Optimize canonicalizer flag checks.
The most important change here relates to type folding: we now check the flags up front, instead of doing it in `inner_fold_ty` after checking the cache and doing a match. This is a small perf win, and matches other similar folders (e.g. `CanonicalInstantiator`).
Likewise for const folding, we now check the flags first. (There is no cache for const folding.)
Elsewhere we don't check flags before folding a predicate (unnecessary, because `fold_predicate` already checks the flags itself before doing anything else), and invert the flag checks in a couple of methods to match the standard order.
r? @lcnr
Explicitly export core and std macros
Currently all core and std macros are automatically added to the prelude via #[macro_use]. However a situation arose where we want to add a new macro `assert_matches` but don't want to pull it into the standard prelude for compatibility reasons. By explicitly exporting the macros found in the core and std crates we get to decide on a per macro basis and can later add them via the rust_20xx preludes.
Closes https://github.com/rust-lang/rust/issues/53977
Unlocks https://github.com/rust-lang/rust/pull/137487
Reference PR:
- https://github.com/rust-lang/reference/pull/2077
# Stabilization report lib
Everything N/A or already covered by lang report except, breaking changes: The unstable and never intended for public use `format_args_nl` macro is no longer publicly accessible as requested by @petrochenkov. Affects <10 crates including dependencies.
# Stabilization report lang
## Summary
Explicitly export core and std macros.
This change if merged would change the code injected into user crates to no longer include #[macro_use] on extern crate core and extern crate std. This change is motivated by a near term goal and a longer term goal. The near term goal is to allow a macro to be defined at the std or core crate root but not have it be part of the implicit prelude. Such macros can then be separately promoted to the prelude in a new edition. Specifically this is blocking the stabilization of assert_matches rust-lang/rust#137487. The longer term goal is to gradually deprecate #[macro_use]. By no longer requiring it for standard library usage, this serves as a step towards that goal. For more information see rust-lang/rust#53977.
PR link: https://github.com/rust-lang/rust/pull/139493
Tracking:
- https://github.com/rust-lang/rust/issues/147319
Reference PRs:
- https://github.com/rust-lang/rust/pull/139493
cc @rust-lang/lang @rust-lang/lang-advisors
### What is stabilized
Stabilization:
* `#[macro_use]` is no longer automatically included in the crate root module. This allows the explicit import of macros in the `core` and `std` prelude e.g. `pub use crate::dbg;`.
* `ambiguous_panic_imports` lint. Code that previously passed without warnings, but included the following or equivalent - only pertaining to core vs std panic - will now receive a warning:
```rust
#![no_std]
extern crate std;
use std::prelude::v1::*;
fn xx() {
panic!(); // resolves to core::panic
//~^ WARNING `panic` is ambiguous
//~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
}
```
This lint is tied to a new exception to the name resolution logic in [compiler/rustc_resolve/src/ident.rs](https://github.com/rust-lang/rust/pull/139493/files#diff-c046507afdba3b0705638f53fffa156cbad72ed17aa01d96d7bd1cc10b8d9bce) similar to an exception added for https://github.com/rust-lang/rust/issues/145575. Specifically this only happens if the import of two builtin macros is ambiguous and they are named `sym::panic`. I.e. this can only happen for `core::panic` and `std::panic`. While there are some tiny differences in what syntax is allowed in `std::panic` vs `core::panic` in editions 2015 and 2018, [see](https://github.com/rust-lang/rust/pull/139493#issuecomment-2796481622). The behavior at runtime will always be the same if it compiles, implying minimal risk in what specific macro is resolved. At worst some closed source project not captured by crater will stop compiling because a different panic is resolved than previously and they were using obscure syntax like `panic!(&String::new())`.
## Design
N/A
### Reference
> What updates are needed to the Reference? Link to each PR. If the Reference is missing content needed for describing this feature, discuss that.
- https://github.com/rust-lang/reference/pull/2077
### RFC history
> What RFCs have been accepted for this feature?
