Point at non-const trait when using them in const context

estebank · estebank · commit 78317f740840 · 2025-07-19T00:40:03.000Z
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 `&lt;() as Trait&gt;::foo` in constant functions
  --&gt; $DIR/inline-incorrect-early-bound-in-ctfe.rs:26:8
   |
LL |     ().foo();
   |        ^^^^^
   |
note: method `foo` is not const because trait `Trait` is not const
  --&gt; $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 {
   |
```
diff --git a/compiler/rustc_const_eval/src/check_consts/ops.rs b/compiler/rustc_const_eval/src/check_consts/ops.rs
@@ -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};
@@ -355,13 +356,58 @@ 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 def_kind = ccx.tcx.def_kind(callee);
+            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)
+                    && !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",
+                        ),
+                    );
+                    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,
+                    );
+                }
+            } 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!(
diff --git a/tests/ui/asm/non-const.stderr b/tests/ui/asm/non-const.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `non_const_fn` in constants
 LL | global_asm!("/* {} */", const non_const_fn(0));
    |                               ^^^^^^^^^^^^^^^
    |
+note: function `non_const_fn` is not const
+  --> $DIR/non-const.rs:8:1
+   |
+LL | fn non_const_fn(x: i32) -> i32 { x }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/borrowck/issue-64453.stderr b/tests/ui/borrowck/issue-64453.stderr
@@ -4,6 +4,8 @@ error[E0015]: cannot call non-const function `format` in statics
 LL | static settings_dir: String = format!("");
    |                               ^^^^^^^^^^^
    |
+note: function `format` is not const
+  --> $SRC_DIR/alloc/src/fmt.rs:LL:COL
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = 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)
diff --git a/tests/ui/consts/const-call.stderr b/tests/ui/consts/const-call.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `f` in constants
 LL |     let _ = [0; f(2)];
    |                 ^^^^
    |
+note: function `f` is not const
+  --> $DIR/const-call.rs:1:1
+   |
+LL | fn f(x: usize) -> usize {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/consts/const-eval/format.stderr b/tests/ui/consts/const-eval/format.stderr
@@ -21,6 +21,8 @@ error[E0015]: cannot call non-const function `_print` in constant functions
 LL |     println!("{:?}", 0);
    |     ^^^^^^^^^^^^^^^^^^^
    |
+note: function `_print` is not const
+  --> $SRC_DIR/std/src/io/stdio.rs:LL:COL
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui/consts/const-extern-fn/const-extern-fn-call-extern-fn.stderr b/tests/ui/consts/const-extern-fn/const-extern-fn-call-extern-fn.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `regular_in_block` in constant func
 LL |         regular_in_block();
    |         ^^^^^^^^^^^^^^^^^^
    |
+note: function `regular_in_block` is not const
+  --> $DIR/const-extern-fn-call-extern-fn.rs:2:5
+   |
+LL |     fn regular_in_block();
+   |     ^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error[E0015]: cannot call non-const function `regular` in constant functions
@@ -12,6 +17,11 @@ error[E0015]: cannot call non-const function `regular` in constant functions
 LL |         regular();
    |         ^^^^^^^^^
    |
+note: function `regular` is not const
+  --> $DIR/const-extern-fn-call-extern-fn.rs:12:1
+   |
+LL | extern "C" fn regular() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 2 previous errors
diff --git a/tests/ui/consts/const-fn-not-safe-for-const.stderr b/tests/ui/consts/const-fn-not-safe-for-const.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `random` in constant functions
 LL |     random()
    |     ^^^^^^^^
    |
+note: function `random` is not const
+  --> $DIR/const-fn-not-safe-for-const.rs:5:1
+   |
+LL | fn random() -> u32 {
+   | ^^^^^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/consts/control-flow/issue-46843.stderr b/tests/ui/consts/control-flow/issue-46843.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `non_const` in constants
 LL | pub const Q: i32 = match non_const() {
    |                          ^^^^^^^^^^^
    |
+note: function `non_const` is not const
+  --> $DIR/issue-46843.rs:6:1
+   |
+LL | fn non_const() -> Thing {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/consts/issue-16538.stderr b/tests/ui/consts/issue-16538.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `Y::foo` in statics
