give correct error message on structural match violation

Author: b-naber

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -121,27 +121,27 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
 
     fn search_for_structural_match_violation(&self, ty: Ty<'tcx>) -> Option<String> {
         traits::search_for_structural_match_violation(self.span, self.tcx(), ty).map(|non_sm_ty| {
-            with_no_trimmed_paths!(match non_sm_ty {
-                traits::NonStructuralMatchTy::Adt(adt) => self.adt_derive_msg(adt),
-                traits::NonStructuralMatchTy::Dynamic => {
+            with_no_trimmed_paths!(match non_sm_ty.kind {
+                traits::NonStructuralMatchTyKind::Adt(adt) => self.adt_derive_msg(adt),
+                traits::NonStructuralMatchTyKind::Dynamic => {
                     "trait objects cannot be used in patterns".to_string()
                 }
-                traits::NonStructuralMatchTy::Opaque => {
+                traits::NonStructuralMatchTyKind::Opaque => {
                     "opaque types cannot be used in patterns".to_string()
                 }
-                traits::NonStructuralMatchTy::Closure => {
+                traits::NonStructuralMatchTyKind::Closure => {
                     "closures cannot be used in patterns".to_string()
                 }
-                traits::NonStructuralMatchTy::Generator => {
+                traits::NonStructuralMatchTyKind::Generator => {
                     "generators cannot be used in patterns".to_string()
                 }
-                traits::NonStructuralMatchTy::Param => {
+                traits::NonStructuralMatchTyKind::Param => {
                     bug!("use of a constant whose type is a parameter inside a pattern")
                 }
-                traits::NonStructuralMatchTy::Projection => {
+                traits::NonStructuralMatchTyKind::Projection => {
                     bug!("use of a constant whose type is a projection inside a pattern")
                 }
-                traits::NonStructuralMatchTy::Foreign => {
+                traits::NonStructuralMatchTyKind::Foreign => {
                     bug!("use of a value of a foreign type inside a pattern")
                 }
             })

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -62,7 +62,7 @@ pub use self::specialize::specialization_graph::FutureCompatOverlapError;
 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
 pub use self::structural_match::search_for_structural_match_violation;
-pub use self::structural_match::NonStructuralMatchTy;
+pub use self::structural_match::{NonStructuralMatchTy, NonStructuralMatchTyKind};
 pub use self::util::{
     elaborate_obligations, elaborate_predicates, elaborate_predicates_with_span,
     elaborate_trait_ref, elaborate_trait_refs,

compiler/rustc_trait_selection/src/traits/structural_match.rs

@@ -11,7 +11,13 @@ use rustc_span::Span;
 use std::ops::ControlFlow;
 
 #[derive(Debug)]
-pub enum NonStructuralMatchTy<'tcx> {
+pub struct NonStructuralMatchTy<'tcx> {
+    pub ty: Ty<'tcx>,
+    pub kind: NonStructuralMatchTyKind<'tcx>,
+}
+
+#[derive(Debug)]
+pub enum NonStructuralMatchTyKind<'tcx> {
     Adt(AdtDef<'tcx>),
     Param,
     Dynamic,
@@ -137,25 +143,32 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
         let (adt_def, substs) = match *ty.kind() {
             ty::Adt(adt_def, substs) => (adt_def, substs),
             ty::Param(_) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Param);
+                let kind = NonStructuralMatchTyKind::Param;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Dynamic(..) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Dynamic);
+                let kind = NonStructuralMatchTyKind::Dynamic;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Foreign(_) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Foreign);
+                let kind = NonStructuralMatchTyKind::Foreign;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Opaque(..) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Opaque);
+                let kind = NonStructuralMatchTyKind::Opaque;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Projection(..) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Projection);
+                let kind = NonStructuralMatchTyKind::Projection;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Closure(..) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Closure);
+                let kind = NonStructuralMatchTyKind::Closure;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::Generator(..) | ty::GeneratorWitness(..) => {
-                return ControlFlow::Break(NonStructuralMatchTy::Generator);
+                let kind = NonStructuralMatchTyKind::Generator;
+                return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
             }
             ty::RawPtr(..) => {
                 // structural-match ignores substructure of
@@ -215,7 +228,8 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for Search<'a, 'tcx> {
 
         if !self.type_marked_structural(ty) {
             debug!("Search found ty: {:?}", ty);
-            return ControlFlow::Break(NonStructuralMatchTy::Adt(adt_def));
+            let kind = NonStructuralMatchTyKind::Adt(adt_def);
+            return ControlFlow::Break(NonStructuralMatchTy { ty, kind });
         }
 
         // structural-match does not care about the

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -827,7 +827,9 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
                     );
                 }
 
-                if traits::search_for_structural_match_violation(param.span, tcx, ty).is_some() {
+                if let Some(non_structural_match_ty) =
+                    traits::search_for_structural_match_violation(param.span, tcx, ty)
+                {
                     // We use the same error code in both branches, because this is really the same
                     // issue: we just special-case the message for type parameters to make it
                     // clearer.
@@ -853,19 +855,23 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
                         )
                         .emit();
                     } else {
-                        struct_span_err!(
+                        let mut diag = struct_span_err!(
                             tcx.sess,
                             hir_ty.span,
                             E0741,
                             "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
                             the type of a const parameter",
-                            ty,
-                        )
-                        .span_label(
-                            hir_ty.span,
-                            format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
-                        )
-                        .emit();
+                            non_structural_match_ty.ty,
+                        );
+
+                        if ty == non_structural_match_ty.ty {
+                            diag.span_label(
+                                hir_ty.span,
+                                format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
+                            );
+                        }
+
+                        diag.emit();
                     }
                 }
             } else {