N/A
### Answers to unresolved questions
N/A
### Post-RFC changes
> What other user-visible changes have occurred since the RFC was accepted? Describe both changes that the lang team accepted (and link to those decisions) as well as changes that are being presented to the team for the first time in this stabilization report.
N/A
### Key points
> What decisions have been most difficult and what behaviors to be stabilized have proved most contentious? Summarize the major arguments on all sides and link to earlier documents and discussions.
- Nothing was really contentious.
### Nightly extensions
> Are there extensions to this feature that remain unstable? How do we know that we are not accidentally committing to those?
N/A
### Doors closed
> What doors does this stabilization close for later changes to the language? E.g., does this stabilization make any other RFCs, lang experiments, or known in-flight proposals more difficult or impossible to do later?
No known doors are closed.
## Feedback
### Call for testing
> Has a "call for testing" been done? If so, what feedback was received?
No.
### Nightly use
> Do any known nightly users use this feature? Counting instances of `#![feature(FEATURE_NAME)]` on GitHub with grep might be informative.
N/A
## Implementation
### Major parts
> Summarize the major parts of the implementation and provide links into the code and to relevant PRs.
>
> See, e.g., this breakdown of the major parts of async closures:
>
> - <https://rustc-dev-guide.rust-lang.org/coroutine-closures.html>
The key change is [compiler/rustc_builtin_macros/src/standard_library_imports.rs](https://github.com/rust-lang/rust/pull/139493/files#diff-be08752823b8f862bb0c7044ef049b0f4724dbde39306b98dea2adb82ec452b0) removing the macro_use inject and the `v1.rs` preludes now explicitly `pub use`ing the macros https://github.com/rust-lang/rust/pull/139493/files#diff-a6f9f476d41575b19b399c6d236197355556958218fd035549db6d584dbdea1d + https://github.com/rust-lang/rust/pull/139493/files#diff-49849ff961ebc978f98448c8990cf7aae8e94cb03db44f016011aa8400170587.
### Coverage
> Summarize the test coverage of this feature.
>
> Consider what the "edges" of this feature are. We're particularly interested in seeing tests that assure us about exactly what nearby things we're not stabilizing. Tests should of course comprehensively demonstrate that the feature works. Think too about demonstrating the diagnostics seen when common mistakes are made and the feature is used incorrectly.
>
> Within each test, include a comment at the top describing the purpose of the test and what set of invariants it intends to demonstrate. This is a great help to our review.
>
> Describe any known or intentional gaps in test coverage.
>
> Contextualize and link to test folders and individual tests.
A variety of UI tests including edge cases have been added.
### Outstanding bugs
> What outstanding bugs involve this feature? List them. Should any block the stabilization? Discuss why or why not.
An old bug is made more noticeable by this change https://github.com/rust-lang/rust/issues/145577 but it was recommended to not block on it https://github.com/rust-lang/rust/pull/139493#issuecomment-3288311495.
### Outstanding FIXMEs
> What FIXMEs are still in the code for that feature and why is it OK to leave them there?
```
// Turn ambiguity errors for core vs std panic into warnings.
// FIXME: Remove with lang team approval.
```
https://github.com/rust-lang/rust/pull/139493/files#diff-c046507afdba3b0705638f53fffa156cbad72ed17aa01d96d7bd1cc10b8d9bce
### Tool changes
> What changes must be made to our other tools to support this feature. Has this work been done? Link to any relevant PRs and issues.
- ~~rustfmt~~
- ~~rust-analyzer~~
- ~~rustdoc (both JSON and HTML)~~
- ~~cargo~~
- ~~clippy~~
- ~~rustup~~
- ~~docs.rs~~
No known changes needed or expected.
### Breaking changes
> If this stabilization represents a known breaking change, link to the crater report, the analysis of the crater report, and to all PRs we've made to ecosystem projects affected by this breakage. Discuss any limitations of what we're able to know about or to fix.
