Move `std::io::Error` into `core` by bushrat011899 · Pull Request #155625 · rust-lang/rust

bushrat011899 · 2026-04-21T23:54:37Z

ACP: rust-lang/libs-team#755
Tracking issue: #154046
Related: #155574
Related: #152918

Description

Moves std::io::Error into core, deferring Box-adjacent methods to incoherent implementations in alloc, and RawOsError methods to std. This requires some substantial changes to the internals of Error, but none of them are breaking changes or externally visible.

Notably, I've replaced usage of Box with a wrapper around a pointer and an appropriate drop function. This requires the addition of quite a few lines of unsafe, but is required to work around Box only being accessible from alloc. Additionally, an atomic pointer to a VTable is used for working with RawOsError in core, since we cannot know the required implementations without std.

Notes

This PR adopts the VTable technique from Move OS-dependent parts of std::io to alloc #152918
This PR builds on my previous PR Move std::io::RawOsError to core::io #155574
No AI tooling of any kind was used during the creation of this PR.

programmerjake · 2026-04-22T04:00:28Z

example of how to link from core's docs to std: https://doc.rust-lang.org/1.95.0/src/core/str/mod.rs.html#200

bushrat011899 · 2026-04-22T04:53:57Z

example of how to link from core's docs to std: https://doc.rust-lang.org/1.95.0/src/core/str/mod.rs.html#200

That's a clever trick I never knew about! Looks like it's also required for linking to incoherent implementations even within the file that creates them too.

programmerjake · 2026-04-22T07:11:54Z

label -O-unix
See

maybe just leave it until you get the PR to work, that way there's less comment spam.

bushrat011899 · 2026-05-14T01:37:31Z

I invite anyone with more experience interpreting a perf run to help me out here, but below is my best attempt at understanding these results. From what I can tell, there is no measurable difference in runtime performance, despite the following two changes which could impact runtime:

Use of an static atomic pointer VTable for information (debug string, error kind, etc.) about OS error codes.
Storing drop function pointers on the heap for custom error types stored within Error.

However it's possible runtime performance along codepaths affected by this PR isn't being measured by the current set of benchmarks. After all, the worst performance is likely to be seen in dropping Error and/or debug printing OS error codes as Error. Since these are both in the failure path for a crate, I consider this acceptable.

Out of the primary compilation benchmarks, it appears the worst regressions come down to spending more time in LLVM optimising, or more time in rustc handling the incoherent methods on Error. I will revisit my use of incoherence to see if I can reduce it, but I'm uncertain if there's much that can be done with regards to LLVM optimisation time. I suspect it comes from needing to work harder around the heap allocated VTable when dropping and atomic operations when creating/printing an Error from an OS error code.

@rustbot label: +perf-regression-triaged

bushrat011899 · 2026-05-18T05:42:24Z

Since there is a measurable performance difference in compilation time, I've split this PR's commits further to hopefully make it easier to review.

Viewing internals wont be possible from `std` once moved into `core`.

Adjust `Error` documentation `core` is more restrictive with documentation quality and linking to other items. Methods that will be implemented through incoherence must also be explicitly linked.

Personal preference that will be used for another module in a later commit. Purely stylistic.

They'll be moved into `core` but must be accessible from `std`.

Incoherence is required to define inherit methods on `Error` from `alloc` and `std` once it is moved into `core`. This is required because: 1. `Box` is part of `Error`'s public API and that is only accessible from `alloc`. 2. `RawOsError` methods must ensure the `OsFunctions` atomic pointer is appropriately set, which can only be done from `std`.

Required to allow `std` access from `core`

This allows `Error` to be moved into `core` while still retaining the ability to store custom error data. This may have performance implications!

