Tweak output on misused `const` parameter to provide more context and more accurate suggestion by estebank · Pull Request #164775 · rust-lang/rust

estebank · 2026-10-23T00:45:36Z

When missing const param in impl, provide complete suggestion:

error[E0107]: missing generics for struct `a::Foo`
  --> $DIR/impl-block-missing-const-generics.rs:6:10
   |
LL |     impl Foo {}
   |          ^^^ expected 1 generic argument
   |
note: struct defined here, with 1 generic parameter: `AHA`
  --> $DIR/impl-block-missing-const-generics.rs:4:12
   |
LL |     struct Foo<const AHA: usize>{}
   |            ^^^ ----------------
help: add missing generic argument
   |
LL |     impl<const AHA: usize> Foo<AHA> {}
   |         ++++++++++++++++++    +++++

Previously we only suggested Foo<AHA> without the impl<const AHA: usize>.

Fix #91119.

Point at const param declaration when constant is not expected type:

error: the constant `N` is not of type `usize`
  --> $DIR/const-in-impl-fn-return-type.rs:12:32
   |
LL |     fn func<const N: u32>() -> [(); N];
   |             ------------       ^^^^^^^ expected `usize`, found `u32`
   |             |
   |             found this const parameter
   |
   = note: the length of array `[(); N]` must be type `usize`

rustbot · 2026-10-23T00:45:42Z

r? @wesleywiser

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wesleywiser · 2026-10-28T22:24:09Z

-   |                             ^^^^^^^^^^^^^^^^ expected `u32`, found `u64`
+   |        ------------         ^^^^^^^^^^^^^^^^ expected `u32`, found `u64`
+   |        |
+   |        found this const parameter


Is there a better message we can give than this? I'm not sure exactly what this note is trying to tell me 🙂

Maybe?

error: the constant `N` is not of type `u32` --> $DIR/mismatched-types-with-generic-in-ace.rs:15:29 | LL | fn bar<const N: u64, T: Foo<ASSOC<N> = { N }>>() {} | ------------ ^^^^^^^^^^^^^^^^ expected `u32`, found `u64` | | | `N` is defined here

View changes since the review

``` error[E0107]: missing generics for struct `a::Foo` --> $DIR/impl-block-missing-const-generics.rs:6:10 | LL | impl Foo {} | ^^^ expected 1 generic argument | note: struct defined here, with 1 generic parameter: `AHA` --> $DIR/impl-block-missing-const-generics.rs:4:12 | LL | struct Foo<const AHA: usize>{} | ^^^ ---------------- help: add missing generic argument | LL | impl<const AHA: usize> Foo<AHA> {} | ++++++++++++++++++ +++++ ``` Previously we only suggested `Foo<AHA>` without the `impl<const AHA: usize>`.

``` error: the constant `N` is not of type `usize` --> $DIR/const-in-impl-fn-return-type.rs:12:32 | LL | fn func<const N: u32>() -> [(); N]; | ------------ ^^^^^^^ expected `usize`, found `u32` | | | found this const parameter | = note: the length of array `[(); N]` must be type `usize` ```

``` error: the constant `B` is not of type `usize` --> $DIR/check_const_arg_type_in_free_alias.rs:15:30 | LL | type ArrLen<const B: bool> = [(); B]; | ------------- ^^^^^^^ expected `usize`, found `bool` | | | found this const parameter | = note: the length of array `[(); B]` must be type `usize` ```

``` error: the constant `N` is not of type `usize` --> $DIR/bad-subst-const-kind.rs:8:26 | LL | impl<const N: u64> Q for [u8; N] { | ------------ ^^^^^^^ expected `usize`, found `u64` | | | `N` defined here | = note: the length of array `[u8; N]` must be type `usize` ```

wesleywiser · 2026-11-05T18:22:34Z

@bors r+ rollup

rust-bors · 2026-11-05T18:22:37Z

📌 Commit ff0444a has been approved by wesleywiser

It is now in the queue for this repository.

estebank · 2026-11-05T18:44:49Z

+LL | impl<T, 'a> Reborrow for PhantomData<'a, T> {}
+   |      ++                                +++


Note to self: need to follow up and suggest <'a, T> instead of <T, 'a>. Applying the suggestion should provide a appropriate suggestion already, but better to handle eagerly if possible. This bug is likely present in more than one suggestion.

