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Update cargo
14 commits in cf53cc54bb593b5ec3dc2be4b1702f50c36d24d5..e75214ea4936d2f2c909a71a1237042cc0e14b07
2024-10-18 13:56:15 +0000 to 2024-10-25 16:34:32 +0000
- refactor(env): remove unnecessary clones (rust-lang/cargo#14730)
- test(install): Verify 2024 edition / resolver=3 doesn't affect resolution (rust-lang/cargo#14724)
- Fix: trace `config` `[env]` table in dep-info. (rust-lang/cargo#14701)
- Added unstable-schema generation for Cargo.toml (rust-lang/cargo#14683)
- fix: add source replacement info when no matching package found (rust-lang/cargo#14715)
- feat(complete): Include descriptions in zsh (rust-lang/cargo#14726)
- refactor(fingerprint): avoid unnecessary fopen calls (rust-lang/cargo#14728)
- docs(resolver): Make room for v3 resolver (rust-lang/cargo#14725)
- test: add fixes in the sat resolver (rust-lang/cargo#14707)
- docs(ci): Don't constrainty latest_deps job by MSRV (rust-lang/cargo#14711)
- refactor: use `Iterator::is_sorted` (rust-lang/cargo#14702)
- refactor(rustfix): minor refactors (rust-lang/cargo#14710)
- chore(deps): update msrv (rust-lang/cargo#14705)
- fix(renovate): Switch matchPackageNames to matchDepNames (rust-lang/cargo#14704)
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Const stability checks v2
The const stability system has served us well ever since `const fn` were first stabilized. It's main feature is that it enforces *recursive* validity -- a stable const fn cannot internally make use of unstable const features without an explicit marker in the form of `#[rustc_allow_const_fn_unstable]`. This is done to make sure that we don't accidentally expose unstable const features on stable in a way that would be hard to take back. As part of this, it is enforced that a `#[rustc_const_stable]` can only call `#[rustc_const_stable]` functions. However, some problems have been coming up with increased usage:
- It is baffling that we have to mark private or even unstable functions as `#[rustc_const_stable]` when they are used as helpers in regular stable `const fn`, and often people will rather add `#[rustc_allow_const_fn_unstable]` instead which was not our intention.
- The system has several gaping holes: a private `const fn` without stability attributes whose inherited stability (walking up parent modules) is `#[stable]` is allowed to call *arbitrary* unstable const operations, but can itself be called from stable `const fn`. Similarly, `#[allow_internal_unstable]` on a macro completely bypasses the recursive nature of the check.
Fundamentally, the problem is that we have *three* disjoint categories of functions, and not enough attributes to distinguish them:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features
Functions in the first two categories cannot use unstable const features and they can only call functions from the first two categories.
This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.
Also, all the holes mentioned above have been closed. There's still one potential hole that is hard to avoid, which is when MIR building automatically inserts calls to a particular function in stable functions -- which happens in the panic machinery. Those need to be manually marked `#[rustc_const_stable_indirect]` to be sure they follow recursive const stability. But that's a fairly rare and special case so IMO it's fine.
The net effect of this is that a `#[unstable]` or unmarked function can be constified simply by marking it as `const fn`, and it will then be const-callable from stable `const fn` and subject to recursive const stability requirements. If it is publicly reachable (which implies it cannot be unmarked), it will be const-unstable under the same feature gate. Only if the function ever becomes `#[stable]` does it need a `#[rustc_const_unstable]` or `#[rustc_const_stable]` marker to decide if this should also imply const-stability.
Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to use unstable const lang features (including intrinsics), or (b) `#[stable]` functions that are not yet intended to be const-stable. Adding `#[rustc_const_stable]` is only needed for functions that are actually meant to be directly callable from stable const code. `#[rustc_const_stable_indirect]` is used to mark intrinsics as const-callable and for `#[rustc_const_unstable]` functions that are actually called from other, exposed-on-stable `const fn`. No other attributes are required.
Also see the updated dev-guide at https://github.com/rust-lang/rustc-dev-guide/pull/2098.
I think in the future we may want to tweak this further, so that in the hopefully common case where a public function's const-stability just exactly mirrors its regular stability, we never have to add any attribute. But right now, once the function is stable this requires `#[rustc_const_stable]`.
