Suggest usize instead of placeholder type for array length constants by lsunsi · Pull Request #160906 · rust-lang/rust

lsunsi · 2026-08-11T12:13:44Z

This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type.

I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed.

Some ideas were pulled from #160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

rustbot · 2026-08-11T12:13:49Z

r? @davidtwco

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davidtwco

LGTM, one suggestion

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davidtwco · 2026-08-19T11:34:53Z

Maybe split this into two diagnostics so that the diagnostics w/ a concrete usize suggestion needn't be "has-placeholders"

@davidtwco Got it, I tried doing just that in 520b339. Does it make sense? The one question I was left with was if the usize branch is now "machine-applicable" instead of "has-placeholder".
I think it looks better this way!

rustbot · 2026-08-19T19:55:37Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

davidtwco · 2026-09-04T13:18:02Z

Thanks, sorry for the wait on the re-review

@bors r+ rollup

rust-bors · 2026-09-04T13:18:05Z

📌 Commit 520b339 has been approved by davidtwco

It is now in the queue for this repository.

Suggest usize instead of placeholder type for array length constants Fixes rust-lang#159487 This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type. I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed. Some ideas were pulled from rust-lang#160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

Rollup of 25 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #161895 (std::sys::pal::sgx: fix mismatched alloc/free alignment) - #161900 (bootstrap: Include feature-gated items in bootstrap tool docs) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

Suggest usize instead of placeholder type for array length constants Fixes rust-lang#159487 This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type. I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed. Some ideas were pulled from rust-lang#160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

Rollup of 25 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #160745 (make closures act like MaybeDangling) - #161895 (std::sys::pal::sgx: fix mismatched alloc/free alignment) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

Suggest usize instead of placeholder type for array length constants Fixes rust-lang#159487 This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type. I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed. Some ideas were pulled from rust-lang#160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

Rollup of 27 pull requests Successful merges: - #159074 ([PAC] FnAbi, llvm.ptrauth.resign and Session API change (2/8)) - #159792 (A more readable debug map for IndexMaps) - #160745 (make closures act like MaybeDangling) - #161940 (Promote `wasm32-wasip3` to a tier 2 target) - #162030 (Prevent `--test` to be used in `rustdoc-html` testsuite) - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1)

…uwer Rollup of 25 pull requests Successful merges: - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162255 (let people change rustc-dev-guide in-tree) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1) - #162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

Suggest usize instead of placeholder type for array length constants Fixes rust-lang#159487 This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type. I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed. Some ideas were pulled from rust-lang#160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

…uwer Rollup of 25 pull requests Successful merges: - #162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - #162179 (type system const items via direct rhs) - #162255 (let people change rustc-dev-guide in-tree) - #162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - #162277 (Introduce `rustc_middle::middel::resolve`) - #162285 (box: fixup map/try_map deallocate calls) - #162286 (string: don't unwind prematurely) - #162289 (alloc: a bunch of safety comments) - #162290 (abby test DSL: AliasTyOutlivesViaEnv) - #162292 (Update `askama` version to `0.16.1`) - #160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - #160906 (Suggest usize instead of placeholder type for array length constants) - #160936 (traits: Represent live alias arguments as bitsets) - #161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - #161400 (Improve diagnostics for references to closures) - #161656 (Suggest mutable references for FnMut closure arguments) - #161711 (Add more splat fn type tests) - #161786 (Make `tcx.def_id_partial_cmp` public) - #161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - #162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - #162212 (Implement `Rng` for `Box`) - #162246 (Fix incorrect meta span) - #162266 (std: fix typo) - #162291 (Add regression test from 1.98.1) - #162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

Rollup merge of #160906 - lsunsi:push-ysqvmnlvvzwr, r=davidtwco Suggest usize instead of placeholder type for array length constants Fixes #159487 This PR makes the type suggestion for a anonymous constant used as an array length or repeat expression less generic: usize instead of placeholder type. I divided the work in several commits so I could track my own progress and test my understanding. I could squash them all into one if needed. Some ideas were pulled from #160155 , which is why the PR might look similar. That said I only used it as reference when I got stuck, because I was prioritizing my own learning instead of just quickly pushing the changes (hence, many commits).