Breaking changes:
* It's possible for user code to invoke an ambiguity by defining their own macros with standard library names and glob importing them, e.g. `use nom::*` importing `nom::dbg`. In practice this happens rarely based on crater data. The 3 public crates where this was an issue, have been fixed. The ambiguous panic import is more common and affects a non-trivial amount of the public - and likely private - crate ecosystem. To avoid a breaking change, a new future incompatible lint was added ambiguous_panic_imports see https://github.com/rust-lang/rust/issues/147319. This allows current code to continue compiling, albeit with a new warning. Future editions of Rust make this an error and future versions of Rust can choose to make this error. Technically this is a breaking change, but crater gives us the confidence that the impact will be at worst a new warning for 99+% of public and private crates.
```rust
#![no_std]
extern crate std;
use std::prelude::v1::*;
fn xx() {
panic!(); // resolves to core::panic
//~^ WARNING `panic` is ambiguous
//~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
}
```
* Code using `#![no_implicit_prelude]` *and* Rust edition 2015 will no longer automatically have access to the prelude macros. The following works on nightly but would stop working with this change:
```rust
#![no_implicit_prelude]
// Uncomment to fix error.
// use std::vec;
fn main() {
let _ = vec![3, 6];
}
```
Inversely with this change the `panic` and `unreachable` macro will always be in the prelude even if `#![no_implicit_prelude]` is specified.
Error matrix when using `#![no_implicit_prelude]`, ✅ means compiler passes 🚫 means compiler error:
Configuration | Rust 2015 | Rust 2018+
--------------|-----------|-----------
Nightly (panic\|unreachable) macro | ✅ | 🚫
PR (panic\|unreachable) macro | ✅ | ✅
Nightly (column\|concat\|file\|line\|module_path\|stringify) macro | ✅ | ✅
PR (column\|concat\|file\|line\|module_path\|stringify) macro | ✅ | ✅
Nightly remaining macros | ✅ | 🚫
PR remaining macros | 🚫 | 🚫
Addressing this issue is deemed expensive.
Crater found no instance of this pattern in use. Affected code can fix the issue by directly importing the macros. The new behavior matches the behavior of `#![no_implicit_prelude]` in Rust editions 2018 and beyond and it's intuitive meaning.
Crater report:
- https://crater-reports.s3.amazonaws.com/pr-139493-2/index.html (latest run, but partial run)
- https://crater-reports.s3.amazonaws.com/pr-139493-1/index.html (previous full run, one fix missing)
Crater analysis:
- Discussed in breaking changes.
PRs to affected crates:
- https://github.com/Michael-F-Bryan/gcode-rs/pull/57
- https://github.com/stbuehler/rust-ipcrypt/pull/1
- https://github.com/jcreekmore/dmidecode/pull/55
## Type system, opsem
### Compile-time checks
> What compilation-time checks are done that are needed to prevent undefined behavior?
>
> Link to tests demonstrating that these checks are being done.
N/A
### Type system rules
> What type system rules are enforced for this feature and what is the purpose of each?
N/A
### Sound by default?
> Does the feature's implementation need specific checks to prevent UB, or is it sound by default and need specific opt-in to perform the dangerous/unsafe operations? If it is not sound by default, what is the rationale?
N/A
### Breaks the AM?
> Can users use this feature to introduce undefined behavior, or use this feature to break the abstraction of Rust and expose the underlying assembly-level implementation? Describe this if so.
N/A
## Common interactions
### Temporaries
> Does this feature introduce new expressions that can produce temporaries? What are the scopes of those temporaries?
N/A
### Drop order
> Does this feature raise questions about the order in which we should drop values? Talk about the decisions made here and how they're consistent with our earlier decisions.
N/A
### Pre-expansion / post-expansion
> Does this feature raise questions about what should be accepted pre-expansion (e.g. in code covered by `#[cfg(false)]`) versus what should be accepted post-expansion? What decisions were made about this?
N/A
### Edition hygiene
> If this feature is gated on an edition, how do we decide, in the context of the edition hygiene of tokens, whether to accept or reject code. E.g., what token do we use to decide?