View changes since the review

The fix is a single line. I'll push to this PR and take your reapproval as a given @wesleywiser :)

rust-bors · 2026-11-05T18:54:37Z

⚠️ A new commit ca9e03561c43015eae0c7b9df9a6ae2679ef509e was pushed.

This pull request was unapproved.

rustbot · 2026-11-05T18:54:37Z

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.

estebank · 2026-11-05T18:54:51Z

@bors r=wesleywiser

rust-bors · 2026-11-05T18:54:55Z

📌 Commit ca9e035 has been tentatively approved by wesleywiser

It will be put into the queue for this repository once PR CI succeeds.

…sleywiser Tweak output on misused `const` parameter to provide more context and more accurate suggestion When missing `const` param in `impl`, provide complete suggestion: ``` error[E0107]: missing generics for struct `a::Foo` --> $DIR/impl-block-missing-const-generics.rs:6:10 | LL | impl Foo {} | ^^^ expected 1 generic argument | note: struct defined here, with 1 generic parameter: `AHA` --> $DIR/impl-block-missing-const-generics.rs:4:12 | LL | struct Foo<const AHA: usize>{} | ^^^ ---------------- help: add missing generic argument | LL | impl<const AHA: usize> Foo<AHA> {} | ++++++++++++++++++ +++++ ``` Previously we only suggested `Foo<AHA>` without the `impl<const AHA: usize>`. Fix rust-lang#91119. Point at `const` param declaration when constant is not expected type: ``` error: the constant `N` is not of type `usize` --> $DIR/const-in-impl-fn-return-type.rs:12:32 | LL | fn func<const N: u32>() -> [(); N]; | ------------ ^^^^^^^ expected `usize`, found `u32` | | | found this const parameter | = note: the length of array `[(); N]` must be type `usize` ```

…uwer Rollup of 6 pull requests Successful merges: - #164775 (Tweak output on misused `const` parameter to provide more context and more accurate suggestion) - #165448 (Update `Vec::retain_unstable` docs to say that this method is generally faster than `Vec::retain_mut`) - #165503 (update Enzyme submodule) - #165505 (DFV: Reject `#[non_exhaustive]` on enum variants with default field values) - #165514 (Remove unused `WhereEqPredicate` and `ScopeId` types) - #165519 (Update books)

…sleywiser Tweak output on misused `const` parameter to provide more context and more accurate suggestion When missing `const` param in `impl`, provide complete suggestion: ``` error[E0107]: missing generics for struct `a::Foo` --> $DIR/impl-block-missing-const-generics.rs:6:10 | LL | impl Foo {} | ^^^ expected 1 generic argument | note: struct defined here, with 1 generic parameter: `AHA` --> $DIR/impl-block-missing-const-generics.rs:4:12 | LL | struct Foo<const AHA: usize>{} | ^^^ ---------------- help: add missing generic argument | LL | impl<const AHA: usize> Foo<AHA> {} | ++++++++++++++++++ +++++ ``` Previously we only suggested `Foo<AHA>` without the `impl<const AHA: usize>`. Fix rust-lang#91119. Point at `const` param declaration when constant is not expected type: ``` error: the constant `N` is not of type `usize` --> $DIR/const-in-impl-fn-return-type.rs:12:32 | LL | fn func<const N: u32>() -> [(); N]; | ------------ ^^^^^^^ expected `usize`, found `u32` | | | found this const parameter | = note: the length of array `[(); N]` must be type `usize` ```

…uwer Rollup of 19 pull requests Successful merges: - #164784 (Introduce OptimizationObjective and fix the entry shim's size attributes) - #165037 (Drop array elements in order after a partial move) - #165078 (Use the `2.38` binutils version in CI) - #159374 (Lower #[no_mangle] to #[symbol_name]/#[link_name]) - #163284 (feat(Reborrow): Allow multiple lifetimes) - #164775 (Tweak output on misused `const` parameter to provide more context and more accurate suggestion) - #165194 (name elided lifetimes in `use<...>` suggestion when `impl Trait` params are present) - #165222 (Anonymize inner binders when checking refining) - #165335 (Fallible type ops) - #165388 (Accept variadic SysV64, Win64, and Aapcs FnAbis in rustc_public) - #165448 (Update `Vec::retain_unstable` docs to say that this method is generally faster than `Vec::retain_mut`) - #165494 (fix Miri ICE on exit with () return type) - #165503 (update Enzyme submodule) - #165505 (DFV: Reject `#[non_exhaustive]` on enum variants with default field values) - #165512 (Fix warnings for the L4Re targets) - #165514 (Remove unused `WhereEqPredicate` and `ScopeId` types) - #165518 (style-guide: Remove ABI explicitness prescription for extern items) - #165519 (Update books) - #165534 (rename MIR_borrow_checking timing scope to less confusing name)

