Recognize potential `impl<const N: usize>` to `impl<N>` mistake by estebank · Pull Request #151077 · rust-lang/rust

estebank · 2026-01-13T19:34:11Z

When encountering code like impl<N> Bar<N> for [u8; N], suggest impl<const N: Type> Bar<N> for [u8; N] as a possibility.

error[E0423]: expected value, found type parameter `T`
  --> $DIR/issue-69654.rs:5:25
   |
LL | impl<T> Bar<T> for [u8; T] {}
   |      -                  ^ not a value
   |      |
   |      found this type parameter
   |
help: you might have meant to write a const parameter here
   |
LL | impl<const T: Type> Bar<T> for [u8; T] {}
   |      +++++  ++++++

Addresses "case 3" from #84327.

rustbot · 2026-01-13T19:34:16Z

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estebank · 2026-01-13T19:36:18Z

The logic could be made more complex to ensure that the type param isn't being used anywhere in the TypeNS, which would tell us that the suggestion would be less likely to be correct.

compiler/rustc_resolve/src/errors.rs

fmease · 2026-01-13T20:18:43Z

compiler/rustc_resolve/src/late/diagnostics.rs

+            }) else {
+                return;
+            };
+            err.subdiagnostic(errors::UnexpectedResChangeImplTyParamToConstParamSugg {


…Impl…

Doesn't this also trigger outside of impls? E.g., on fn f<N>(_: [(); N]) {}?

Maybe rename the preexisting diag struct (the one for N: Type→const N: Type) to …BoundedTyParam… & remove Impl from this one? Idk, sth. like that

fmease · 2026-01-13T20:25:14Z

@bors rollup

Thanks! r=me with nit addressed in one way or another

When encountering code like `impl<N> Bar<N> for [u8; N]`, suggest `impl<const N: Type> Bar<N> for [u8; N]` as a possibility. ``` error[E0423]: expected value, found type parameter `T` --> $DIR/issue-69654.rs:5:25 | LL | impl<T> Bar<T> for [u8; T] {} | - ^ not a value | | | found this type parameter | help: you might have meant to write a const parameter here | LL | impl<const T: /* Type */> Bar<T> for [u8; T] {} | +++++ ++++++++++++ ```

estebank · 2026-01-14T01:22:19Z

@bors r=fmease

rust-bors · 2026-01-14T01:22:23Z

📌 Commit 15d8e9e has been approved by fmease

It is now in the queue for this repository.

Recognize potential `impl<const N: usize>` to `impl<N>` mistake When encountering code like `impl<N> Bar<N> for [u8; N]`, suggest `impl<const N: Type> Bar<N> for [u8; N]` as a possibility. ``` error[E0423]: expected value, found type parameter `T` --> $DIR/issue-69654.rs:5:25 | LL | impl<T> Bar<T> for [u8; T] {} | - ^ not a value | | | found this type parameter | help: you might have meant to write a const parameter here | LL | impl<const T: Type> Bar<T> for [u8; T] {} | +++++ ++++++ ``` Addresses "case 3" from rust-lang#84327.

@ghost

…uwer Rollup of 12 pull requests Successful merges: - #149408 (refactor: remove Ord bound from BinaryHeap::new etc) - #150406 (Change some `matches!(.., .. if ..)` with let-chains) - #150723 (std: move `errno` and related functions into `sys::io`) - #150877 (resolve: Refactor away the side table `decl_parent_modules`) - #150902 (Update to_uppercase docs to avoid ß->SS example) - #150962 (Remove `FeedConstTy` and provide ty when lowering const arg) - #151034 (std: Change UEFI env vars to volatile storage) - #151036 (Better handle when trying to iterate on a `Range` of a type that isn't `Step`) - #151067 (Avoid should-fail in two ui tests and a codegen-llvm test) - #151072 (also handle ENOTTY ioctl errors when checking pidfd -> pid support) - #151077 (Recognize potential `impl<const N: usize>` to `impl<N>` mistake) - #151096 (Remove `Deref`/`DerefMut` impl for `Providers`.) Failed merges: - #150939 (resolve: Relax some asserts in glob overwriting and add tests) r? @ghost

@ghost

…uwer Rollup of 11 pull requests Successful merges: - #149408 (refactor: remove Ord bound from BinaryHeap::new etc) - #150406 (Change some `matches!(.., .. if ..)` with let-chains) - #150723 (std: move `errno` and related functions into `sys::io`) - #150877 (resolve: Refactor away the side table `decl_parent_modules`) - #150902 (Update to_uppercase docs to avoid ß->SS example) - #151034 (std: Change UEFI env vars to volatile storage) - #151036 (Better handle when trying to iterate on a `Range` of a type that isn't `Step`) - #151067 (Avoid should-fail in two ui tests and a codegen-llvm test) - #151072 (also handle ENOTTY ioctl errors when checking pidfd -> pid support) - #151077 (Recognize potential `impl<const N: usize>` to `impl<N>` mistake) - #151096 (Remove `Deref`/`DerefMut` impl for `Providers`.) Failed merges: - #150939 (resolve: Relax some asserts in glob overwriting and add tests) r? @ghost

Rollup merge of #151077 - issue-84327, r=fmease Recognize potential `impl<const N: usize>` to `impl<N>` mistake When encountering code like `impl<N> Bar<N> for [u8; N]`, suggest `impl<const N: Type> Bar<N> for [u8; N]` as a possibility. ``` error[E0423]: expected value, found type parameter `T` --> $DIR/issue-69654.rs:5:25 | LL | impl<T> Bar<T> for [u8; T] {} | - ^ not a value | | | found this type parameter | help: you might have meant to write a const parameter here | LL | impl<const T: Type> Bar<T> for [u8; T] {} | +++++ ++++++ ``` Addresses "case 3" from #84327.