Stored in a `static` `AtomicPtr` to allow definition in `core` and setting in `std`. Should be replaced with Externally Implemented Items (EII) or similar once stable.

lygstate · 2026-05-18T06:38:24Z

+#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
+pub use self::os_functions::{decode_error_kind, format_os_error, is_interrupted, set_functions};
 use crate::{error, fmt};



Maybe split this commit out first and testing(merge it first) it if have performance regression?
I think this commit have maximal possibility have performance issue.

View changes since the review

It's tricky because both these changes are purely a negative until Error is moved into core, so it's hard to justify splitting them out.

Now that `Error` lives in `core::io`, doc links to it from `core` can be simplified.

Move `IoSlice` and `IoSliceMut` to `core::io` ACP: rust-lang/libs-team#755 Tracking issue: #154046 Related: #152918 Related: #155625 ## Description Moves `std::io::IoSlice` and `std::io::IoSliceMut` into `core::io`. This is required for the `Read` and `Write` traits to be moved into `alloc` and/or `core`, as they contain stable methods which work with IO slices. Similar to #155574, this PR inlines the `std::sys` types required to create an ABI compatible type for the platforms where such compatibility is guaranteed. Additionally, I've moved the relevant tests out of `std::io::tests` into `coretests::io::io_slice`. --- ## Notes * This PR overlaps with #152918, but goes further than moving the IO slice types to `alloc` and instead moves them straight to `core`. Since these types have no interaction with allocation, and doing so will allow `Write` to move to `core::io`, I consider this a better home for these types. * Some discussion around the decision to not use a `core::sys` module can be found [here](#155574 (comment)). * I've renamed `unsupported` to `generic` to better reflect that `IoSlice(Mut)` _is_ supported by all platforms, it just doesn't have a special ABI-compatible type. I don't want to imply that parts of `core` "don't work" depending on the target; `IoSlice(Mut)` works exactly as expected on all targets. * I've made `pub` items within each platform-specific representation `pub(super)` to highlight that everything within `core::io::io_slice` is an internal implementation detail not meant for any other part of the crate to be aware of. * No AI tooling of any kind was used during the creation of this PR.

rust-bors · 2026-05-30T17:16:45Z

☔ The latest upstream changes (presumably #155849) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Apr 21, 2026

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Comment thread library/core/src/io/error/os_functions_atomic.rs

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rustbot added the perf-regression-triaged The performance regression has been triaged. label May 14, 2026

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bushrat011899 added 9 commits May 18, 2026 15:52

Adjust std::io::Error tests to only assess public API

e8017c9

Viewing internals wont be possible from `std` once moved into `core`.

Adjust Error documentation

9e385be

Adjust `Error` documentation `core` is more restrictive with documentation quality and linking to other items. Methods that will be implemented through incoherence must also be explicitly linked.

Change repr module definition to use path attribute

94575a8

Personal preference that will be used for another module in a later commit. Purely stylistic.

Setup alloc::io

4bdcda3

Make common Error constants public and hidden

e3039ae

They'll be moved into `core` but must be accessible from `std`.

Make Error::is_interrupted public and hidden

c58a2c0

Required to allow `std` access from `core`

Remove usage of Box from Error internals

e879567

This allows `Error` to be moved into `core` while still retaining the ability to store custom error data. This may have performance implications!

Delegate RawOsError functionality to OsFunctions

7c97791

Stored in a `static` `AtomicPtr` to allow definition in `core` and setting in `std`. Should be replaced with Externally Implemented Items (EII) or similar once stable.

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bushrat011899 added 3 commits May 18, 2026 20:30

Move std::io::Error and friends into core and alloc

8fc91fa

Reduce usage of unstable public items

6ca7170

Simplify core::io documentation links

7c41c8e

Now that `Error` lives in `core::io`, doc links to it from `core` can be simplified.

bushrat011899 force-pushed the core_io_error branch from 55b632f to 7c41c8e Compare May 18, 2026 10:31

taiki-e mentioned this pull request May 26, 2026

no_std support for futures-io rust-lang/futures-rs#860

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