N/A
### SemVer implications
> Does this feature create any new ways in which library authors must take care to prevent breaking downstreams when making minor-version releases? Describe these. Are these new hazards "major" or "minor" according to [RFC 1105](https://rust-lang.github.io/rfcs/1105-api-evolution.html)?
No.
### Exposing other features
> Are there any other unstable features whose behavior may be exposed by this feature in any way? What features present the highest risk of that?
No.
## History
> List issues and PRs that are important for understanding how we got here.
- This change was asked for here https://github.com/rust-lang/rust/pull/137487#issuecomment-2770801974
## Acknowledgments
> Summarize contributors to the feature by name for recognition and so that those people are notified about the stabilization. Does anyone who worked on this *not* think it should be stabilized right now? We'd like to hear about that if so.
More or less solo developed by @Voultapher with some help from @petrochenkov.
## Open items
> List any known items that have not yet been completed and that should be before this is stabilized.
None.
Don't suggest replacing closure parameter with type name
When a closure has an inferred parameter type like `|ch|` and the expected type differs in borrowing (e.g., `char` vs `&char`), the suggestion would incorrectly propose `|char|` instead of something valid like `|ch: char|`.
This happened because the code couldn't walk explicit `&` references in the HIR when the type is inferred, and fell back to replacing the entire parameter span with the expected type name.
Fix by only emitting the suggestion when we can properly identify the `&` syntax to remove.
Fixesrust-lang/rust#150693
Update books
## rust-lang/nomicon
2 commits in 5b3a9d084cbc64e54da87e3eec7c7faae0e48ba9..050c002a360fa45b701ea34feed7a860dc8a41bf
2026-01-10 15:05:01 UTC to 2026-01-09 23:34:23 UTC
- Correct false typo fix in safe-unsafe-meaning.md (rust-lang/nomicon#518)
- Fix grammar and typos in safe-unsafe-meaning.md (rust-lang/nomicon#517)
## rust-lang/reference
2 commits in 6363385ac4ebe1763f1e6fb2063c0b1db681a072..28b5a54419985f03db5294de5eede71b6665b594
2026-01-03 19:09:24 UTC to 2026-01-03 18:10:24 UTC
- RISC-V Extensions update including 29 extensions to stabilize (rust-lang/reference#1987)
- Update note about shebang removal in `include`d files (rust-lang/reference#2127)
## rust-lang/rust-by-example
1 commits in 2e02f22a10e7eeb758e6aba484f13d0f1988a3e5..8de6ff811315ac3a96ebe01d74057382e42ffdee
2026-01-06 00:16:33 UTC to 2026-01-06 00:16:33 UTC
- Update comments on iterator behavior in closure examples (rust-lang/rust-by-example#1983)
Remove a workaround for a bug (take 2)
I don't think it is necessary anymore. As I understand it from issue 39504 the original problem was that rustbuild changed a hardlink in the cargo build dir to point to copy in the sysroot while cargo may have hardlinked it to the original first. I don't think this happens anymore and as such this workaround is no longer necessary
I can't reproduce the CI failure in https://github.com/rust-lang/rust/pull/122186 locally with `./x.py test library/core`. It is possible this got fixed by https://github.com/rust-lang/rust/pull/149273.
core: ptr: split_at_mut: fix typo in safety doc
Removes the double subject "it" in the safety documentation of `core::ptr::split_at_mut` for raw slice pointers, as it does not refer to anything.
Reported-by: Johnathan Van Why <jrvanwhy@betterbytes.org>
Move some checks from `check_doc_attrs` directly into `rustc_attr_parsing`
Part of https://github.com/rust-lang/rust/issues/149865.
I also used this opportunity to remove the `id_is_crate_root` method.
Once this is merged, I think I'll try to see if we can remove `target_id` as well.
r? @JonathanBrouwer
Improve span for "unresolved intra doc link" on `deprecated` attribute
Follow-up of rust-lang/rust#150721.
