust1", since = "1.0.0")]
  fn consume(&mut self, amt: usize);
 
+ /// Check if the underlying `Read` has any data left to be read.
+ ///
+ /// This function may fill the buffer to check for data,
+ /// so this functions returns `Result<bool>`, not `bool`.
+ ///
+ /// Default implementation calls `fill_buf` and checks that
+ /// returned slice is empty (which means that there is no data left,
+ /// since EOF is reached).
+ ///
+ /// Examples
+ ///
+ /// ```
+ /// #![feature(buf_read_has_data_left)]
+ /// use std::io;
+ /// use std::io::prelude::*;
+ ///
+ /// let stdin = io::stdin();
+ /// let mut stdin = stdin.lock();
+ ///
+ /// while stdin.has_data_left().unwrap() {
+ /// let mut line = String::new();
+ /// stdin.read_line(&mut line).unwrap();
+ /// // work with line
+ /// println!("{:?}", line);
+ /// }
+ /// ```
+ #[unstable(feature = "buf_read_has_data_left", reason = "recently added", issue = "86423")]
+ fn has_data_left(&mut self) -> Result<bool> {
+ self.fill_buf().map(|b| !b.is_empty())
+ }
+
  /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
  ///
  /// This function will read bytes from the underlying stream until the

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""[..]);