### Open question
There is one point I could see we might want to do differently, and that is putting `#[rustc_const_unstable]` functions (but not intrinsics) in category 2 by default, and requiring an extra attribute for `#[rustc_const_not_exposed_on_stable]` or so. This would require a bunch of extra annotations, but would have the advantage that turning a `#[rustc_const_unstable]` into `#[rustc_const_stable]` will never change the way the function is const-checked. Currently, we often discover in the const stabilization PR that a function needs some other unstable const things, and then we rush to quickly deal with that. In this alternative universe, we'd work towards getting rid of the `rustc_const_not_exposed_on_stable` before stabilization, and once that is done stabilization becomes a trivial matter. `#[rustc_const_stable_indirect]` would then only be used for intrinsics.
I think I like this idea, but might want to do it in a follow-up PR, as it will need a whole bunch of annotations in the standard library. Also, we probably want to convert all const intrinsics to the "new" form (`#[rustc_intrinsic]` instead of an `extern` block) before doing this to avoid having to deal with two different ways of declaring intrinsics.
Cc `@rust-lang/wg-const-eval` `@rust-lang/libs-api`
Part of https://github.com/rust-lang/rust/issues/129815 (but not finished since this is not yet sufficient to safely let us expose `const fn` from hashbrown)
Fixes https://github.com/rust-lang/rust/issues/131073 by making it so that const-stable functions are always stable
try-job: test-various
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Rollup of 3 pull requests
Successful merges:
- #132106 (Pass Ident by reference in ast Visitor)
- #132130 (remove `change-id` from CI script)
- #132137 (library: consistently use American spelling for 'behavior')
r? `@ghost`
`@rustbot` modify labels: rollup
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Then we can rename the _raw functions to drop their suffix, and instead
explicitly use is_stable_const_fn for the few cases where that is really what
you want.
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Run the full stage 2 `run-make` test suite in `x86_64-gnu-debug`
Run the full `run-make` test suite in the `x86_64-gnu-debug` CI job. This is currently the *only* CI job where `//@ needs-force-clang-based-test` will be satisfied, so some `run-make` tests will literally never be run otherwise. Before this PR, the CI job only ran `run-make` tests which contains the substring `clang` in its test name, which is both (1) a footgun because it's very easy to forget and (2) it masks tests that would otherwise fail (even failing to compile) because the test is skipped if doesn't have a `clang` in its test name.
With the environment of `x86_64-gnu-debug`, two `run-make` tests failed before this PR:
1. `tests/run-make/issue-84395-lto-embed-bitcode/rmake.rs`: this was broken for a long time because `objcopy` in llvm bin tools was renamed to `llvm-objcopy`. This test was converted into a rmake.rs test, rather straight forward.
2. `tests/run-make/cross-lang-lto-riscv-abi/rmake.rs`: this was broken for a long time and never worked. The old version inspected human-readable output of `llvm-readobj --file-header` looking for substring `EF_RISCV_FLOAT_ABI_DOUBLE`, but the human-readable output will only contain something like `Flags: 0x5, RVC, double-float ABI`, hence it will never match. This test was fixed by instead using the `object` crate to actually decode the ELF headers looking for the specific `e_flags` based on reading the RISCV ELF psABI docs.
This PR is best reviewed commit-by-commit, two commits setup the support library for functionality and two commits are for each of the failing `run-make` tests.
I had to bump the `x86_64-gnu-debug` job to be ran with a runner with larger disk space.
Part of #132034.
try-job: x86_64-gnu-debug
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library: consistently use American spelling for 'behavior'
We use "behavior" a lot more often than "behaviour", but some "behaviour" have even snuck into user-facing docs. This makes the spelling consistent.
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remove `change-id` from CI script
It's not necessary to set `change-id` for CI since https://github.com/rust-lang/rust/pull/130356.
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Pass Ident by reference in ast Visitor
`MutVisitor`'s version of `visit_ident` passes around `&Ident`, but `Visitor` copies `Ident`. This PR changes that
r? `@petrochenkov`
related to #128974
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Fundamentally, we have *three* disjoint categories of functions:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features
This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.
Also, several holes in recursive const stability checking are being closed.
There's still one potential hole that is hard to avoid, which is when MIR
building automatically inserts calls to a particular function in stable
functions -- which happens in the panic machinery. Those need to *not* be
`rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be
sure they follow recursive const stability. But that's a fairly rare and special
case so IMO it's fine.
