Provide additional context to errors involving const traits #144194

estebank · 2025-07-19T21:46:27Z

When encountering an unmet Ty: [const] Trait bound, if Trait is #[const_trait] and there's an impl Trait for Ty point at it. If local, suggest impl const Trait for Ty, otherwise just point at it.

error[E0277]: the trait bound `NonConstAdd: [const] Add` is not satisfied
  --> $DIR/assoc-type.rs:37:16
   |
LL |     type Bar = NonConstAdd;
   |                ^^^^^^^^^^^
   |
note: required by a bound in `Foo::Bar`
  --> $DIR/assoc-type.rs:33:15
   |
LL |     type Bar: [const] Add;
   |               ^^^^^^^^^^^ required by this bound in `Foo::Bar`
help: make the `impl` of trait `Add` `const`
   |
LL | impl const Add for NonConstAdd {
   |      +++++

error[E0277]: the trait bound `T: [const] PartialEq` is not satisfied
    --> tests/ui/traits/const-traits/call-generic-method-fail.rs:5:5
     |
5    |     *t == *t
     |     ^^^^^^^^
     |
note: trait `PartialEq` is implemented but not `const`
    --> /home/gh-estebank/rust/library/core/src/ptr/const_ptr.rs:1590:1
     |
1590 | impl<T: PointeeSized> PartialEq for *const T {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: trait `PartialEq` is implemented but not `const`
    --> /home/gh-estebank/rust/library/core/src/ptr/mut_ptr.rs:2011:1
     |
2011 | impl<T: PointeeSized> PartialEq for *mut T {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Point at trait and associated item when that associated item is used in a const context. Suggest making the trait #[const_trait]. Point at the const context where the const expression was found.

error[E0015]: cannot call non-const method `<() as Trait>::foo` in constant functions
  --> $DIR/inline-incorrect-early-bound-in-ctfe.rs:26:8
   |
LL | const fn foo() {
   | -------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
LL |     ().foo();
   |        ^^^^^
   |
note: method `foo` is not const because trait `Trait` is not const
  --> $DIR/inline-incorrect-early-bound-in-ctfe.rs:13:1
   |
LL | trait Trait {
   | ^^^^^^^^^^^ this trait is not const
LL |     fn foo(self);
   |     ------------- this method is not const
help: consider making trait `Trait` const
   |
LL + #[const_trait]
LL | trait Trait {
   |

rustbot · 2025-07-19T21:46:32Z

r? @oli-obk

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rustbot · 2025-07-19T21:46:34Z

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

This PR modifies run-make tests.

cc @jieyouxu

When encountering an unmet `Ty: [const] Trait` bound, if `Trait` is `#[const_trait]` and there's an `impl Trait for Ty` point at it. If local, suggest `impl const Trait for Ty`, otherwise just point at it. ``` error[E0277]: the trait bound `NonConstAdd: [const] Add` is not satisfied --> $DIR/assoc-type.rs:37:16 | LL | type Bar = NonConstAdd; | ^^^^^^^^^^^ | note: required by a bound in `Foo::Bar` --> $DIR/assoc-type.rs:33:15 | LL | type Bar: [const] Add; | ^^^^^^^^^^^ required by this bound in `Foo::Bar` help: make the `impl` of trait `Add` `const` | LL | impl const Add for NonConstAdd { | +++++ ``` ``` error[E0277]: the trait bound `T: [const] PartialEq` is not satisfied --> tests/ui/traits/const-traits/call-generic-method-fail.rs:5:5 | 5 | *t == *t | ^^^^^^^^ | note: trait `PartialEq` is implemented but not `const` --> /home/gh-estebank/rust/library/core/src/ptr/const_ptr.rs:1590:1 | 1590 | impl<T: PointeeSized> PartialEq for *const T { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ note: trait `PartialEq` is implemented but not `const` --> /home/gh-estebank/rust/library/core/src/ptr/mut_ptr.rs:2011:1 | 2011 | impl<T: PointeeSized> PartialEq for *mut T { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ```