@fmease

Recover parse gracefully from `<const N>` When a const param doesn't have a `: Type`, recover the parser state and provide a structured suggestion. This not only provides guidance on what was missing, but it also makes subsuequent errors to be emitted that would otherwise be silenced. ``` error: expected `:`, found `>` --> $DIR/incorrect-const-param.rs:26:16 | LL | impl<T, const N> From<[T; N]> for VecWrapper<T> | ^ expected `:` | help: you might have meant to write the type of the const parameter here | LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T> | ++++++++++++ ``` r? @fmease Follow up to rust-lang#151077. Fix rust-lang#84327.

@fmease

Recover parse gracefully from `<const N>` When a const param doesn't have a `: Type`, recover the parser state and provide a structured suggestion. This not only provides guidance on what was missing, but it also makes subsuequent errors to be emitted that would otherwise be silenced. ``` error: expected `:`, found `>` --> $DIR/incorrect-const-param.rs:26:16 | LL | impl<T, const N> From<[T; N]> for VecWrapper<T> | ^ expected `:` | help: you might have meant to write the type of the const parameter here | LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T> | ++++++++++++ ``` r? @fmease Follow up to rust-lang#151077. Fix rust-lang#84327.

@fmease

Rollup merge of #151099 - issue-84327-2, r=fmease Recover parse gracefully from `<const N>` When a const param doesn't have a `: Type`, recover the parser state and provide a structured suggestion. This not only provides guidance on what was missing, but it also makes subsuequent errors to be emitted that would otherwise be silenced. ``` error: expected `:`, found `>` --> $DIR/incorrect-const-param.rs:26:16 | LL | impl<T, const N> From<[T; N]> for VecWrapper<T> | ^ expected `:` | help: you might have meant to write the type of the const parameter here | LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T> | ++++++++++++ ``` r? @fmease Follow up to #151077. Fix #84327.

@ghost

…uwer Rollup of 11 pull requests Successful merges: - rust-lang/rust#149408 (refactor: remove Ord bound from BinaryHeap::new etc) - rust-lang/rust#150406 (Change some `matches!(.., .. if ..)` with let-chains) - rust-lang/rust#150723 (std: move `errno` and related functions into `sys::io`) - rust-lang/rust#150877 (resolve: Refactor away the side table `decl_parent_modules`) - rust-lang/rust#150902 (Update to_uppercase docs to avoid ß->SS example) - rust-lang/rust#151034 (std: Change UEFI env vars to volatile storage) - rust-lang/rust#151036 (Better handle when trying to iterate on a `Range` of a type that isn't `Step`) - rust-lang/rust#151067 (Avoid should-fail in two ui tests and a codegen-llvm test) - rust-lang/rust#151072 (also handle ENOTTY ioctl errors when checking pidfd -> pid support) - rust-lang/rust#151077 (Recognize potential `impl<const N: usize>` to `impl<N>` mistake) - rust-lang/rust#151096 (Remove `Deref`/`DerefMut` impl for `Providers`.) Failed merges: - rust-lang/rust#150939 (resolve: Relax some asserts in glob overwriting and add tests) r? @ghost

@fmease

Recover parse gracefully from `<const N>` When a const param doesn't have a `: Type`, recover the parser state and provide a structured suggestion. This not only provides guidance on what was missing, but it also makes subsuequent errors to be emitted that would otherwise be silenced. ``` error: expected `:`, found `>` --> $DIR/incorrect-const-param.rs:26:16 | LL | impl<T, const N> From<[T; N]> for VecWrapper<T> | ^ expected `:` | help: you might have meant to write the type of the const parameter here | LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T> | ++++++++++++ ``` r? @fmease Follow up to rust-lang/rust#151077. Fix rust-lang/rust#84327.

rustbot assigned petrochenkov Jan 13, 2026

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 Jan 13, 2026

estebank mentioned this pull request Jan 13, 2026

Provide const generics hints when a token is forgotten #84327

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fmease reviewed Jan 13, 2026

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estebank force-pushed the issue-84327 branch from 665d910 to b86e945 Compare January 13, 2026 19:49

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fmease assigned fmease and unassigned petrochenkov Jan 13, 2026

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 Jan 13, 2026

estebank force-pushed the issue-84327 branch from b86e945 to 15d8e9e Compare January 13, 2026 23:47

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 Jan 14, 2026

estebank mentioned this pull request Jan 14, 2026

Recover parse gracefully from <const N> #151099

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JonathanBrouwer mentioned this pull request Jan 14, 2026

Rollup of 12 pull requests #151104

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Rollup of 11 pull requests #151107

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rust-bors bot merged commit 18c62b5 into rust-lang:main Jan 14, 2026
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Recognize potential `impl<const N: usize>` to `impl<N>` mistake#151077

Recognize potential `impl<const N: usize>` to `impl<N>` mistake#151077
rust-bors[bot] merged 1 commit intorust-lang:mainfrom
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