The net effect of this is that a `#[unstable]` or unmarked function can be
constified simply by marking it as `const fn`, and it will then be
const-callable from stable `const fn` and subject to recursive const stability
requirements. If it is publicly reachable (which implies it cannot be unmarked),
it will be const-unstable under the same feature gate. Only if the function ever
becomes `#[stable]` does it need a `#[rustc_const_unstable]` or
`#[rustc_const_stable]` marker to decide if this should also imply
const-stability.
Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to
use unstable const lang features (including intrinsics), or (b) `#[stable]`
functions that are not yet intended to be const-stable. Adding
`#[rustc_const_stable]` is only needed for functions that are actually meant to
be directly callable from stable const code. `#[rustc_const_stable_indirect]` is
used to mark intrinsics as const-callable and for `#[rustc_const_unstable]`
functions that are actually called from other, exposed-on-stable `const fn`. No
other attributes are required.
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Emit future-incompatibility lint when calling/declaring functions with vectors that require missing target feature
On some architectures, vector types may have a different ABI depending on whether the relevant target features are enabled. (The ABI when the feature is disabled is often not specified, but LLVM implements some de-facto ABI.)
As discussed in https://github.com/rust-lang/lang-team/issues/235, this turns out to very easily lead to unsound code.
This commit makes it a post-monomorphization error to declare or call functions using those vector types in a context in which the corresponding target features are disabled, if using an ABI for which the difference is relevant. This ensures that these functions are always called with a consistent ABI.
See the [nomination comment](https://github.com/rust-lang/rust/pull/127731#issuecomment-2288558187) for more discussion.
r? RalfJung
Part of https://github.com/rust-lang/rust/issues/116558
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ci: aarch64-gnu-debug job
- Adds a new CI job which checks that the compiler builds with `--enable-debug` and tests that `needs-force-clang-based-tests` pass (where cross-language LTO is tested).
- Add a test confirming that `-Zbranch-protection=pac-ret` and cross-language LTO work together.
r? `@Mark-Simulacrum`
try-job: aarch64-gnu-debug
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Signed-off-by: onur-ozkan <work@onurozkan.dev>
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Rollup of 4 pull requests
Successful merges:
- #131457 (Expand `ptr::fn_addr_eq()` documentation.)
- #132085 (Update StableMIR doc to reflect current status)
- #132118 (Add support for `~const` item bounds)
- #132125 (coverage: Emit LLVM intrinsics using the normal helper method)
r? `@ghost`
`@rustbot` modify labels: rollup
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On some architectures, vector types may have a different ABI when
relevant target features are enabled.
As discussed in https://github.com/rust-lang/lang-team/issues/235, this
turns out to very easily lead to unsound code.
This commit makes it an error to declare or call functions using those
vector types in a context in which the corresponding target features are
disabled, if using an ABI for which the difference is relevant.
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coverage: Emit LLVM intrinsics using the normal helper method
Codegen already has convenient ways to declare and emit LLVM intrinsics, so there's no need for coverage instrumentation to jump through hoops to emit them manually.
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Add support for `~const` item bounds
Supports the only missing capability of `~const` associated types that I can think of now (this is obviously excluding `~const` opaques, which I see as an extension to this; I'll probably do that next).
r? ``@lcnr`` mostly b/c it changes candidate assembly, or reassign
cc ``@fee1-dead``
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Update StableMIR doc to reflect current status
Update stable-mir documentation, since we no longer use git subtree, and we have 2 different crates.
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Expand `ptr::fn_addr_eq()` documentation.
* Describe more clearly what is (not) guaranteed, and de-emphasize the description of rustc implementation details.
* Explain what you *can* reliably use it for.
Tracking issue for `ptr_fn_addr_eq`: #129322
The motivation for this PR is that I just learned that `ptr::fn_addr_eq()` exists, read the documentation, and thought: “*I* know what this means, but someone not already familiar with how `rustc` works could be left wondering whether this is even good for anything.” Fixing that seems especially important if we’re going to recommend people use it instead of `==` (as per #118833).
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[rustdoc] Do not consider nested functions as main function even if named `main` in doctests
Fixes #131893.
