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Provide additional context to errors involving const traits #144194

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23cecd7
Point at non-const trait impl when encountering unmet `[const]` bound
estebank Jul 18, 2025
0fc3d5f
Point at non-const trait when using them in const context
estebank Jul 19, 2025
8b89cb3
Only suggest `#[const_trait]` only on local traits
estebank Jul 19, 2025
90ff38b
Point at the enclosing const context
estebank Jul 19, 2025
fd07c3f
Account for `#![feature(const_trait_impl)]` state
estebank Jul 19, 2025
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90 changes: 77 additions & 13 deletions compiler/rustc_const_eval/src/check_consts/ops.rs
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Original file line number Diff line number Diff line change
@@ -1,9 +1,10 @@
//! Concrete error types for all operations which may be invalid in a certain const context.
use hir::{ConstContext, LangItem};
use rustc_errors::Diag;
use rustc_errors::codes::*;
use rustc_errors::{Applicability, Diag, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
Expand Down Expand Up @@ -211,6 +212,7 @@ fn build_error_for_const_call<'tcx>(

debug!(?call_kind);

let mut note = true;
let mut err = match call_kind {
CallKind::Normal { desugaring: Some((kind, self_ty)), .. } => {
macro_rules! error {
Expand Down Expand Up @@ -355,20 +357,82 @@ fn build_error_for_const_call<'tcx>(
non_or_conditionally,
})
}
_ => ccx.dcx().create_err(errors::NonConstFnCall {
span,
def_descr: ccx.tcx.def_descr(callee),
def_path_str: ccx.tcx.def_path_str_with_args(callee, args),
kind: ccx.const_kind(),
non_or_conditionally,
}),
_ => {
let def_descr = ccx.tcx.def_descr(callee);
let mut err = ccx.dcx().create_err(errors::NonConstFnCall {
span,
def_descr,
def_path_str: ccx.tcx.def_path_str_with_args(callee, args),
kind: ccx.const_kind(),
non_or_conditionally,
});
let context_span = ccx.tcx.def_span(ccx.def_id());
err.span_label(context_span, format!(
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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.

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This line is pretty long for a label too.

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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.

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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.

"calls in {}s are limited to constant functions, tuple structs and tuple variants",
ccx.const_kind(),
));
note = false;
let def_kind = ccx.tcx.def_kind(callee);
if let DefKind::AssocTy | DefKind::AssocConst | DefKind::AssocFn = def_kind {
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Callees are always AssocFns, why did you add DefKind::AssocTy | DefKind::AssocConst too?

let parent = ccx.tcx.parent(callee);
if let DefKind::Trait = ccx.tcx.def_kind(parent)
Comment on lines +376 to +378
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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.

&& !ccx.tcx.is_const_trait(parent)
{
let assoc_span = ccx.tcx.def_span(callee);
let assoc_name = ccx.tcx.item_name(callee);
let mut span: MultiSpan = ccx.tcx.def_span(parent).into();
span.push_span_label(assoc_span, format!("this {def_descr} is not const"));
let trait_descr = ccx.tcx.def_descr(parent);
let trait_span = ccx.tcx.def_span(parent);
let trait_name = ccx.tcx.item_name(parent);
span.push_span_label(trait_span, format!("this {trait_descr} is not const"));
err.span_note(
span,
format!(
"{def_descr} `{assoc_name}` is not const because {trait_descr} \
`{trait_name}` is not const",
),
);
if parent.is_local() && ccx.tcx.sess.is_nightly_build() {
if !ccx.tcx.features().const_trait_impl() {
err.help(
"add `#![feature(const_trait_impl)]` to the crate attributes to \
enable `#[const_trait]`",
);
}
let indentation = ccx
.tcx
.sess
.source_map()
.indentation_before(trait_span)
.unwrap_or_default();
err.span_suggestion_verbose(
trait_span.shrink_to_lo(),
format!("consider making trait `{trait_name}` const"),
format!("#[const_trait]\n{indentation}"),
Applicability::MachineApplicable,
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This is not MachineApplicable, since it doesn't fix the bug completely.

