MGCA: Syntactically distinguish anon const const args by BoxyUwU · Pull Request #149136 · rust-lang/rust

BoxyUwU · 2025-11-20T10:44:35Z

r? oli-obk

tracking issue: #132980

This PR requires that when feature(min_generic_const_args) is enabled, anon const const args are syntactically distinguishable from other kinds of args. We use const { ... } in const argument position to denote an anon const:

#![feature(min_generic_const_args)]// no longer allowed as `1 + 1` is represented via an anon const and// there is no syntactic markertypeFoo = [();1 + 1];// allowed, `const { ... }` indicates an anon const representationtypeFoo = [();const{1 + 1}];

This restriction is only placed when mgca is enabled. There should be no effect on stable. This restriction is not enforced for unbraced literals which we continue to implicitly wrap in an anon const: tests/ui/const-generics/mgca/explicit_anon_consts_literals_hack.rs

This restriction allows us to create DefIds for anon consts only when actually required. When it is syntactically ambiguous whether a const argument is an anon const or not we are forced to conservatively create a DefId for every const argument even if it doesn't wind up needing one.

This works fine on stable but under mgca we can wind up with anon consts nested inside non-anon-const const arguments resulting in a broken DefId tree. See #148838 where an anon const arg inside of a path arg winds up with a parent of a conservatively created DefId that doesn't actually correspond to an anon const, resulting in an ICE.

With #149114 every field initialiser in a const argument would become a place where there could possibly be an anon const. This would also get worse once we support tuple constructors- now every function argument is a place where there could possibly be an anon const.

We introduce this restriction to avoid creating massive amounts of unused DefIds that make the parent tree significantly more complicated, and to avoid having to paper over this issue in things like generics_of.

Fixes#148838

It also must be syntactically clear from context whether '_ means an inference lifetime or an elided lifetime parameter. This restriction will allow us to properly resolve '_ in const arguments in mgca. This PR doesn't actually fix handle this, but we could do so trivially after this lands.

rustbot · 2025-11-20T10:44:38Z

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

rustbot · 2025-11-20T10:44:41Z

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2025-11-21T03:54:00Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

bors · 2025-11-27T20:43:49Z

☔ The latest upstream changes (presumably #149387) made this pull request unmergeable. Please resolve the merge conflicts.

oli-obk · 2025-12-10T14:35:21Z

r=me with commits cleaned up

BoxyUwU · 2025-12-10T18:36:53Z

@bors r=oli-obk rollup=never

bors · 2025-12-10T18:36:57Z

📌 Commit 0c86a41 has been approved by oli-obk

It is now in the queue for this repository.

bors · 2025-12-12T15:51:44Z

📌 Commit acc3a0e has been approved by oli-obk

It is now in the queue for this repository.

BoxyUwU · 2025-12-12T15:51:52Z

@bors r-

actually should wait for CI to pass given rebase

BoxyUwU · 2025-12-12T17:34:44Z

@bors r=oli-obk

bors · 2025-12-12T17:34:47Z

📌 Commit acc3a0e has been approved by oli-obk

It is now in the queue for this repository.

bors · 2025-12-12T21:35:35Z

⌛ Testing commit acc3a0e with merge dc47a69...

bors · 2025-12-13T00:48:15Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing dc47a69 to main...

github-actions · 2025-12-13T00:51:21Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing fa5eda1 (parent) -> dc47a69 (this PR)

Test differences

Show 56 test diffs

Stage 1

[ui] tests/ui/associated-type-bounds/const-projection-err.rs: [missing] -> pass (J1)
[ui] tests/ui/associated-type-bounds/const-projection-err.rs#gce: pass -> [missing] (J1)
[ui] tests/ui/associated-type-bounds/const-projection-err.rs#stock: pass -> [missing] (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts_literals_hack.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/multi_braced_direct_const_args.rs: [missing] -> pass (J1)
[ui] tests/ui/const-generics/mgca/unbraced_const_block_const_arg_gated.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/associated-type-bounds/const-projection-err.rs: [missing] -> pass (J0)
[ui] tests/ui/associated-type-bounds/const-projection-err.rs#gce: pass -> [missing] (J0)
[ui] tests/ui/associated-type-bounds/const-projection-err.rs#stock: pass -> [missing] (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/explicit_anon_consts_literals_hack.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/multi_braced_direct_const_args.rs: [missing] -> pass (J0)
[ui] tests/ui/const-generics/mgca/unbraced_const_block_const_arg_gated.rs: [missing] -> pass (J0)