Rollup of 15 pull requests Successful merges: - #165037 (Drop array elements in order after a partial move) - #165550 (Make `Literal::cstr_value` return a `CString`) - #159374 (Lower #[no_mangle] to #[symbol_name]/#[link_name]) - #163284 (feat(Reborrow): Allow multiple lifetimes) - #164775 (Tweak output on misused `const` parameter to provide more context and more accurate suggestion) - #165194 (name elided lifetimes in `use<...>` suggestion when `impl Trait` params are present) - #165222 (Anonymize inner binders when checking refining) - #165335 (Fallible type ops) - #165388 (Accept variadic SysV64, Win64, and Aapcs FnAbis in rustc_public) - #165448 (Update `Vec::retain_unstable` docs to say that this method is generally faster than `Vec::retain_mut`) - #165494 (fix Miri ICE on exit with () return type) - #165505 (DFV: Reject `#[non_exhaustive]` on enum variants with default field values) - #165512 (Fix warnings for the L4Re targets) - #165514 (Remove unused `WhereEqPredicate` and `ScopeId` types) - #165518 (style-guide: Remove ABI explicitness prescription for extern items)

Rollup merge of #164775 - estebank:missing-const-param, r=wesleywiser Tweak output on misused `const` parameter to provide more context and more accurate suggestion When missing `const` param in `impl`, provide complete suggestion: ``` error[E0107]: missing generics for struct `a::Foo` --> $DIR/impl-block-missing-const-generics.rs:6:10 | LL | impl Foo {} | ^^^ expected 1 generic argument | note: struct defined here, with 1 generic parameter: `AHA` --> $DIR/impl-block-missing-const-generics.rs:4:12 | LL | struct Foo<const AHA: usize>{} | ^^^ ---------------- help: add missing generic argument | LL | impl<const AHA: usize> Foo<AHA> {} | ++++++++++++++++++ +++++ ``` Previously we only suggested `Foo<AHA>` without the `impl<const AHA: usize>`. Fix #91119. Point at `const` param declaration when constant is not expected type: ``` error: the constant `N` is not of type `usize` --> $DIR/const-in-impl-fn-return-type.rs:12:32 | LL | fn func<const N: u32>() -> [(); N]; | ------------ ^^^^^^^ expected `usize`, found `u32` | | | found this const parameter | = note: the length of array `[(); N]` must be type `usize` ```

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 Oct 23, 2026

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estebank force-pushed the missing-const-param branch 2 times, most recently from aa81524 to d5fa09c Compare October 23, 2026 17:40

rustbot added the WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) label Oct 23, 2026

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estebank force-pushed the missing-const-param branch 2 times, most recently from 69d8e14 to 8cb71cd Compare October 26, 2026 03:04

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wesleywiser reviewed Oct 28, 2026

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estebank force-pushed the missing-const-param branch from 8cb71cd to ff0444a Compare November 4, 2026 19:24

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estebank added 5 commits November 5, 2026 17:50

Tweak span of WF errors in type aliases

1be208e

Reword label

3fceff3

``` error: the constant `N` is not of type `usize` --> $DIR/bad-subst-const-kind.rs:8:26 | LL | impl<const N: u64> Q for [u8; N] { | ------------ ^^^^^^^ expected `usize`, found `u64` | | | `N` defined here | = note: the length of array `[u8; N]` must be type `usize` ```

estebank mentioned this pull request Nov 5, 2026

Point at array length instead of whole array type #165516

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wesleywiser approved these changes Nov 5, 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 Nov 5, 2026

estebank commented Nov 5, 2026

View reviewed changes

Fix suggestion span

b4f8df1

Fix rebase

ca9e035

estebank force-pushed the missing-const-param branch from ff0444a to ca9e035 Compare November 5, 2026 18:54

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JonathanBrouwer mentioned this pull request Nov 5, 2026

Rollup of 6 pull requests #165526

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JonathanBrouwer mentioned this pull request Nov 6, 2026

Rollup of 13 pull requests #165540

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Rollup of 19 pull requests #165548

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Zalathar mentioned this pull request Nov 7, 2026

Rollup of 15 pull requests #165579

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rust-bors Bot merged commit 29e9edf into rust-lang:main Nov 7, 2026
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