 LL | static foo: &Y::X = &*Y::foo(Y::x as *const Y::X);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
+note: function `foo` is not const
+  --> $DIR/issue-16538.rs:6:5
+   |
+LL |     pub fn foo(value: *const X) -> *const X {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
diff --git a/tests/ui/consts/issue-32829-2.stderr b/tests/ui/consts/issue-32829-2.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `invalid` in constants
 LL |         invalid();
    |         ^^^^^^^^^
    |
+note: function `invalid` is not const
+  --> $DIR/issue-32829-2.rs:68:1
+   |
+LL | fn invalid() {}
+   | ^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error[E0015]: cannot call non-const function `invalid` in statics
@@ -12,6 +17,11 @@ error[E0015]: cannot call non-const function `invalid` in statics
 LL |         invalid();
    |         ^^^^^^^^^
    |
+note: function `invalid` is not const
+  --> $DIR/issue-32829-2.rs:68:1
+   |
+LL | fn invalid() {}
+   | ^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
@@ -21,6 +31,11 @@ error[E0015]: cannot call non-const function `invalid` in statics
 LL |         invalid();
    |         ^^^^^^^^^
    |
+note: function `invalid` is not const
+  --> $DIR/issue-32829-2.rs:68:1
+   |
+LL | fn invalid() {}
+   | ^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
diff --git a/tests/ui/consts/issue-43105.stderr b/tests/ui/consts/issue-43105.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `xyz` in constants
 LL | const NUM: u8 = xyz();
    |                 ^^^^^
    |
+note: function `xyz` is not const
+  --> $DIR/issue-43105.rs:1:1
+   |
+LL | fn xyz() -> u8 { 42 }
+   | ^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/consts/mir_check_nonconst.stderr b/tests/ui/consts/mir_check_nonconst.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `bar` in statics
 LL | static foo: Foo = bar();
    |                   ^^^^^
    |
+note: function `bar` is not const
+  --> $DIR/mir_check_nonconst.rs:4:1
+   |
+LL | fn bar() -> Foo {
+   | ^^^^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
diff --git a/tests/ui/error-codes/E0015.stderr b/tests/ui/error-codes/E0015.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `create_some` in constants
 LL | const FOO: Option<u8> = create_some();
    |                         ^^^^^^^^^^^^^
    |
+note: function `create_some` is not const
+  --> $DIR/E0015.rs:1:1
+   |
+LL | fn create_some() -> Option<u8> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/explicit-tail-calls/constck.stderr b/tests/ui/explicit-tail-calls/constck.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `not_const` in constant functions
 LL |         become not_const();
    |                ^^^^^^^^^^^
    |
+note: function `not_const` is not const
+  --> $DIR/constck.rs:18:1
+   |
+LL | fn not_const() {}
+   | ^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error[E0015]: cannot call non-const function `not_const` in constant functions
@@ -12,6 +17,11 @@ error[E0015]: cannot call non-const function `not_const` in constant functions
 LL |         become yes_const(not_const());
    |                          ^^^^^^^^^^^
    |
+note: function `not_const` is not const
+  --> $DIR/constck.rs:18:1
+   |
+LL | fn not_const() {}
+   | ^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 2 previous errors
diff --git a/tests/ui/resolve/issue-39559-2.stderr b/tests/ui/resolve/issue-39559-2.stderr
@@ -4,15 +4,39 @@ error[E0015]: cannot call non-const associated function `<Dim3 as Dim>::dim` in
 LL |     let array: [usize; Dim3::dim()]
    |                        ^^^^^^^^^^^
    |
+note: associated function `dim` is not const because trait `Dim` is not const
+  --> $DIR/issue-39559-2.rs:1:1
+   |
+LL | trait Dim {
+   | ^^^^^^^^^ this trait is not const
+LL |     fn dim() -> usize;
+   |     ------------------ this associated function is not const
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
+help: consider making trait `Dim` const
+   |
+LL + #[const_trait]
+LL | trait Dim {
+   |
 
 error[E0015]: cannot call non-const associated function `<Dim3 as Dim>::dim` in constants
   --> $DIR/issue-39559-2.rs:16:15
    |
 LL |         = [0; Dim3::dim()];
    |               ^^^^^^^^^^^
    |
+note: associated function `dim` is not const because trait `Dim` is not const
+  --> $DIR/issue-39559-2.rs:1:1
+   |
+LL | trait Dim {
+   | ^^^^^^^^^ this trait is not const
+LL |     fn dim() -> usize;
+   |     ------------------ this associated function is not const
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
