rustc_resolve: improve const generic errors by Unique-Usman · Pull Request #152913 · rust-lang/rust

Unique-Usman · 2026-02-20T14:49:03Z

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rustbot · 2026-02-20T14:49:08Z

rustbot has assigned @JonathanBrouwer.
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Unique-Usman · 2026-02-20T14:49:21Z

r? @estebank

compiler/rustc_resolve/src/late/diagnostics.rs

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

compiler/rustc_resolve/src/late/diagnostics.rs

tests/ui/const-generics/const-generic-function.stderr

tests/ui/const-generics/const-arg-in-const-arg.min.stderr

compiler/rustc_resolve/src/build_reduced_graph.rs

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

estebank · 2026-03-06T17:28:43Z

tests/ui/missing/missing-items/missing-type-parameter2.stderr

-LL + impl X<X> {}
-   |
-help: you might be missing a type parameter
+help: you might have meant to write a const parameter here


Suggested change

help: you might have meant to write a const parameter here

help: you might have meant to introduce a const parameter on the `impl`

the const parameter can be in other place apart from impl, I think we should leave it as here instead of the impl or we can just make it varies based on where it appears.

estebank · 2026-03-06T17:29:24Z

tests/ui/missing/missing-items/missing-type-parameter2.stderr

+  --> $DIR/missing-type-parameter2.rs:5:28
   |
+LL | struct X<const N: u8>();
+   |          ----------- const parameter `N` is defined here


Suggested change

| ----------- const parameter `N` is defined here

| ----------- const parameter `N` is defined on the type

Make sense.

estebank · 2026-03-06T17:29:37Z

tests/ui/missing/missing-items/missing-type-parameter2.stderr

   |
 LL | struct X<const N: u8>();
-   | ------------------------ similarly named struct `X` defined here
+   |          ----------- const parameter `N` is defined here


Suggested change

| ----------- const parameter `N` is defined here

| ----------- const parameter `N` is defined on the type

estebank · 2026-03-06T17:31:53Z

compiler/rustc_resolve/src/build_reduced_graph.rs

                        .struct_constructors
                        .insert(local_def_id, (ctor_res, ctor_vis.to_def_id(), ret_fields));
                }
+                self.r.struct_generics.insert(local_def_id, generics.clone());


Let's see if we can struct_generics store references. That should keep mem usage to a minimum.

estebank · 2026-03-06T17:32:57Z

compiler/rustc_resolve/src/late.rs

+                if let Some(const_err) = const_err {
+                    err.cancel();
+                    err = const_err;
+                }


This is currently inactive, right? const_err isn't None anywhere right now?

const_err is actually always None except in the case of where we add the error we just introduce see it here -> https://github.com/rust-lang/rust/pull/152913/changes#diff-2c81e6d8048a31a3ce43f30181aaa037e0ad0076e7aee67792ca993f28c75ac3R3440-R3441

it is to prevent the type error from showing up.

estebank · 2026-03-06T17:34:22Z

tests/ui/const-generics/early/invalid-const-arguments.stderr

 LL - impl<const N: u8> Foo for C<N, T> {}
-LL + impl<const N: u8> Foo for C<N, A> {}
-   |
-help: you might be missing a type parameter
+LL + impl<const T: u8> Foo for C<N, T> {}
   |
-LL | impl<const N: u8, T> Foo for C<N, T> {}
-   |                 +++
-
-error[E0747]: unresolved item provided when a constant was expected
-  --> $DIR/invalid-const-arguments.rs:5:16
-   |
-LL | impl Foo for A<N> {}
-   |                ^
-   |
-help: if this generic argument was intended as a const parameter, surround it with braces
-   |
-LL | impl Foo for A<{ N }> {}
-   |                +   +

 error[E0747]: type provided when a constant was expected
-  --> $DIR/invalid-const-arguments.rs:10:19
+  --> $DIR/invalid-const-arguments.rs:9:19
   |
 LL | impl<N> Foo for B<N> {}


We're going to have to account for "there's already an impl generics list" to suggest adding, instead of replacing.

Yeah, noted. make sense.

rustbot · 2026-03-06T17:35:33Z

HIR ty lowering was modified

cc @fmease

rust-log-analyzer · 2026-03-06T17:48:34Z

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

/dev/sda15      105M  6.2M   99M   6% /boot/efi
tmpfs           1.6G   12K  1.6G   1% /run/user/1001
================================================================================

Sufficient disk space available (94885672KB >= 52428800KB). Skipping cleanup.
##[group]Run echo "[CI_PR_NUMBER=$num]"
echo "[CI_PR_NUMBER=$num]"
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}

rustbot assigned JonathanBrouwer Feb 20, 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 Feb 20, 2026

rustbot assigned estebank and unassigned JonathanBrouwer Feb 20, 2026

Unique-Usman marked this pull request as draft February 20, 2026 14:49

rustbot 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 Feb 20, 2026

estebank reviewed Feb 23, 2026

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Unique-Usman force-pushed the ua/constnottype branch from 00fa325 to 98d6219 Compare February 28, 2026 21:28

Unique-Usman requested a review from estebank February 28, 2026 21:31

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 28, 2026

Unique-Usman commented Feb 28, 2026

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rustc_resolve: improve const generic errors

b162b4b

Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>

Unique-Usman force-pushed the ua/constnottype branch from 98d6219 to b162b4b Compare March 6, 2026 17:25

estebank reviewed Mar 6, 2026

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Unique-Usman requested a review from estebank March 6, 2026 17:35

Unique-Usman marked this pull request as ready for review March 6, 2026 17:35

	help: you might have meant to write a const parameter here
	help: you might have meant to introduce a const parameter on the `impl`

	\| ----------- const parameter `N` is defined here
	\| ----------- const parameter `N` is defined on the type

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Unique-Usman commented Feb 20, 2026

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rustbot commented Mar 6, 2026

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