Implement io::Seek for io::Empty

library/std/src/io/util.rs

@@ -4,7 +4,7 @@
 mod tests;
 
 use crate::fmt;
-use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Write};
+use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
 
 /// A reader which is always at EOF.
 ///
@@ -58,6 +58,21 @@ impl BufRead for Empty {
     fn consume(&mut self, _n: usize) {}
 }
 
+#[stable(feature = "empty_seek", since = "1.51.0")]
+impl Seek for Empty {
+    fn seek(&mut self, _pos: SeekFrom) -> io::Result<u64> {
+        Ok(0)
+    }
+
+    fn stream_len(&mut self) -> io::Result<u64> {
+        Ok(0)
+    }
+
+    fn stream_position(&mut self) -> io::Result<u64> {
+        Ok(0)
+    }
+}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl fmt::Debug for Empty {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

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

@@ -1,5 +1,5 @@
 use crate::io::prelude::*;
-use crate::io::{copy, empty, repeat, sink, Empty, Repeat, Sink};
+use crate::io::{copy, empty, repeat, sink, Empty, Repeat, SeekFrom, Sink};
 
 #[test]
 fn copy_copies() {
@@ -29,6 +29,26 @@ fn empty_reads() {
     assert_eq!(e.by_ref().read(&mut [0; 1024]).unwrap(), 0);
 }
 
+#[test]
+fn empty_seeks() {
+    let mut e = empty();
+    assert!(matches!(e.seek(SeekFrom::Start(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Start(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Start(u64::MAX)), Ok(0)));
+
+    assert!(matches!(e.seek(SeekFrom::End(i64::MIN)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(-1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(i64::MAX)), Ok(0)));
+
+    assert!(matches!(e.seek(SeekFrom::Current(i64::MIN)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(-1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(i64::MAX)), Ok(0)));
+}
+
 #[test]
 fn repeat_repeats() {
     let mut r = repeat(4);