Additionally, 42 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
test-dashboard dc47a69ed94bc88b10b7d500cceacf29b87bcbbe --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-gnu-llvm-21-1: 3238.0s -> 3753.9s (+15.9%)
aarch64-gnu-llvm-20-2: 2251.0s -> 2580.7s (+14.6%)
aarch64-gnu-llvm-20-1: 3378.5s -> 3871.9s (+14.6%)
pr-check-1: 1704.8s -> 1944.8s (+14.1%)
x86_64-gnu-gcc: 3135.0s -> 3539.2s (+12.9%)
dist-x86_64-apple: 7034.7s -> 7918.2s (+12.6%)
test-various: 6532.0s -> 7318.2s (+12.0%)
x86_64-rust-for-linux: 2870.8s -> 3208.8s (+11.8%)
dist-apple-various: 4078.6s -> 3617.3s (-11.3%)
dist-aarch64-llvm-mingw: 6546.5s -> 5988.1s (-8.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-12-13T01:30:27Z

Finished benchmarking commit (dc47a69): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary 0.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.8%	[0.8%, 0.9%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -4.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.0%	[-4.0%, -4.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 473.976s -> 475.622s (0.35%)
Artifact size: 389.30 MiB -> 389.31 MiB (0.00%)

…li-obk dont create unnecessary `DefId`s under mgca Fixesrust-lang#149977Fixesrust-lang#148838 Accidentally left this out of rust-lang#149136 even though being able to do this was a large part of the point of the PR :3 First ICE was caused by the fact that we create a defid but never lower the nodeid associated with it to a hirid which later parts of the compiler can't handle. See test for second ICE r? oli-obk

Rollup merge of #150025 - BoxyUwU:mgca_no_unused_defids, r=oli-obk dont create unnecessary `DefId`s under mgca Fixes#149977Fixes#148838 Accidentally left this out of #149136 even though being able to do this was a large part of the point of the PR :3 First ICE was caused by the fact that we create a defid but never lower the nodeid associated with it to a hirid which later parts of the compiler can't handle. See test for second ICE r? oli-obk

dont create unnecessary `DefId`s under mgca Fixesrust-lang/rust#149977Fixesrust-lang/rust#148838 Accidentally left this out of rust-lang/rust#149136 even though being able to do this was a large part of the point of the PR :3 First ICE was caused by the fact that we create a defid but never lower the nodeid associated with it to a hirid which later parts of the compiler can't handle. See test for second ICE r? oli-obk

rustbot assigned oli-obk Nov 20, 2025

BoxyUwU mentioned this pull request Nov 20, 2025
MGCA: Support struct expressions without intermediary anon consts #149114
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Comment threadcompiler/rustc_ast_lowering/src/lib.rs

BoxyUwU force-pushed the mgca_explicit_anon_consts branch 2 times, most recently from a786e30 to 09d75e3Compare November 21, 2025 01:38

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BoxyUwU force-pushed the mgca_explicit_anon_consts branch from 09d75e3 to 5c64af2Compare November 21, 2025 03:53

rustbot added the T-clippy Relevant to the Clippy team. label Nov 21, 2025

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Full Const Generics rust-lang/goals#100
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Comment threadcompiler/rustc_ast_lowering/src/lib.rs Outdated
Comment threadcompiler/rustc_ast_lowering/src/lib.rs
Comment threadcompiler/rustc_parse/src/parser/expr.rs Outdated
Comment threadsrc/tools/clippy/tests/ui/trait_duplication_in_bounds_assoc_const_eq.fixed Outdated

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 9, 2025

BoxyUwU force-pushed the mgca_explicit_anon_consts branch from 5c64af2 to 8f74277Compare December 10, 2025 07:46

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Comment threadtests/ui/const-generics/associated_const_equality/coherence.rs Outdated

BoxyUwU force-pushed the mgca_explicit_anon_consts branch from 8f74277 to 0c86a41Compare December 10, 2025 16:25

bors removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Dec 10, 2025

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github-actionsBot mentioned this pull request Dec 15, 2025
Toolchain upgrade to nightly-2026-02-09 flux-rs/flux#1345
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BoxyUwU mentioned this pull request Dec 15, 2025
dont create unnecessary DefIds under mgca #150025
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BoxyUwU mentioned this pull request Dec 28, 2025
Tracking Issue for Generic Constant Arguments MVP #132980
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29 tasks

This was referenced Jan 8, 2026
ICE: unhandled node ConstArg(ConstArg #143358
Closed
ICE: mgca: Val(ZeroSized ..) has parameters, but no args were provided in instantiate#147415
Closed
ICE: primitive read not possible for type: fn(usize) -> bool {not_one}#137656
Closed

reddevilmidzy mentioned this pull request Jan 22, 2026
ICE: cannot find M/#0 in param-env with feature(min_generic_const_args)#148822
Closed

AprilNEA mentioned this pull request Feb 9, 2026
Add regression test for nested const arg ICE under mGCA #152406
Closed

matthiaskrgr mentioned this pull request Mar 29, 2026
[ICE]: mgca: No HirId for DefId#154539
Closed

chenyukang mentioned this pull request Jun 29, 2026
Handle primitive constants in const arguments for MGCA #158590
Open

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