To make this work, I replaced the `Symbol` by an `Ident` to keep the `Span` information.
cc @folkertdev
r? @camelid
Add `f16` inline ASM support for s390x
tracking issue: https://github.com/rust-lang/rust/issues/116909
cc https://github.com/rust-lang/rust/issues/125398
Support the `f16x8` type in inline assembly. Only with the `nnp-assist` feature are there any instructions that make use of this type. Based on the riscv implementation I now cast to `i16x8` when that feature is not enabled.
As far as I'm aware there are no instructions operating on `f16` scalar values. Should we still add support for using them in inline assembly?
r? @tgross35
cc @uweigand
Destabilise `target-spec-json`
Per rust-lang/compiler-team#944:
> Per https://github.com/rust-lang/rust/issues/71009, the ability to load target spec JSONs was stabilised accidentally. Within the team, we've always considered the format to be unstable and have changed it freely. This has been feasible as custom targets can only be used with core, like any other target, and so custom targets de-facto require nightly to be used (i.e. to build core manually or use Cargo's -Zbuild-std).
>
> Current build-std RFCs (https://github.com/rust-lang/rfcs/pull/3873, https://github.com/rust-lang/rfcs/pull/3874) propose a mechanism for building core on stable (at the request of Rust for Linux), which combined with a stable target-spec-json format, permit the current format to be used much more widely on stable toolchains. This would prevent us from improving the format - making it less tied to LLVM, switching to TOML, enabling keys in the spec to be stabilised individually, etc.
>
> De-stabilising the format gives us the opportunity to improve the format before it is too challenging to do so. Internal company toolchains and projects like Rust for Linux already use target-spec-json, but must use nightly at some point while doing so, so while it could be inconvenient for those users to destabilise this, it is hoped that an minimal alternative that we could choose to stabilise can be proposed relatively quickly.
Currently all core and std macros are automatically added to the prelude
via #[macro_use]. However a situation arose where we want to add a new macro
`assert_matches` but don't want to pull it into the standard prelude for
compatibility reasons. By explicitly exporting the macros found in the core and
std crates we get to decide on a per macro basis and can later add them via
the rust_20xx preludes.
Dogfood `-Zno-embed-metadata` in the standard library
This PR dogfoods the [`-Zno-embed-metadata`](https://github.com/rust-lang/cargo/issues/15495) flag in the standard library. This removes the .rmeta portion out of the `libstd.so` file, thus reducing its filesize on disk. Notably, this reduces the amount of MiB that we ship to people who download the standard library.
I think that the only way to find out what this breaks is to try to run full CI, and then try to land it on nightly :)
r? @ghost
When a closure has an inferred parameter type like `|ch|` and the
expected type differs in borrowing (e.g., `char` vs `&char`), the
suggestion code would incorrectly suggest `|char|` instead of the
valid `|ch: char|`.
This happened because the code couldn't walk explicit `&` references
in the HIR when the type is inferred, and fell back to replacing the
entire parameter span with the expected type name.
Fix by only emitting the suggestion when we can properly identify the
`&` syntax to remove.
mGCA: Move tests for assoc const bindings (formerly ACE) into dedicated directory & replace more mentions of ACE
Split out of PR rust-lang/rust#150843.
As discussed.
Somewhat obvious underlying principle: If the test checks basic or core parts of assoc const bindings and nothing else, move it, otherwise leave it even if it contains ACEs.
Motivation: It makes a lot easier for me to continue working on ACE efficiently.
r? @BoxyUwU
mGCA: Support array expression as direct const arguments
tracking issue: rust-lang/rust#132980resolve: rust-lang/rust#150612
Support array expression as direct const arguments (e. g. [1, 2, N]) in min_generic_const_args.
todo:
* [x] Rebase another mGCA PR
* [x] Add more test case
* [x] Modify clippy code
Remove `S-waiting-on-bors` after a PR is merged
I just noticed that we have 50k+ PRs marked as waiting on bors, even though they have been merged, lol.
Piotr Osiewicz