If a nested function is called `main`, it is not considered as the entry point of the program. Therefore, doctests should not consider such functions as such either.
r? `@notriddle`
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Rollup of 6 pull requests
Successful merges:
- #131851 ([musl] use posix_spawn if a directory change was requested)
- #132048 (AIX: use /dev/urandom for random implementation )
- #132093 (compiletest: suppress Windows Error Reporting (WER) for `run-make` tests)
- #132101 (Avoid using imports in thread_local_inner! in static)
- #132113 (Provide a default impl for Pattern::as_utf8_pattern)
- #132115 (rustdoc: Extend fake_variadic to "wrapped" tuples)
r? `@ghost`
`@rustbot` modify labels: rollup
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Clarify that the translation between unstable and stable items is currently done in the `rustc_smir` crate.
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Co-authored-by: Michael Goulet <michael@errs.io>
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Update books
## rust-lang/edition-guide
7 commits in c7ebae25cb4801a31b6f05353f6d85bfa6feedd1..1f07c242f8162a711a5ac5a4ea8fa7ec884ee7a9
2024-10-21 14:29:49 UTC to 2024-10-19 19:08:20 UTC
- 2024: Add reserved syntax (rust-lang/edition-guide#326)
- Update stdout of `cargo new` (rust-lang/edition-guide#327)
- Don't run doctests on rustfmt ident sorting page
- add rustfmt raw identifer sorting doc (rust-lang/edition-guide#321)
- Add some tips for what to be careful of with rustdoc-doctests (rust-lang/edition-guide#323)
- Remove cargo-remove-implicit-features (rust-lang/edition-guide#324)
- Rename doctest standalone tag (rust-lang/edition-guide#325)
## rust-embedded/book
1 commits in f40a8b420ec4b4505d9489965e261f1d5c28ba23..ddbf1b4e2858fedb71b7c42eb15c4576517dc125
2024-10-13 19:53:37 UTC to 2024-10-13 19:53:37 UTC
- Add link to Cortex-M comparison to install.md (rust-embedded/book#378)
## rust-lang/reference
29 commits in c64e52a3d306eac0129f3ad6c6d8806ab99ae2e9..23ce619966541bf2c80d45fdfeecf3393e360a13
2024-10-05 00:33:03 +0000 to 2024-10-22 21:34:51 +0000
- Mention `--print cfg` under set configuration options (rust-lang/reference#1636)
- Fix `pat` fragment specifier to be the "current" edition (rust-lang/reference#1640)
- Add restriction for cfg_attr with crate_type and crate_name (rust-lang/reference#1649)
- Sort macro fragment specifiers (rust-lang/reference#1641)
- Document mixed-site hygiene (rust-lang/reference#1656)
- Clarify that "macro attributes" refers to proc macros (rust-lang/reference#1660)
- mdbook-spec: Fix Spec::new creation (rust-lang/reference#1658)
- Fix stdcall example broken by recent rustc change (rust-lang/reference#1659)
- Add spec identifiers to const_eval.md (rust-lang/reference#1569)
- Add identifier syntax to trait-bounds.md (rust-lang/reference#1631)
- Add identifier syntax to macro-ambiguity.md (rust-lang/reference#1634)
- Add spec identifier syntax to conditional-compilation.md (rust-lang/reference#1564)
- Add spec identifiers to behaviour-considered-undefined.md (rust-lang/reference#1562)
- Add test linking (rust-lang/reference#1646)
- Allow `deny` inside `forbid` as a no-op (rust-lang/reference#1655)
- Add identifier syntax to identifiers.md (rust-lang/reference#1583)
- Add spec identifiers to crates-and-source-files.md (rust-lang/reference#1570)
- Add identifier syntax to linkage.md (rust-lang/reference#1633)
- Add identifier syntax to type-coercions.md (rust-lang/reference#1632)
- Add identifiers to variables.md (rust-lang/reference#1626)
- Add identifier syntax to lexer chapters (rust-lang/reference#1620)
- Add spec identifier syntax to `unsafe-keyword.md` and `unsafety.md` (rust-lang/reference#1619)
- Add identifier syntax to types and subchapters. (rust-lang/reference#1618)
- Add identifier syntax to subtyping.md (rust-lang/reference#1613)
- Add identifier syntax to statements.md (rust-lang/reference#1611)
- Add identifier syntax to type-layout.md (rust-lang/reference#1614)
- Clarify definition of "immutable bytes" (rust-lang/reference#1637)
- Add preview artifacts in CI (rust-lang/reference#1647)
- trait bounds grammar: make `?` and `for<>` mutually exclusive (rust-lang/reference#1650)
## rust-lang/rustc-dev-guide
5 commits in 07bc9ca9eb1cd6d9fbbf758c2753b748804a134f..59d94ea75a0b157e148af14c73c2dd60efb7b60a
2024-10-21 02:30:07 UTC to 2024-10-07 21:12:09 UTC
- Add the WASM | WASI | Emscripten notification groups (rust-lang/rustc-dev-guide#2100)
- Update bootstrapping.md (rust-lang/rustc-dev-guide#1900)
- Rename `needs-profiler-support` to `needs-profiler-runtime` (rust-lang/rustc-dev-guide#2095)
- Document compiletest directives `ignore-coverage-map` and `ignore-coverage-run` (rust-lang/rustc-dev-guide#2094)
- Purge `run-pass-valgrind` mentions (rust-lang/rustc-dev-guide#2091)
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r=GuillaumeGomez,notriddle
rustdoc: Extend fake_variadic to "wrapped" tuples
This allows impls such as `impl QueryData for OneOf<(T,)>` to be displayed as variadic: `impl QueryData for OneOf<(T₁, T₂, …, Tₙ)>`.
