Support function args const generics by bit-aloo · Pull Request #162727 · rust-lang/rust

bit-aloo · 2026-09-13T14:27:49Z

This PR adds initial support for argument-position const generics behind the function_arg_const_generics feature gate.

It allows const generics to be declared directly in function arguments. The PR adds the basic plumbing across AST, HIR, and ty, parses these parameters into the function's generics, lowers call arguments into consts. For now, we lower literals, negated literals, const parameters, const items, unit variants, and etc, This works for functions, methods and a couple of more cases.

r? @BoxyUwU
cc: @khyperia

rustbot · 2026-09-24T17:29:43Z

Requested reviewer is already assigned to this pull request.

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rustbot · 2026-09-24T17:29:57Z

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

HIR ty lowering was modified

cc @fmease

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

ytmimi · 2026-09-24T17:39:30Z

            ref ty,
            span,
            default,
+            arg_pos: _,


What is arg_pos, and does rustfmt need to handle formatting it in some way?

Probably good to add a #![feature(function_arg_const_generics] test case to rustfmt to make sure things are getting formatted as expected.

View changes since the review

khyperia

I'm feeling a bit overwhelmed and struggling to review this, and haven't gotten through the parser changes yet. Still though, wanted to comment for what I've seen so far. I think I'd appreciate talking over how to deal with lowering here in zulip or something, I haven't fully thought through the theory space. If you have any direction/resources/etc. on lowering (e.g. whether we fundamentally must lower from hir instead of ast, what potential issues there are from doing so), I'd love to see.

View changes since this review

rustbot · 2026-10-07T10:41:21Z

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

cc @ZuseZ4

khyperia

rawr, nice! looks good to me :3 just a couple nits!

... I am realizing now that you poking in Zulip maybe didn't imply you wanted a rereview but I only realized this just now after writing all this, so uuuh, get reviewed I guess

View changes since this review

…enerics, r=<try> Support function args const generics

khyperia

nice, looks good to me, thanks so much! ❤️ r=me if perf isn't red

View changes since this review

rust-bors · 2026-10-09T12:49:04Z

☀️ Try build successful (CI)
Build commit: 93791e6 (93791e636c2db4b72084ef8121f766d75d9a4971)
Base parent: 76c9095 (76c90957b7e422c4b9c45192b0197214d7de5a54)

rust-timer · 2026-10-09T15:11:40Z

Finished benchmarking commit (93791e6): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.1%, 0.2%]	10
Regressions ❌ (secondary)	0.3%	[0.1%, 0.9%]	22
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.1%]	5
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	40
All ❌✅ (primary)	0.0%	[-0.5%, 0.2%]	15

Max RSS (memory usage)

Results (primary 1.4%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	2.3%	[1.6%, 3.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Cycles

Results (primary -1.7%, secondary 12.5%)

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)	12.5%	[12.5%, 12.5%]	1
Improvements ✅ (primary)	-1.7%	[-2.2%, -1.2%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.7%	[-2.2%, -1.2%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 490.84s -> 486.205s (-0.94%)
Artifact size: 406.48 MiB -> 406.67 MiB (0.05%)

fmease · 2026-10-09T16:41:27Z

+            let mut is_const = false;
+            if fn_parse_mode.allow_const
+                && this.check_keyword(exp!(Const))
+                && this.look_ahead(1, |token| token.is_ident())


This lookahead is insufficient.

It makes Rust 2015 snippets like trait P { fn f(const dyn Trait); }, trait P { fn f(const impl Trait); } or trait P { fn f(const fn()); } syntactically legal modulo feature gating which is hardly intentional.

Moreover it alters the meaning of unstable Rust 2015 snippet trait P { fn f(const Fn()); }. On nightly, it means trait P { fn f(_: dyn const Fn()); } essentially but on your branch it now basically means trait P { fn f(_: dyn Fn()); } dropping the const trait bound modifier.

View changes since the review

Damn! Thanks for mentioning this. I guess explicitly checking that the first token is const, followed by a non-reserved identifier and then a colon, should do the trick, unless I'm missing something. Added this: 624858f

Make arg_pos to Option<u16> instead of Option<u32> And add a has_arg_pos_const in generics, so we can have shortcircuit

bit-aloo · 2026-10-10T15:29:29Z

Lets try and check perf again:

@bors try @rust-timer queue

…enerics, r=<try> Support function args const generics

rust-bors · 2026-10-10T16:28:34Z

☀️ Try build successful (CI)
Build commit: c48133e (c48133e73987fcf93fccd047b86595883c001ac0)
Base parent: 5ec56e3 (5ec56e3902656f2460259fc3ac0f6e2255f94417)

rust-timer · 2026-10-10T17:59:10Z

Finished benchmarking commit (c48133e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	3
Regressions ❌ (secondary)	0.3%	[0.1%, 0.4%]	10
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	39
All ❌✅ (primary)	-0.1%	[-0.6%, 0.1%]	8

