Tweak "use array's length as const param" suggestion by estebank · Pull Request #163040 · rust-lang/rust

estebank · 2026-09-19T22:42:47Z

Don't suggest braces unnecessarily for numeric literals
Use verbose suggestion
Tweak messages

error[E0747]: type provided when a constant was expected
  --> $DIR/suggest_const_for_array.rs:6:15
   |
LL |     example::<[usize; 3]>();
   |               ^^^^^^^^^^ array type provided where a `usize` was expected
   |
help: you might have meant to use the array's length's value
   |
LL -     example::<[usize; 3]>();
LL +     example::<3>();
   |

rustbot · 2026-09-19T22:42:50Z

HIR ty lowering was modified

cc @fmease

rustbot · 2026-09-19T22:42:52Z

r? @adwinwhite

rustbot has assigned @adwinwhite.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 77 candidates
Random selection from 20 candidates

fmease

r=me some nitpicks & some optional suggestions

View changes since this review

fmease · 2026-09-19T22:52:13Z

+                let sugg = if let hir::ConstArgKind::Anon(hir::AnonConst { body, .. }) = len.kind
+                    && let hir::ExprKind::Lit(..) = tcx.hir_body(*body).value.kind


I'm not really up to speed with (m)GCA. Does this check fail if feature min_generic_const_args / generic_const_args is enabled since the length is no longer an AnonConst but a direct ConstArgKind::Lit?

If so, we might want to account for that, too. Of course, not high priority so feel free to ignore.

Added revisions for all three cases, as well as the additional check, but the check doesn't trigger today (yet?).

fmease · 2026-09-19T22:55:06Z

            GenericParamDefKind::Const { .. },
        ) if tcx.type_of(param.def_id).skip_binder() == tcx.types.usize => {
+            err.span_label(arg.span(), "array type provided where a `usize` was expected");
            let snippet = sess.source_map().span_to_snippet(tcx.hir_span(len.hir_id));


(preexisting) It should be possible to eliminate the use of span_to_snippet by using a multi-part suggestion, Span::{until,to,between,…} and the span of the entire HIR type above. 🤷 Feel free to ignore

I'm just concerned about spans through multiple macro scopes.

- Don't suggest braces unnecessarily for numeric literals - Use verbose suggestion - Tweak messages ``` error[E0747]: type provided when a constant was expected --> $DIR/suggest_const_for_array.rs:6:15 | LL | example::<[usize; 3]>(); | ^^^^^^^^^^ array type provided where a `usize` was expected | help: you might have meant to use the array's length's value | LL - example::<[usize; 3]>(); LL + example::<3>(); | ```

estebank · 2026-09-20T03:17:01Z

@bors r=fmease

rust-bors · 2026-09-20T03:17:04Z

📌 Commit 5f33ac3 has been approved by fmease

It is now in the queue for this repository.

…anted, r=fmease Tweak "use array's length as const param" suggestion - Don't suggest braces unnecessarily for numeric literals - Use verbose suggestion - Tweak messages ``` error[E0747]: type provided when a constant was expected --> $DIR/suggest_const_for_array.rs:6:15 | LL | example::<[usize; 3]>(); | ^^^^^^^^^^ array type provided where a `usize` was expected | help: you might have meant to use the array's length's value | LL - example::<[usize; 3]>(); LL + example::<3>(); | ```

…uwer Rollup of 15 pull requests Successful merges: - #162726 (std: fix unix socket address panic on a full sun_path) - #163016 (Don't claim that escaping value is a reference in diagnostics) - #163040 (Tweak "use array's length as const param" suggestion) - #163060 (Point to fields that introduce trait requirements) - #163066 (don't mark `f128` as reliable on AIX) - #162098 (Tweak `Infallible` docs) - #162854 (Add safety section for atomic_load/store) - #163015 (add `minicore::ffi::VaList`) - #163021 (`va_arg`: pass in `TyAndLayout`) - #163036 ([rustdoc] Correctly handle `dyn` trait methods linking for jump to def feature) - #163042 (Remove redundant output from suggestion) - #163046 (Use verbose suggestion for `const _`) - #163050 (Use verbose suggestion for similarly named label suggestion) - #163052 (Use verbose suggestion for wrong primitive type names) - #163055 (Use the full path of `bug_impl` to avoid bogus errors in rust-analyzer)

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 Sep 19, 2026

rustbot assigned adwinwhite Sep 19, 2026

estebank mentioned this pull request Sep 19, 2026

Tracking Issue for improving suggestion rendering in diagnostics #141973

Open

3 tasks

fmease approved these changes Sep 19, 2026

View reviewed changes

fmease 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 Sep 19, 2026

fmease assigned fmease and unassigned adwinwhite Sep 19, 2026

fmease changed the title ~~Tweak "use array's lenght as const param" suggestion~~ Tweak "use array's length as const param" suggestion Sep 19, 2026

estebank force-pushed the array-length-when-usize-wanted branch from 56a01b2 to f14d55b Compare September 20, 2026 00:21

estebank force-pushed the array-length-when-usize-wanted branch from f14d55b to 5f33ac3 Compare September 20, 2026 00:22

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 20, 2026

JonathanBrouwer mentioned this pull request Sep 20, 2026

Rollup of 15 pull requests #163069

Closed

JonathanBrouwer mentioned this pull request Sep 20, 2026

Rollup of 15 pull requests #163071

Open

Navigation Menu

Uh oh!

Tweak "use array's length as const param" suggestion - #163040

Tweak "use array's length as const param" suggestion#163040
estebank wants to merge 1 commit into
rust-lang:mainfrom
estebank:array-length-when-usize-wanted

estebank commented Sep 19, 2026

Uh oh!

rustbot commented Sep 19, 2026

Uh oh!

rustbot commented Sep 19, 2026

Uh oh!

fmease left a comment •

edited by rustbot

Loading

Uh oh!

Uh oh!

Uh oh!

fmease Sep 19, 2026

Uh oh!

estebank Sep 20, 2026

Uh oh!

fmease Sep 19, 2026 •

edited

Loading

Uh oh!

estebank Sep 20, 2026

Uh oh!

estebank commented Sep 20, 2026

Uh oh!

rust-bors Bot commented Sep 20, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

		let sugg = if let hir::ConstArgKind::Anon(hir::AnonConst { body, .. }) = len.kind
		&& let hir::ExprKind::Lit(..) = tcx.hir_body(*body).value.kind

Uh oh!

Conversation

estebank commented Sep 19, 2026

Uh oh!

rustbot commented Sep 19, 2026

Uh oh!

rustbot commented Sep 19, 2026

Uh oh!

fmease left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

fmease Sep 19, 2026

Choose a reason for hiding this comment

Uh oh!

estebank Sep 20, 2026

Choose a reason for hiding this comment

Uh oh!

fmease Sep 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

estebank Sep 20, 2026

Choose a reason for hiding this comment

Uh oh!

estebank commented Sep 20, 2026

Uh oh!

rust-bors Bot commented Sep 20, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

fmease left a comment •

edited by rustbot

Loading

fmease Sep 19, 2026 •

edited

Loading