See question on [zulip](https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/Make.20.60.23.5Bdoc.28fake_variadic.29.5D.60.20more.20useful).
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Provide a default impl for Pattern::as_utf8_pattern
Newly added ```Pattern::as_utf8_pattern()``` causes needless breakage for crates that implement Pattern. This provides a default implementation instead.
r? `@BurntSushi`
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Avoid using imports in thread_local_inner! in static
Fixes #131863 for wasm targets
All other macros were done in #131866, but this sub module is missed.
r? `@jieyouxu`
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compiletest: suppress Windows Error Reporting (WER) for `run-make` tests
WER by default will show a *bunch* of error dialogues for missing DLLs on Windows for `run-make` tests. We address that by:
1. Guarding `run-make` test process spawning with `disable_error_reporting`.
2. Fixing `disable_error_reporting` to also add the [`SEM_FAILCRITICALERRORS` flag to `SetErrorMode`][SetErrorMode]. Just `SEM_NOGPFAULTERRORBOX` was not sufficient to suppress error dialogues for e.g. missing DLLs.
Fixes #132092. In particular, refer to that issue for the necessary conditions to observe these dialogues from popping up in the first place.
I was only able to manually test this locally in my "native" Windows msvc environment and it prevents the WER dialogues from popping up, I don't think it's possible to really test this automatically.
[SetErrorMode]: https://learn.microsoft.com/en-us/windows/win32/api/errhandlingapi/nf-errhandlingapi-seterrormode?redirectedfrom=MSDN#parameters
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AIX: use /dev/urandom for random implementation
On AIX, we can poll `/dev/urandom` for cryptographically secure random output to implement `fill_bytes` because we don't have equivalent syscalls like other platforms. https://www.ibm.com/docs/en/aix/7.3?topic=files-random-urandom-devices
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[musl] use posix_spawn if a directory change was requested
Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol.
Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in https://github.com/rust-lang/libc/pull/3949 -- use it here.
I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at
61f5ee514f8f60ed8f04c6494bdf36c19e7a8126:
Before:
```
Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped
```
After:
```
Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped
```
Fixes #99740.
try-job: dist-various-1
try-job: dist-various-2
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Fixes #131863 for wasm targets
All other macros were done in #131866, but this sub module is missed.
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Rollup of 4 pull requests
Successful merges:
- #131790 (Document textual format of SocketAddrV{4,6})
- #131983 (Stabilize shorter-tail-lifetimes)
- #132097 (sanitizer.md: LeakSanitizer is not supported on aarch64 macOS)
- #132107 (Remove visit_expr_post from ast Visitor)
r? `@ghost`
`@rustbot` modify labels: rollup
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doctests
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Remove visit_expr_post from ast Visitor
`visit_expr_post` is only present in the immutable version of ast Visitors and its default implementation is a noop.
Given that its only implementer is on `rustc_lint/src/early.rs` and its name follows the same naming convention as some other lints (`_post`), it seems that `visit_expr_post` being in `Visitor` was a little mistake.
r? `@petrochenkov`
related to #128974
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sanitizer.md: LeakSanitizer is not supported on aarch64 macOS
related to #98473
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r=lcnr
Stabilize shorter-tail-lifetimes
Close #131445
Tracked by #123739
We found a test case `tests/ui/drop/drop_order.rs` that had not been covered by the change. The test fixture is fixed now with the correct expectation.
