Implement Seek::stream_position() for BufReader

Optimization over BufReader::seek() for getting the current position
without flushing the internal buffer.

Related to #31100. Based on code in #70577.

library/std/src/io/buffered.rs

@@ -386,6 +386,51 @@ impl<R: Seek> Seek for BufReader<R> {
         self.discard_buffer();
         Ok(result)
     }
+
+    /// Returns the current seek position from the start of the stream.
+    ///
+    /// The value returned is equivalent to `self.seek(SeekFrom::Current(0))`
+    /// but does not flush the internal buffer. Due to this optimization the
+    /// function does not guarantee that calling `.into_inner()` immediately
+    /// afterwards will yield the underlying reader at the same position. Use
+    /// [`BufReader::seek`] instead if you require that guarantee.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the position of the inner reader is smaller
+    /// than the amount of buffered data. That can happen if the inner reader
+    /// has an incorrect implementation of [`Seek::stream_position`], or if the
+    /// position has gone out of sync due to calling [`Seek::seek`] directly on
+    /// the underlying reader.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// #![feature(seek_convenience)]
+    /// use std::{
+    ///     io::{self, BufRead, BufReader, Seek},
+    ///     fs::File,
+    /// };
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = BufReader::new(File::open("foo.txt")?);
+    ///
+    ///     let before = f.stream_position()?;
+    ///     f.read_line(&mut String::new())?;
+    ///     let after = f.stream_position()?;
+    ///
+    ///     println!("The first line was {} bytes long", after - before);
+    ///     Ok(())
+    /// }
+    /// ```
+    fn stream_position(&mut self) -> io::Result<u64> {
+        let remainder = (self.cap - self.pos) as u64;
+        self.inner.stream_position().map(|pos| {
+            pos.checked_sub(remainder).expect(
+                "overflow when subtracting remaining buffer size from inner stream position",
+            )
+        })
+    }
 }
 
 /// Wraps a writer and buffers its output.

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

@@ -1,5 +1,6 @@
 use crate::io::prelude::*;
 use crate::io::{self, BufReader, BufWriter, ErrorKind, IoSlice, LineWriter, SeekFrom};
+use crate::panic;
 use crate::sync::atomic::{AtomicUsize, Ordering};
 use crate::thread;
 
@@ -86,6 +87,47 @@ fn test_buffered_reader_seek_relative() {
     assert_eq!(reader.fill_buf().ok(), Some(&[2, 3][..]));
 }
 
+#[test]
+fn test_buffered_reader_stream_position() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, io::Cursor::new(inner));
+
+    assert_eq!(reader.stream_position().ok(), Some(0));
+    assert_eq!(reader.seek(SeekFrom::Start(3)).ok(), Some(3));
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    // relative seeking within the buffer and reading position should keep the buffer
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert!(reader.seek_relative(0).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    assert_eq!(reader.buffer(), &[0, 1][..]);
+    assert!(reader.seek_relative(1).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(4));
+    assert_eq!(reader.buffer(), &[1][..]);
+    assert!(reader.seek_relative(-1).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    assert_eq!(reader.buffer(), &[0, 1][..]);
+    // relative seeking outside the buffer will discard it
+    assert!(reader.seek_relative(2).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(5));
+    assert_eq!(reader.buffer(), &[][..]);
+}
+
+#[test]
+fn test_buffered_reader_stream_position_panic() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(4, io::Cursor::new(inner));
+
+    // cause internal buffer to be filled but read only partially
+    let mut buffer = [0, 0];
+    assert!(reader.read_exact(&mut buffer).is_ok());
+    // rewinding the internal reader will cause buffer to loose sync
+    let inner = reader.get_mut();
+    assert!(inner.seek(SeekFrom::Start(0)).is_ok());
+    // overflow when subtracting the remaining buffer size from current position
+    let result = panic::catch_unwind(panic::AssertUnwindSafe(|| reader.stream_position().ok()));
+    assert!(result.is_err());
+}
+
 #[test]
 fn test_buffered_reader_invalidated_after_read() {
     let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];