+help: consider making trait `Dim` const
+   |
+LL + #[const_trait]
+LL | trait Dim {
+   |
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/static/static-vec-repeat-not-constant.stderr b/tests/ui/static/static-vec-repeat-not-constant.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `foo` in statics
 LL | static a: [isize; 2] = [foo(); 2];
    |                         ^^^^^
    |
+note: function `foo` is not const
+  --> $DIR/static-vec-repeat-not-constant.rs:1:1
+   |
+LL | fn foo() -> isize { 23 }
+   | ^^^^^^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
diff --git a/tests/ui/statics/check-values-constraints.stderr b/tests/ui/statics/check-values-constraints.stderr
@@ -35,6 +35,13 @@ error[E0015]: cannot call non-const method `<str as ToString>::to_string` in sta
 LL |     field2: SafeEnum::Variant4("str".to_string()),
    |                                      ^^^^^^^^^^^
    |
+note: method `to_string` is not const because trait `ToString` is not const
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+   = note: this trait is not const
+  ::: $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+   = note: this method is not const
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
    = note: consider wrapping this expression in `std::sync::LazyLock::new(|| ...)`
 
diff --git a/tests/ui/traits/const-traits/const-check-fns-in-const-impl.stderr b/tests/ui/traits/const-traits/const-check-fns-in-const-impl.stderr
@@ -4,6 +4,11 @@ error[E0015]: cannot call non-const function `non_const` in constant functions
 LL |     fn foo() { non_const() }
    |                ^^^^^^^^^^^
    |
+note: function `non_const` is not const
+  --> $DIR/const-check-fns-in-const-impl.rs:11:1
+   |
+LL | fn non_const() {}
+   | ^^^^^^^^^^^^^^
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/traits/const-traits/inline-incorrect-early-bound-in-ctfe.stderr b/tests/ui/traits/const-traits/inline-incorrect-early-bound-in-ctfe.stderr
@@ -13,7 +13,19 @@ error[E0015]: cannot call non-const method `<() as Trait>::foo` in constant func
 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: aborting due to 2 previous errors
 
diff --git a/tests/ui/traits/const-traits/issue-79450.stderr b/tests/ui/traits/const-traits/issue-79450.stderr
@@ -4,6 +4,8 @@ error[E0015]: cannot call non-const function `_print` in constant functions
 LL |         println!("lul");
    |         ^^^^^^^^^^^^^^^
    |
+note: function `_print` is not const
+  --> $SRC_DIR/std/src/io/stdio.rs:LL:COL
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui/traits/const-traits/issue-88155.stderr b/tests/ui/traits/const-traits/issue-88155.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-2.nn.stderr b/tests/ui/traits/const-traits/super-traits-fail-2.nn.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-2.ny.stderr b/tests/ui/traits/const-traits/super-traits-fail-2.ny.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-3.nnn.stderr b/tests/ui/traits/const-traits/super-traits-fail-3.nnn.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-3.nny.stderr b/tests/ui/traits/const-traits/super-traits-fail-3.nny.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-3.ynn.stderr b/tests/ui/traits/const-traits/super-traits-fail-3.ynn.stderr
diff --git a/tests/ui/traits/const-traits/super-traits-fail-3.yny.stderr b/tests/ui/traits/const-traits/super-traits-fail-3.yny.stderr
diff --git a/tests/ui/typeck/typeck_type_placeholder_item.stderr b/tests/ui/typeck/typeck_type_placeholder_item.stderr

Original file line number	Diff line number	Diff line change
@@ -4,6 +4,8 @@ error[E0015]: cannot call non-const function `format` in statics
`4`	`4`	`LL \| static settings_dir: String = format!("");`
`5`	`5`	`\| ^^^^^^^^^^^`
`6`	`6`	`\|`
	`7`	+note: function `format` is not const
	`8`	`+ --> $SRC_DIR/alloc/src/fmt.rs:LL:COL`
`7`	`9`	`= note: calls in statics are limited to constant functions, tuple structs and tuple variants`
`8`	`10`	= note: consider wrapping this expression in `std::sync::LazyLock::new(\|\| ...)`
`9`	`11`	= note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
Original file line number	Diff line number	Diff line change
@@ -21,6 +21,8 @@ error[E0015]: cannot call non-const function `_print` in constant functions
`21`	`21`	`LL \| println!("{:?}", 0);`
`22`	`22`	`\| ^^^^^^^^^^^^^^^^^^^`
`23`	`23`	`\|`
	`24`	+note: function `_print` is not const
	`25`	`+ --> $SRC_DIR/std/src/io/stdio.rs:LL:COL`
`24`	`26`	`= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants`
`25`	`27`	= note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
`26`	`28`