give a hint which feature is missing

Author: RalfJung

Diff for: compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -89,12 +89,15 @@ impl<'tcx> NonConstOp<'tcx> for ConditionallyConstCall<'tcx> {
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::ConditionallyConstCall {
-            span,
-            def_path_str: ccx.tcx.def_path_str_with_args(self.callee, self.args),
-            def_descr: ccx.tcx.def_descr(self.callee),
-            kind: ccx.const_kind(),
-        })
+        ccx.tcx.sess.create_feature_err(
+            errors::ConditionallyConstCall {
+                span,
+                def_path_str: ccx.tcx.def_path_str_with_args(self.callee, self.args),
+                def_descr: ccx.tcx.def_descr(self.callee),
+                kind: ccx.const_kind(),
+            },
+            sym::const_trait_impl,
+        )
     }
 }
 
@@ -417,15 +420,8 @@ impl<'tcx> NonConstOp<'tcx> for Coroutine {
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         let msg = format!("{:#}s are not allowed in {}s", self.0, ccx.const_kind());
-        if let hir::CoroutineKind::Desugared(
-            hir::CoroutineDesugaring::Async,
-            hir::CoroutineSource::Block,
-        ) = self.0
-        {
-            ccx.tcx.sess.create_feature_err(
-                errors::UnallowedOpInConstContext { span, msg },
-                sym::const_async_blocks,
-            )
+        if let Status::Unstable { gate, .. } = self.status_in_item(ccx) {
+            ccx.tcx.sess.create_feature_err(errors::UnallowedOpInConstContext { span, msg }, gate)
         } else {
             ccx.dcx().create_err(errors::UnallowedOpInConstContext { span, msg })
         }

Diff for: compiler/rustc_session/src/parse.rs

@@ -155,6 +155,7 @@ pub fn feature_warn_issue(
 }
 
 /// Adds the diagnostics for a feature to an existing error.
+/// Must be a language feature!
 pub fn add_feature_diagnostics<G: EmissionGuarantee>(
     err: &mut Diag<'_, G>,
     sess: &Session,

Diff for: compiler/rustc_session/src/session.rs

@@ -300,6 +300,7 @@ impl Session {
         self.opts.test
     }
 
+    /// `feature` must be a language feature.
     #[track_caller]
     pub fn create_feature_err<'a>(&'a self, err: impl Diagnostic<'a>, feature: Symbol) -> Diag<'a> {
         let mut err = self.dcx().create_err(err);

Diff for: tests/ui/traits/const-traits/cross-crate.stock.stderr

@@ -1,8 +1,13 @@
-error: cannot call conditionally-const method `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
+error[E0658]: cannot call conditionally-const method `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:22:5
    |
 LL |     Const.func();
    |     ^^^^^^^^^^^^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.

Diff for: tests/ui/traits/const-traits/cross-crate.stocknc.stderr

@@ -1,14 +1,23 @@
-error: cannot call conditionally-const method `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
+error[E0658]: cannot call conditionally-const method `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:19:5
    |
 LL |     NonConst.func();
    |     ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: cannot call conditionally-const method `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
+error[E0658]: cannot call conditionally-const method `<cross_crate::Const as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:22:5
    |
 LL |     Const.func();
    |     ^^^^^^^^^^^^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0658`.

Diff for: tests/ui/traits/const-traits/staged-api-user-crate.stderr

@@ -1,8 +1,13 @@
-error: cannot call conditionally-const associated function `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
+error[E0658]: cannot call conditionally-const associated function `<staged_api::Unstable as staged_api::MyTrait>::func` in constant functions
   --> $DIR/staged-api-user-crate.rs:12:5
    |
 LL |     Unstable::func();
    |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.