Max RSS (memory usage)

Results (primary -1.1%, secondary -2.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-2.1%	[-2.7%, -1.6%]	2
Improvements ✅ (secondary)	-8.7%	[-8.7%, -8.7%]	1
All ❌✅ (primary)	-1.1%	[-2.7%, 1.1%]	3

Cycles

Results (primary 2.4%, secondary 1.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.1%, 2.6%]	2
Regressions ❌ (secondary)	3.3%	[2.1%, 4.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	2.4%	[2.1%, 2.6%]	2

Binary size

Results (primary -0.0%, secondary -0.0%)

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)	-0.0%	[-0.1%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	2
All ❌✅ (primary)	-0.0%	[-0.1%, -0.0%]	4

Bootstrap: 487.044s -> 489.815s (0.57%)
Artifact size: 406.45 MiB -> 406.75 MiB (0.07%)

bit-aloo · 2026-10-10T19:56:31Z

@bors try @rust-timer queue

…enerics, r=<try> Support function args const generics

rust-bors · 2026-10-10T21:00:30Z

☀️ Try build successful (CI)
Build commit: 5415718 (5415718efb0ab088d913bf332f4581c437f1cd84)
Base parent: 6e4cba3 (6e4cba3380a6f9e1e4d7672e11e40372bf04e873)

rust-timer · 2026-10-10T22:27:40Z

Finished benchmarking commit (5415718): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.3%	[0.1%, 0.3%]	8
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	6
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	39
All ❌✅ (primary)	-0.2%	[-0.6%, 0.1%]	7

Max RSS (memory usage)

Results (primary 1.3%, secondary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.9%	[0.6%, 2.8%]	8
Regressions ❌ (secondary)	3.8%	[0.9%, 9.8%]	8
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-2.8%	[-3.2%, -2.2%]	3
All ❌✅ (primary)	1.3%	[-2.9%, 2.8%]	9

Cycles

Results (secondary 4.4%)

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)	7.5%	[2.8%, 11.5%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-3.8%, -2.8%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	123
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	105
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	123

Bootstrap: 488.311s -> 489.736s (0.29%)
Artifact size: 407.22 MiB -> 406.75 MiB (-0.12%)

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BoxyUwU self-assigned this Sep 13, 2026

fmease reviewed Sep 13, 2026

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Comment thread compiler/rustc_parse/src/parser/function.rs Outdated

Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs Outdated

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rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch from 069cb06 to 0816e30 Compare September 14, 2026 10:55

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bit-aloo mentioned this pull request Sep 24, 2026

Tracking Issue for function_arg_const_generics #163261

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rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch from 0816e30 to 74ba4f6 Compare September 24, 2026 13:50

rustbot added the T-clippy Relevant to the Clippy team. label Sep 24, 2026

bit-aloo marked this pull request as ready for review September 24, 2026 17:29

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BoxyUwU assigned khyperia Sep 28, 2026

khyperia self-requested a review September 28, 2026 15:11

BoxyUwU mentioned this pull request Sep 28, 2026

Full Const Generics rust-lang/goals#100

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khyperia reviewed Oct 2, 2026

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Comment thread compiler/rustc_ast/src/ast.rs Outdated

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Comment thread compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch from 74ba4f6 to e0fc639 Compare October 7, 2026 10:41

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khyperia 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 Oct 8, 2026

rust-cloud-vms Bot force-pushed the 2026-09-06-function-argument-const-generics branch 2 times, most recently from 9d3480a to 08a6a30 Compare October 9, 2026 09:08

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rust-bors Bot pushed a commit that referenced this pull request Oct 9, 2026

Auto merge of #162727 - bit-aloo:2026-09-06-function-argument-const-g…

93791e6

…enerics, r=<try> Support function args const generics

khyperia approved these changes Oct 9, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Oct 9, 2026

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bit-aloo added 2 commits October 10, 2026 13:38

Tighten const argument lookahead to preserve Rust 2015 syntax

624858f

Lets not allocate in lower_fn_decl, and have iterator from inputs

153aa1a

Make arg_pos to Option<u16> instead of Option<u32> And add a has_arg_pos_const in generics, so we can have shortcircuit

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rust-bors Bot pushed a commit that referenced this pull request Oct 10, 2026

Auto merge of #162727 - bit-aloo:2026-09-06-function-argument-const-g…

c48133e

…enerics, r=<try> Support function args const generics

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 10, 2026

use is-keyword instead of check keyword

66b0ea0

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rust-bors Bot pushed a commit that referenced this pull request Oct 10, 2026

Auto merge of #162727 - bit-aloo:2026-09-06-function-argument-const-g…

5415718

…enerics, r=<try> Support function args const generics

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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