Rollup merge of #70904 - LukasKalbertodt:stabilize-seek-convenience, r=m-ou-se

Stabilize `Seek::stream_position` (feature `seek_convenience`)

Tracking issue: #59359

Unresolved questions from tracking issue:
- "Override `stream_len` for `File`?" → we can do that in the future, this does not block stabilization.
- "Rename to `len` and `position`?" → as noted in the tracking issue, both of these shorter names have problems (`len` is usually a cheap getter, `position` clashes with `Cursor`). I do think the current names are perfectly fine.
- "Rename `stream_position` to `tell`?" → as mentioned in [the comment bringing this up](https://github.com/rust-lang/rust/issues/59359#issuecomment-559541545), `stream_position` is more descriptive. I don't think `tell` would be a good name.

What remains to decide, is whether or not adding these methods is worth it.

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

@@ -410,7 +410,6 @@ impl<R: Seek> Seek for BufReader<R> {
     /// # Example
     ///
     /// ```no_run
-    /// #![feature(seek_convenience)]
     /// use std::{
     ///     io::{self, BufRead, BufReader, Seek},
     ///     fs::File,

library/std/src/io/mod.rs

@@ -1671,7 +1671,7 @@ pub trait Seek {
     /// # Example
     ///
     /// ```no_run
-    /// #![feature(seek_convenience)]
+    /// #![feature(seek_stream_len)]
     /// use std::{
     ///     io::{self, Seek},
     ///     fs::File,
@@ -1685,7 +1685,7 @@ pub trait Seek {
     ///     Ok(())
     /// }
     /// ```
-    #[unstable(feature = "seek_convenience", issue = "59359")]
+    #[unstable(feature = "seek_stream_len", issue = "59359")]
     fn stream_len(&mut self) -> Result<u64> {
         let old_pos = self.stream_position()?;
         let len = self.seek(SeekFrom::End(0))?;
@@ -1706,7 +1706,6 @@ pub trait Seek {
     /// # Example
     ///
     /// ```no_run
-    /// #![feature(seek_convenience)]
     /// use std::{
     ///     io::{self, BufRead, BufReader, Seek},
     ///     fs::File,
@@ -1723,7 +1722,7 @@ pub trait Seek {
     ///     Ok(())
     /// }
     /// ```
-    #[unstable(feature = "seek_convenience", issue = "59359")]
+    #[stable(feature = "seek_convenience", since = "1.51.0")]
     fn stream_position(&mut self) -> Result<u64> {
         self.seek(SeekFrom::Current(0))
     }