);
} else if !ccx.tcx.sess.is_nightly_build() {
err.help("const traits are not yet supported on stable Rust");
}
}
} else if ccx.tcx.constness(callee) != hir::Constness::Const {
let name = ccx.tcx.item_name(callee);
err.span_note(
ccx.tcx.def_span(callee),
format!("{def_descr} `{name}` is not const"),
);
}
err
}
};

err.note(format!(
"calls in {}s are limited to constant functions, \
tuple structs and tuple variants",
ccx.const_kind(),
));
if note {
err.note(format!(
"calls in {}s are limited to constant functions, tuple structs and tuple variants",
ccx.const_kind(),
));
}

err
}
Expand Down
28 changes: 28 additions & 0 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -806,6 +806,34 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
)) {
diag.downgrade_to_delayed_bug();
}
for candidate in self.find_similar_impl_candidates(trait_ref) {
let CandidateSimilarity::Exact { .. } = candidate.similarity else { continue };
let impl_did = candidate.impl_def_id;
let trait_did = candidate.trait_ref.def_id;
let impl_span = self.tcx.def_span(impl_did);
let trait_name = self.tcx.item_name(trait_did);

if self.tcx.is_const_trait(trait_did) && !self.tcx.is_const_trait_impl(impl_did) {
if let Some(impl_did) = impl_did.as_local()
&& let item = self.tcx.hir_expect_item(impl_did)
&& let hir::ItemKind::Impl(item) = item.kind
&& let Some(of_trait) = item.of_trait
{
// trait is const, impl is local and not const
diag.span_suggestion_verbose(
of_trait.path.span.shrink_to_lo(),
format!("make the `impl` of trait `{trait_name}` `const`"),
"const ".to_string(),
Applicability::MachineApplicable,
);
} else {
diag.span_note(
impl_span,
format!("trait `{trait_name}` is implemented but not `const`"),
);
}
}
}
diag
}

Expand Down
16 changes: 15 additions & 1 deletion tests/run-make/const-trait-stable-toolchain/const-super-trait-nightly-disabled.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -55,10 +55,24 @@ LL | #[const_trait] trait Bar: ~const Foo {}
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();
| ^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: method `a` is not const because trait `Foo` is not const
--> const-super-trait.rs:3:1
|
LL | trait Foo {
| ^^^^^^^^^ this trait is not const
LL | fn a(&self);
| ------------ this method is not const
= help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
help: consider making trait `Foo` const
|
LL + #[const_trait]
LL | trait Foo {
|

error: aborting due to 6 previous errors

Expand Down
15 changes: 14 additions & 1 deletion tests/run-make/const-trait-stable-toolchain/const-super-trait-nightly-enabled.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -35,10 +35,23 @@ LL | #[const_trait] trait Bar: ~const Foo {}
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();
| ^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: method `a` is not const because trait `Foo` is not const
--> const-super-trait.rs:3:1
|
LL | trait Foo {
| ^^^^^^^^^ this trait is not const
LL | fn a(&self);
| ------------ this method is not const
help: consider making trait `Foo` const
|
LL + #[const_trait]
LL | trait Foo {
|

error: aborting due to 4 previous errors

Expand Down
11 changes: 10 additions & 1 deletion tests/run-make/const-trait-stable-toolchain/const-super-trait-stable-disabled.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -53,10 +53,19 @@ note: `Bar` can't be used with `[const]` because it isn't `const`
error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
--> const-super-trait.rs:10:7
|
9 | const fn foo<T: ~const Bar>(x: &T) {
| ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
10 | x.a();
| ^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: method `a` is not const because trait `Foo` is not const
--> const-super-trait.rs:3:1
|
3 | trait Foo {
| ^^^^^^^^^ this trait is not const
4 | fn a(&self);
| ------------ this method is not const
= help: const traits are not yet supported on stable Rust

error: aborting due to 6 previous errors

Expand Down
11 changes: 10 additions & 1 deletion tests/run-make/const-trait-stable-toolchain/const-super-trait-stable-enabled.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -43,10 +43,19 @@ note: `Bar` can't be used with `[const]` because it isn't `const`
error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
--> const-super-trait.rs:10:7
|
9 | const fn foo<T: ~const Bar>(x: &T) {
| ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
10 | x.a();
| ^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: method `a` is not const because trait `Foo` is not const
--> const-super-trait.rs:3:1
|
3 | trait Foo {
| ^^^^^^^^^ this trait is not const
4 | fn a(&self);
| ------------ this method is not const
= help: const traits are not yet supported on stable Rust

error: aborting due to 5 previous errors

Expand Down
8 changes: 6 additions & 2 deletions tests/ui/asm/non-const.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,9 +2,13 @@ error[E0015]: cannot call non-const function `non_const_fn` in constants
--> $DIR/non-const.rs:10:31
|
LL | global_asm!("/* {} */", const non_const_fn(0));
| ^^^^^^^^^^^^^^^
| ^^^^^^^^^^^^^^^ calls in constants are limited to constant functions, tuple structs and tuple variants
|
= note: calls in constants are limited to constant functions, tuple structs and tuple variants
note: function `non_const_fn` is not const
--> $DIR/non-const.rs:8:1
|
LL | fn non_const_fn(x: i32) -> i32 { x }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

