Add Seek instance for io::Take

[wangbj]

Library tracking issue #97227.

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[wangbj]

Note I added

#[unstable(feature = "seek_io_take", issue = "37214")]

Befreo impl<T: Seek> for Take<T> but rustc complaines:

     = note: `#[deny(ineffective_unstable_trait_impl)]` on by default
     = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information

[the8472]

SeekFrom allows seeking beyond the end of an object. This could be observable by doing (&file).take(n).seek(m) and then dropping the Take but continuing to operate on the file.

Since some branches clamp the seek value via min I think test-cases and documentation should cover what happens when the underlying source starts with a seek position beyond its end and when one tries to seek past its end. Some of those cases might be surprising or inconsistent with what SeekFrom says.

[wangbj]

@the8472 You are right if .take(N) return fewer than N bytes indeed cause issues (when calling seek). What would you suggest for the path forward? Do you think we could save stream_position() in struct Take?

[compiler-errors]

This is an impl of a public trait on a public type, and therefore insta-stable. Not sure if needs a tracking issue, but it is a T-libs-api issue instead of a T-libs one, per this comment.

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[compiler-errors]

@wangbj heads up that merge commits are not allowed in PRs. I recommend git pull --rebase instead in the future, and you should probably try to rework your git history so that merge commit is no longer included. See: https://rustc-dev-guide.rust-lang.org/git.html#no-merge-policy

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/library/std/src/io/tests.rs at line 618:
         assert_eq!(bytes, *b"01");
         assert_eq!(stream.stream_position().unwrap(), 2);
         assert_eq!(stream.seek(SeekFrom::Current(2)).unwrap(), 4);
-        assert_eq!(stream.seek(SeekFrom::Current(-5)).unwrap_err().kind(), io::ErrorKind::InvalidInput);
+        assert_eq!(
+            stream.seek(SeekFrom::Current(-5)).unwrap_err().kind(),
+            io::ErrorKind::InvalidInput
+        );
         assert_eq!(stream.seek(SeekFrom::Start(1)).unwrap(), 1);
         assert!(stream.read_exact(&mut bytes).is_ok());
         assert_eq!(bytes, *b"12");
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2021" "--unstable-features" "--skip-children" "--check" "/checkout/library/std/src/io/readbuf/tests.rs" "/checkout/library/std/src/io/util/tests.rs" "/checkout/library/std/src/io/cursor/tests.rs" "/checkout/library/std/src/io/readbuf.rs" "/checkout/library/std/src/io/stdio.rs" "/checkout/library/std/src/io/tests.rs" "/checkout/library/std/src/io/stdio/tests.rs" "/checkout/library/std/src/sys_common/thread_local_key/tests.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.

[wangbj]

Close in flavor of #97807 97807