Add `read_buf` equivalents for positioned reads

[niklasf]

Adds the following items under the ~~read_buf (rust-lang/rust#78485)~~ read_buf_at (rust-lang/rust#140771) feature:

• std::os::unix::fs::FileExt::read_buf_at
• std::os::unix::fs::FileExt::read_buf_exact_at
• std::os::windows::fs::FileExt::seek_read_buf

try-job: x86_64-msvc* try-job: test-various* try-job: dist-various*

[rustbot]

r? @tgross35

rustbot has assigned @tgross35. They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[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)

    |
585 |     pub fn read_buf(&self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: no method named `as_nut` found for struct `BorrowedCursor` in the current scope
   --> library/std/src/sys/pal/windows/handle.rs:141:42
    |
141 |             self.synchronous_read(cursor.as_nut().as_mut_ptr(), cursor.capacity(), Some(offset))
    |                                          ^^^^^^
    |
help: there is a method `as_mut` with a similar name
    |
141 -             self.synchronous_read(cursor.as_nut().as_mut_ptr(), cursor.capacity(), Some(offset))
141 +             self.synchronous_read(cursor.as_mut().as_mut_ptr(), cursor.capacity(), Some(offset))
    |

error[E0061]: this method takes 2 arguments but 1 argument was supplied
   --> library/std/src/sys/fs/windows.rs:590:21
    |
590 |         self.handle.read_buf_at(cursor)
    |                     ^^^^^^^^^^^-------- argument #2 of type `u64` is missing
    |
note: method defined here
   --> library/std/src/sys/pal/windows/handle.rs:139:12
    |
139 |     pub fn read_buf_at(&self, mut cursor: BorrowedCursor<'_>, offset: u64) -> io::Result<()> {
    |            ^^^^^^^^^^^                                        -----------
help: provide the argument
    |
590 |         self.handle.read_buf_at(cursor, /* u64 */)
    |                                       +++++++++++

Some errors have detailed explanations: E0061, E0599.
For more information about an error, try `rustc --explain E0061`.
[RUSTC-TIMING] std test:false 2.650

[tgross35]

Have these changes been discussed with libs-api at all? Usually changes to unstable API need to be proposed at https://github.com/rust-lang/libs-team/ with the ACP issue template.

This needs some tests as well.

[tgross35]

For the above, @rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Kivooeo]

For consistency and clarity, could we include safety comments on each unsafe block? Even brief notes are helpful to understand the assumptions being made

[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)

[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)

[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)

[niklasf]

Thank you both.

• Opened an APC that was accepted, but items to be added under a new feature instead of read_buf. Moved to read_buf_at with new tracking issue #140771.
• Added safety comments also for FFI calls.
• Added tests.

@rustbot ready

[tgross35]

@bors try

[bors]

:hourglass: Trying commit 85e1da142056d8591d972f156f4a9ca16171c98b with merge f76928546c97438a758c27b2f9381b78f73fcbc0...

[rust-log-analyzer]

The job x86_64-msvc-1 failed! Check out the build log: (web) (plain)

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

Updating files:  98% (52406/53474)
Updating files:  99% (52940/53474)
Updating files: 100% (53474/53474)
Updating files: 100% (53474/53474), done.
branch 'try' set up to track 'origin/try'.
Switched to a new branch 'try'
##[endgroup]
[command]"C:\Program Files\Git\bin\git.exe" log -1 --format=%H
f76928546c97438a758c27b2f9381b78f73fcbc0
##[group]Run src/ci/scripts/setup-environment.sh
src/ci/scripts/setup-environment.sh
---
file:.git/config remote.origin.url=https://github.com/rust-lang/rust
file:.git/config remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
file:.git/config gc.auto=0
file:.git/config http.https://github.com/.extraheader=AUTHORIZATION: basic ***
file:.git/config branch.try.remote=origin
file:.git/config branch.try.merge=refs/heads/try
file:.git/config submodule.library/backtrace.active=true
file:.git/config submodule.library/backtrace.url=https://github.com/rust-lang/backtrace-rs.git
file:.git/config submodule.library/stdarch.active=true
file:.git/config submodule.library/stdarch.url=https://github.com/rust-lang/stdarch.git
file:.git/config submodule.src/doc/book.active=true
---
failures:

---- fs::tests::test_seek_read_buf stdout ----

thread 'fs::tests::test_seek_read_buf' panicked at library\std\src\fs\tests.rs:635:9:
assertion `left == right` failed
  left: 9
 right: 0


failures:
    fs::tests::test_seek_read_buf

test result: FAILED. 561 passed; 1 failed; 5 ignored; 0 measured; 0 filtered out; finished in 56.85s

error: test failed, to rerun pass `-p std --lib`
Build completed unsuccessfully in 1:47:18
make: *** [Makefile:113: ci-msvc-py] Error 1
  local time: Thu May 29 01:02:38 CUT 2025
  network time: Thu, 29 May 2025 01:02:38 GMT
##[error]Process completed with exit code 2.
Post job cleanup.
[command]"C:\Program Files\Git\bin\git.exe" version

[bors]

:broken_heart: Test failed - checks-actions

[rustbot]

:warning: Warning :warning:

• There are issue links (such as #123) in the commit messages of the following commits. Please remove them as they will spam the issue with references to the commit.
  • 05dcef0608ae14d0de2d96112a7c6eec933be650

[niklasf]

Ready for the next round of review.

The failed test on Windows was an assertion that I had (blindly) added about the seek position after seek_read_buf() with an empty buffer. Turns out it's a noop, but neither the existing seek_read() nor the current underlying Windows API guarantees it (afaict), so I just removed the assertion.

@rustbot ready

[tgross35]

Running another try, including a few other unix platforms

@bors2 try

[rust-bors[bot]]

:hourglass: Trying commit e8a35d4acf34ca1c93b9538a95034ed001f3682b with merge 2e522c2e8590b7b074d7a35dea51085165d83c67…

To cancel the try build, run the command @bors2 try cancel.

[tgross35]

@rustbot author

[rust-bors[bot]]

:sunny: Try build successful (CI) Build commit: 2e522c2e8590b7b074d7a35dea51085165d83c67 (2e522c2e8590b7b074d7a35dea51085165d83c67)

[tgross35]

Hey @niklasf gentle ping: this only needs a few small changes if you're able to get it over the line

[bors]

:umbrella: The latest upstream changes (presumably #145334) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[niklasf]

Back after a while, rebased to resolve conflicts, and ready for the next round of review.

@rustbot ready

[rust-log-analyzer]

The job pr-check-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

     |        -------- the method is available for `&Handle` here
     |
    ::: library/std/src/sys/pal/windows/handle.rs:20:1
     |
  20 | pub struct Handle(OwnedHandle);
     | ----------------- method `read_buf` not found for this struct
     |
     = help: items from traits can only be used if the trait is implemented and in scope
note: `io::Read` defines an item `read_buf`, perhaps you need to implement it
    --> library/std/src/io/mod.rs:732:1
     |
---
     |        -------- the method is available for `&Handle` here
     |
    ::: library/std/src/sys/pal/windows/handle.rs:20:1
     |
  20 | pub struct Handle(OwnedHandle);
     | ----------------- method `read_buf` not found for this struct
     |
     = help: items from traits can only be used if the trait is implemented and in scope
note: `io::Read` defines an item `read_buf`, perhaps you need to implement it
    --> library/std/src/io/mod.rs:732:1
     |
---

error[E0599]: no method named `read_buf` found for struct `Handle` in the current scope
    --> library/std/src/sys/pal/windows/handle.rs:347:18
     |
  20 | pub struct Handle(OwnedHandle);
     | ----------------- method `read_buf` not found for this struct
...
 347 |         (**self).read_buf(buf)
     |                  ^^^^^^^^
     |
    ::: library/std/src/io/mod.rs:1057:8
     |
1057 |     fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {

[niklasf]

@rustbot ready

[tgross35]

Going to rerun the try job just in case, but LGTM. Thanks for the updates

@bors2 try

[rust-bors[bot]]

:hourglass: Trying commit c914c471b9d83f9e6c74b0e6c2c8044efd3a57dd with merge f0baf1cd9fa552c3e2ab50b1610b11ce86350ef2…

To cancel the try build, run the command @bors try cancel.

[rust-bors[bot]]

:sunny: Try build successful (CI) Build commit: f0baf1cd9fa552c3e2ab50b1610b11ce86350ef2 (f0baf1cd9fa552c3e2ab50b1610b11ce86350ef2, parent: fd75a9c32d643f39c8c61df770d2cff60b3fefd5)

[tgross35]

@bors r+