Expand Down
7 changes: 5 additions & 2 deletions tests/ui/borrowck/issue-64453.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,9 +2,12 @@ error[E0015]: cannot call non-const function `format` in statics
--> $DIR/issue-64453.rs:4:31
|
LL | static settings_dir: String = format!("");
| ^^^^^^^^^^^
| --------------------------- ^^^^^^^^^^^
| |
| calls in statics are limited to constant functions, tuple structs and tuple variants
|
= note: calls in statics are limited to constant functions, tuple structs and tuple variants
note: function `format` is not const
--> $SRC_DIR/alloc/src/fmt.rs:LL:COL
= note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
= note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)

Expand Down
15 changes: 11 additions & 4 deletions tests/ui/const-generics/nested-type.full.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,17 @@
error[E0015]: cannot call non-const associated function `Foo::{constant#0}::Foo::<17>::value` in constants
--> $DIR/nested-type.rs:15:5
|
LL | Foo::<17>::value()
| ^^^^^^^^^^^^^^^^^^
|
= note: calls in constants are limited to constant functions, tuple structs and tuple variants
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

error: aborting due to 1 previous error

Expand Down
15 changes: 11 additions & 4 deletions tests/ui/const-generics/nested-type.min.stderr
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Original file line number Diff line number Diff line change
@@ -1,10 +1,17 @@
error[E0015]: cannot call non-const associated function `Foo::{constant#0}::Foo::<17>::value` in constants
--> $DIR/nested-type.rs:15:5
|
LL | Foo::<17>::value()
| ^^^^^^^^^^^^^^^^^^
|
= note: calls in constants are limited to constant functions, tuple structs and tuple variants
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

error: `[u8; {
struct Foo<const N: usize>;
Expand Down
8 changes: 6 additions & 2 deletions tests/ui/consts/const-call.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,9 +2,13 @@ error[E0015]: cannot call non-const function `f` in constants
--> $DIR/const-call.rs:6:17
|
LL | let _ = [0; f(2)];
| ^^^^
| ^^^^ calls in constants are limited to constant functions, tuple structs and tuple variants
|
= note: calls in constants are limited to constant functions, tuple structs and tuple variants
note: function `f` is not const
--> $DIR/const-call.rs:1:1
|
LL | fn f(x: usize) -> usize {
| ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

Expand Down
5 changes: 4 additions & 1 deletion tests/ui/consts/const-eval/format.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -18,10 +18,13 @@ LL | println!("{:?}", 0);
error[E0015]: cannot call non-const function `_print` in constant functions
--> $DIR/format.rs:7:5
|
LL | const fn print() {
| ---------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
LL | println!("{:?}", 0);
| ^^^^^^^^^^^^^^^^^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: function `_print` is not const
--> $SRC_DIR/std/src/io/stdio.rs:LL:COL
= note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0015]: cannot call non-const formatting macro in constant functions
Expand Down
18 changes: 16 additions & 2 deletions tests/ui/consts/const-extern-fn/const-extern-fn-call-extern-fn.stderr
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Original file line number Diff line number Diff line change
@@ -1,18 +1,32 @@
error[E0015]: cannot call non-const function `regular_in_block` in constant functions
--> $DIR/const-extern-fn-call-extern-fn.rs:7:9
|
LL | const extern "C" fn bar() {
| ------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
LL | unsafe {
LL | regular_in_block();
| ^^^^^^^^^^^^^^^^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: function `regular_in_block` is not const
--> $DIR/const-extern-fn-call-extern-fn.rs:2:5
|
LL | fn regular_in_block();
| ^^^^^^^^^^^^^^^^^^^^^^

error[E0015]: cannot call non-const function `regular` in constant functions
--> $DIR/const-extern-fn-call-extern-fn.rs:16:9
|
LL | const extern "C" fn foo() {
| ------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
LL | unsafe {
LL | regular();
| ^^^^^^^^^
|
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
note: function `regular` is not const
--> $DIR/const-extern-fn-call-extern-fn.rs:12:1
|
LL | extern "C" fn regular() {}
| ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

Expand Down
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