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Document textual format of SocketAddrV{4,6}
This commit adds new "Textual representation" documentation sections to SocketAddrV4 and SocketAddrV6, by analogy to the existing "textual representation" sections of Ipv4Addr and Ipv6Addr.
Rationale: Without documentation about which formats are actually accepted, it's hard for a programmer to be sure that their code will actually behave as expected when implementing protocols that require support (or rejection) for particular representations. This lack of clarity can in turn can lead to ambiguities and security problems like those discussed in RFC 6942.
(I've tried to describe the governing RFCs or standards where I could, but it's possible that the actual implementers had something else in mind. I could not find any standards that corresponded _exactly_ to the one implemented in SocketAddrv6, but I have linked the relevant documents that I could find.)
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Represent trait constness as a distinct predicate
cc `@rust-lang/project-const-traits`
r? `@ghost` for now
Also mirrored everything that is written below on this hackmd here: https://hackmd.io/`@compiler-errors/r12zoixg1l`
# Tl;dr:
* This PR removes the bulk of the old effect desugaring.
* This PR reimplements most of the effect desugaring as a new predicate and set of a couple queries. I believe it majorly simplifies the implementation and allows us to move forward more easily on its implementation.
I'm putting this up both as a request for comments and a vibe-check, but also as a legitimate implementation that I'd like to see land (though no rush of course on that last part).
## Background
### Early days
Once upon a time, we represented trait constness in the param-env and in `TraitPredicate`. This was very difficult to implement correctly; it had bugs and was also incomplete; I don't think this was anyone's fault though, it was just the limit of experimental knowledge we had at that point.
Dealing with `~const` within predicates themselves meant dealing with constness all throughout the trait solver. This was difficult to keep track of, and afaict was not handled well with all the corners of candidate assembly.
Specifically, we had to (in various places) remap constness according to the param-env constness:
https://github.com/rust-lang/rust/blob/574b64a97f52162f965bc201e47f0af8279ca65d/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1498
This was annoying and manual and also error prone.
### Beginning of the effects desugaring
Later on, #113210 reimplemented a new desugaring for const traits via a `<const HOST: bool>` predicate. This essentially "reified" the const checking and separated it from any of the remapping or separate tracking in param-envs. For example, if I was in a const-if-const environment, but I wanted to call a trait that was non-const, this reification would turn the constness mismatch into a simple *type* mismatch of the effect parameter.
While this was a monumental step towards straightening out const trait checking in the trait system, it had its own issues, since that meant that the constness of a trait (or any item within it, like an associated type) was *early-bound*. This essentially meant that `<T as Trait>::Assoc` was *distinct* from `<T as ~const Trait>::Assoc`, which was bad.
### Associated-type bound based effects desugaring
After this, #120639 implemented a new effects desugaring. This used an associated type to more clearly represent the fact that the constness is not an input parameter of a trait, but a property that could be computed of a impl. The write-up linked in that PR explains it better than I could.
However, I feel like it really reached the limits of what can comfortably be expressed in terms of associated type and trait calculus. Also, `<const HOST: bool>` remains a synthetic const parameter, which is observable in nested items like RPITs and closures, and comes with tons of its own hacks in the astconv and middle layer.
For example, there are pieces of unintuitive code that are needed to represent semantics like elaboration, and eventually will be needed to make error reporting intuitive, and hopefully in the future assist us in implementing built-in traits (eventually we'll want something like `~const Fn` trait bounds!).
elaboration hack: https://github.com/rust-lang/rust/blob/8069f8d17a6c86a8fd881939fcce359a90c57ff2/compiler/rustc_type_ir/src/elaborate.rs#L133-L195
trait bound remapping hack for diagnostics: https://github.com/rust-lang/rust/blob/8069f8d17a6c86a8fd881939fcce359a90c57ff2/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs#L2370-L2413
I want to be clear that I don't think this is a issue of implementation quality or anything like that; I think it's simply a very clear sign that we're using types and traits in a way that they're not fundamentally supposed to be used, especially given that constness deserves to be represented as a first-class concept.
### What now?
This PR implements a new desugaring for const traits. Specifically, it introduces a `HostEffect` predicate to represent the obligation an impl is const, rather than using associated type bounds and the compat trait that exists for effects today.
### `HostEffect` predicate
A `HostEffect` clause has two parts -- the `TraitRef` we're trying to prove, and a `HostPolarity::{Maybe, Const}`.