Point at trait and associated item when that associated item is used in a const context. Suggest making the trait `#[const_trait]`. ``` error[E0015]: cannot call non-const method `<() as Trait>::foo` in constant functions --> $DIR/inline-incorrect-early-bound-in-ctfe.rs:26:8 | LL | ().foo(); | ^^^^^ | note: method `foo` is not const because trait `Trait` is not const --> $DIR/inline-incorrect-early-bound-in-ctfe.rs:13:1 | LL | trait Trait { | ^^^^^^^^^^^ this trait is not const LL | fn foo(self); | ------------- this method is not const = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants help: consider making trait `Trait` const | LL + #[const_trait] LL | trait Trait { | ```

``` error[E0015]: cannot call non-const associated function `Foo::{constant#0}::Foo::<17>::value` in constants --> $DIR/nested-type.rs:15:5 | LL | struct Foo<const N: [u8; { | __________________________- LL | | struct Foo<const N: usize>; LL | | LL | | impl<const N: usize> Foo<N> { ... | LL | | Foo::<17>::value() | | ^^^^^^^^^^^^^^^^^^ LL | | LL | | }]>; | |_- calls in constants are limited to constant functions, tuple structs and tuple variants ```

oli-obk · 2025-07-20T05:39:14Z

tests/ui/traits/const-traits/call-generic-method-fail.stderr

+note: trait `PartialEq` is implemented but not `const`
+  --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+note: trait `PartialEq` is implemented but not `const`
+  --> $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL


Why is it pointing at the raw ptr impls?

I think that they are incorrectly detected as CandidateSimilarity::Exact, which is incorrect. I can skip them explicitly on this PR by adding some other check, or look into it at another time.

compiler-errors · 2025-07-20T16:50:14Z

compiler/rustc_const_eval/src/check_consts/ops.rs

+                            trait_span.shrink_to_lo(),
+                            format!("consider making trait `{trait_name}` const"),
+                            format!("#[const_trait]\n{indentation}"),
+                            Applicability::MachineApplicable,


This is not MachineApplicable, since it doesn't fix the bug completely.

compiler-errors · 2025-07-20T16:51:02Z

compiler/rustc_const_eval/src/check_consts/ops.rs

+            ));
+            note = false;
+            let def_kind = ccx.tcx.def_kind(callee);
+            if let DefKind::AssocTy | DefKind::AssocConst | DefKind::AssocFn = def_kind {


Callees are always AssocFns, why did you add DefKind::AssocTy | DefKind::AssocConst too?

compiler-errors · 2025-07-20T16:51:35Z

compiler/rustc_const_eval/src/check_consts/ops.rs

+            if let DefKind::AssocTy | DefKind::AssocConst | DefKind::AssocFn = def_kind {
+                let parent = ccx.tcx.parent(callee);
+                if let DefKind::Trait = ccx.tcx.def_kind(parent)


This whole check could be simplified if you looked at opt_associated_item and checked both the AssocKind and the AssocItemContainer, rather than using manual parenting logic.

compiler-errors · 2025-07-20T16:53:30Z

compiler/rustc_const_eval/src/check_consts/ops.rs

+                non_or_conditionally,
+            });
+            let context_span = ccx.tcx.def_span(ccx.def_id());
+            err.span_label(context_span, format!(


Why did you change this to a label only for this case, and keep it as a note for the rest? Having special logic changing it between a note and a label seems too special cased. I'd prefer if you reverted it.

This line is pretty long for a label too.

I wanted to point at the constant context in the output in some way, as it isn't always trivially obvious why a constant function was expected. As we already had a note talking about the const context I moved that same message over from the note to the label so we wouldn't need both.

It was

error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions --> const-super-trait.rs:10:7 | LL | const fn foo<T: ~const Bar>(x: &T) { | ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants LL | x.a(); | ^^^

vs

error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions --> const-super-trait.rs:10:7 | LL | x.a(); | ^^^ | = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

We could instead have

error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions --> const-super-trait.rs:10:7 | LL | const fn foo<T: ~const Bar>(x: &T) { | ---------------------------------- required by this constant function LL | x.a(); | ^^^ | = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

I was just trying to keep the verbosity increase to a minimum.

I wanted to point at the constant context in the output in some way, as it isn't always trivially obvious why a constant function was expected.

I don't think the users need this to be a label. There's a tradeoff here with verbosity and I think this both complicates the implementation by having that note boolean and also splits the presentation of the diagnostic.

rustbot assigned oli-obk Jul 19, 2025

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs 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 Jul 19, 2025

estebank added 5 commits July 20, 2025 02:29

Only suggest #[const_trait] only on local traits

8b89cb3

Account for #![feature(const_trait_impl)] state

fd07c3f

estebank force-pushed the const-traits branch from 7b5d122 to fd07c3f Compare July 20, 2025 02:29

oli-obk reviewed Jul 20, 2025

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compiler-errors requested changes Jul 20, 2025

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Provide additional context to errors involving const traits #144194

Provide additional context to errors involving const traits #144194

estebank commented Jul 19, 2025

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Provide additional context to errors involving const traits #144194

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