rust-timer · 2026-09-05T20:43:14Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (e3fa2db): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -3.2%, secondary 2.2%)

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

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-6.5%	[-10.3%, -2.7%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.2%	[-10.3%, 3.5%]	3

Cycles

Results (secondary -2.2%)

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)	-2.2%	[-2.4%, -2.0%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: missing data
Artifact size: 403.29 MiB -> 403.37 MiB (0.02%)

…uwer Rollup of 25 pull requests Successful merges: - rust-lang/rust#162072 (Add new Tier-3 target: `powerpc64-sony-ps3`) - rust-lang/rust#162179 (type system const items via direct rhs) - rust-lang/rust#162255 (let people change rustc-dev-guide in-tree) - rust-lang/rust#162262 (Avoid manually instantiating some binders in error reporting with `-Znext-solver`) - rust-lang/rust#162277 (Introduce `rustc_middle::middel::resolve`) - rust-lang/rust#162285 (box: fixup map/try_map deallocate calls) - rust-lang/rust#162286 (string: don't unwind prematurely) - rust-lang/rust#162289 (alloc: a bunch of safety comments) - rust-lang/rust#162290 (abby test DSL: AliasTyOutlivesViaEnv) - rust-lang/rust#162292 (Update `askama` version to `0.16.1`) - rust-lang/rust#160509 (Remove `RegionExt`; move methods to `Region` in `rustc_type_ir`) - rust-lang/rust#160906 (Suggest usize instead of placeholder type for array length constants) - rust-lang/rust#160936 (traits: Represent live alias arguments as bitsets) - rust-lang/rust#161394 (fix `is_homogeneous_aggregate`: use unit size, not total size ) - rust-lang/rust#161400 (Improve diagnostics for references to closures) - rust-lang/rust#161656 (Suggest mutable references for FnMut closure arguments) - rust-lang/rust#161711 (Add more splat fn type tests) - rust-lang/rust#161786 (Make `tcx.def_id_partial_cmp` public) - rust-lang/rust#161953 (sanitizers: Implicitly disable mutually exclusive sanitizers) - rust-lang/rust#162155 (add suggestion for `rustc_allowed_through_unstable_modules` attribute) - rust-lang/rust#162212 (Implement `Rng` for `Box`) - rust-lang/rust#162246 (Fix incorrect meta span) - rust-lang/rust#162266 (std: fix typo) - rust-lang/rust#162291 (Add regression test from 1.98.1) - rust-lang/rust#162303 (Fix a minor spelling error in `library/std/src/sys/fs/unix.rs`)

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 11, 2026

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lsunsi commented Aug 11, 2026

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Comment thread compiler/rustc_resolve/src/diagnostics/mod.rs Outdated

lsunsi changed the title ~~Push ysqvmnlvvzwr~~ Suggest usize instead of placeholder type for array length constants Aug 11, 2026

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lsunsi force-pushed the push-ysqvmnlvvzwr branch from 0094f3f to 29a62a7 Compare August 11, 2026 18:16

davidtwco approved these changes Aug 19, 2026

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lsunsi added 6 commits August 19, 2026 16:54

add ui test for usize suggestion on array literal

0e3ef39

add AnonConstKind::ArrayLength to be used forwards

745606c

add ConstantSpecificType to pass information forward

ae53151

wire specific type into diagnostic

edcfc0d

x fmt (so tidy checks)

5f56642

Split diagnostic to avoid has-placeholder on fixed one

520b339

lsunsi force-pushed the push-ysqvmnlvvzwr branch from 29a62a7 to 520b339 Compare August 19, 2026 19:55

lsunsi requested a review from davidtwco August 20, 2026 17:58

davidtwco approved these changes Sep 4, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 4, 2026

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rust-bors Bot merged commit a108d58 into rust-lang:main Sep 5, 2026
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rustbot added this to the 1.100.0 milestone Sep 5, 2026

lsunsi deleted the push-ysqvmnlvvzwr branch September 7, 2026 11:20

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Suggest usize instead of placeholder type for array length constants - #160906

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