const_eval: allow function pointer signatures containing &mut T in const contexts

[EvanMerlock]

potentially fixes #114994

We utilize a TypeVisitor here in order to more easily handle control flow.

• In the event the typekind the Visitor sees is a function pointer, we skip over it
• However, otherwise we do one of two things:
  • If we find a mutable reference, check it, then continue visiting types
  • If we find any other type, continue visiting types

This means we will check if the function pointer itself is mutable, but not if any of the types within are.

[rustbot]

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[cjgillot]

cc @oli-obk as it touches const-fn checks.

[oli-obk]

Nominating for T-lang FCP as this enables more code to compile. Specifically this allows mutable references in const fn, as long as they are part of a function pointer signature. We already support function pointers on stable (but not calling them), and we forbid all mutable references. This check was overzealous and also forbade function pointers that had mutable references as arguments, even though there is no risk associated with that at all, as function pointers can't be used at compile-time anyway (except to pass them around).

[tmandry]

@rfcbot fcp merge

[rfcbot]

Team member @tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[nikomatsakis]

@oli-obk I'm fine with this PR, but it'd be nice to have examples of other places where &mut is disallowed (and why) so that we can see that those cases don't apply here.

[nikomatsakis]

@rfcbot reviewed

[compiler-errors]

@nikomatsakis: See #57349 for the remaining blockers for &mut in const, there seems to be some outstanding issue with statics (de)duplication.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[RalfJung]

Cc @rust-lang/wg-const-eval

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[oli-obk]

@bors r+

[bors]

📌 Commit d975ae5 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit d975ae5 with merge 2104b0d...

[rust-log-analyzer]

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

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

tests\ui-toml\undocumented_unsafe_blocks\undocumented_unsafe_blocks.rs (default) ... ok
tests\ui-toml\undocumented_unsafe_blocks\undocumented_unsafe_blocks.rs (disabled) ... ok

tests\ui-toml\doc_valid_idents_append\doc_markdown.rs FAILED:
command: "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\clippy-driver.exe" "--error-format=json" "--emit=metadata" "-Aunused" "-Ainternal_features" "-Zui-testing" "-Dwarnings" "-Ldependency=C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps" "--extern=clippy_lints=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libclippy_lints-5e8d785e054edb28.rlib" "--extern=clippy_utils=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libclippy_utils-0cf228b0c1780fd5.rlib" "--extern=futures=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libfutures-a074388a1bf2e009.rlib" "--extern=if_chain=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libif_chain-ba0253edeba7a8f0.rlib" "--extern=itertools=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libitertools-6894b4f3e8c5523f.rlib" "--extern=parking_lot=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libparking_lot-f87e1900e520ff9b.rlib" "--extern=quote=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libquote-3ebce4a1d5d43cf1.rlib" "--extern=regex=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libregex-3e46ad2dfdd0c657.rlib" "--extern=serde=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libserde-12500b7f802b2beb.rlib" "--extern=serde_derive=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\release\\deps\\serde_derive-67ed154847593e2b.dll" "--extern=syn=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libsyn-bc3ce53f417d339c.rlib" "--extern=tokio=\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\x86_64-pc-windows-msvc\\release\\deps\\libtokio-159ee9fbda356223.rlib" "-Ldependency=C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\release\\deps" "--out-dir" "\\\\?\\C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\stage2-tools\\ui_test" "tests\\ui-toml\\doc_valid_idents_append\\doc_markdown.fixed" "--edition" "2021" "--crate-name" "doc_markdown"
called `Result::unwrap()` on an `Err` value: tests failed

Location:
    C:\Users\runneradmin\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ui_test-0.20.0\src\lib.rs:362:13
    C:\Users\runneradmin\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ui_test-0.20.0\src\lib.rs:362:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: test failed, to rerun pass `--test compile-test`

rustfix failed with exit code: 1

full stderr:
error: failed to write \\?\C:\a\rust\rust\build\x86_64-pc-windows-msvc\stage2-tools\ui_test\libdoc_markdown.rmeta: Access is denied. (os error 5)
error: aborting due to previous error


full stdout:

[bors]

💔 Test failed - checks-actions

[oli-obk]

@bors retry

[bors]

⌛ Testing commit d975ae5 with merge 139f63a...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 139f63a to master...

[rust-timer]

Finished benchmarking commit (139f63a): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	1.6%	[1.3%, 2.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 627.527s -> 628.233s (0.11%)
Artifact size: 271.26 MiB -> 271.32 MiB (0.03%)