`HostPolarity::Const` corresponds to `T: const Trait` bounds, which must *always* be proven as const, and which can be written in any context. These are lowered directly into the predicates of an item, since they're not "context-specific".
On the other hand, `HostPolarity::Maybe` corresponds to `T: ~const Trait` bounds which must only exist in a conditionally-const context like a method in a `#[const_trait]`, or a `const fn` free function. We do not lower these immediately into the predicates of an item; instead, we collect them into a new query called the **`const_conditions`**. These are the set of trait refs that we need to prove have const implementations for an item to be const.
Notably, they're represented as bare (poly) trait refs because they are meant to be paired back together with a `HostPolarity` when they're being registered in typeck (see next section).
For example, given:
```rust
const fn foo<T: ~const A + const B>() {}
```
`foo`'s const conditions would contain `T: A`, but not `T: B`. On the flip side, foo's predicates (`predicates_of`) query would contain `HostEffect(T: B, HostPolarity::Const)` but not `HostEffect(T: A, HostPolarity::Maybe)` since we don't need to prove that predicate in a non-const environment (and it's not even the right predicate to prove in an unconditionally const environment).
### Type checking const bodies
When type checking bodies in HIR, when we encounter a call expression, we additionally register the callee item's const conditions with the `HostPolarity` from the body we're typechecking (`Const` for unconditionally const things like `const`/`static` items, and `Maybe` for conditionally const things like const fns; and we don't register `HostPolarity` predicates for non-const bodies).
When type-checking a conditionally const body, we augment its param-env with `HostEffect(..., Maybe)` predicates.
### Checking that const impls are WF
We extend the logic in `compare_method_predicate_entailment` to also check the const-conditions of the impl method, to make sure that we error for:
```rust
#[const_trait] Bar {}
#[const_trait] trait Foo {
fn method<T: Bar>();
}
impl Foo for () {
fn method<T: ~const Bar>() {} // stronger assumption!
}
```
We also extend the WF check for impls to register the const conditions of the trait that is being implemented. This is to make sure we error for:
```rust
#[const_trait] trait Bar {}
#[const_trait] trait Foo<T> where T: ~const Bar {}
impl<T> const Foo<T> for () {}
//~^ `T: ~const Bar` is missing!
```
### Proving a `HostEffect` predicate
We have several ways of proving a `HostEffect` predicate:
1. Matching a `HostEffect` predicate from the param-env
2. From an impl - we do impl selection very similar to confirming a trait goal, except we filter for only const impls, and we additionally register the impl's const conditions (i.e. the impl's `~const` where clauses).
Later I expect that we will add more built-in implementations for things like `Fn`.
## What next?
After this PR, I'd like to split out the work more so it can proceed in parallel and probably amongst others that are not me.
* Register `HostEffect` goal for places in HIR typeck that correspond to call terminators, like autoderef.
* Make traits in libstd const again.
* Probably need to impl host effect preds in old solver.
* Implement built-in `HostEffect` rules for traits like `Fn`.
* Rip out const checking from MIR altogether.
## So what?
This ends up being super convenient basically everywhere in the compiler. Due to the design of the new trait solver, we end up having an almost parallel structure to the existing trait and projection predicates for assembling `HostEffect` predicates; adding new candidates and especially new built-in implementations is now basically trivial, and it's quite straightforward to understand the confirmation logic for these predicates.
Same with diagnostics reporting; since we have predicates which represent the obligation to prove an impl is const, we can simplify and make these diagnostics richer without having to write a ton of logic to intercept and rewrite the existing `Compat` trait errors.
Finally, it gives us a much more straightforward path for supporting the const effect on the old trait solver. I'm personally quite passionate about getting const trait support into the hands of users without having to wait until the new solver lands[^1], so I think after this PR lands we can begin to gauge how difficult it would be to implement constness in the old trait solver too. This PR will not do this yet.
[^1]: Though this is not a prerequisite or by any means the only justification for this PR.
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This allows impls such as `impl QueryData for OneOf<(T,)>`
to be displayed as variadic: `impl QueryData for OneOf<(T₁, T₂, …, Tₙ)>`.
See question on zulip:
<https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/Make.20.60.23.5Bdoc.28fake_variadic.29.5D.60.20more.20useful>
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Add a test confirming that `-Zbranch-protection=pac-ret` and
cross-language LTO work together.
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