Split diagnostic details out into a separate function and fluent files

compiler/rustc_error_messages/locales/en-US/hir_analysis.ftl

@@ -137,3 +137,12 @@ hir_analysis_expected_used_symbol = expected `used`, `used(compiler)` or `used(l
 hir_analysis_missing_parentheses_in_range = can't call method `{$method_name}` on type `{$ty_str}`
 
 hir_analysis_add_missing_parentheses_in_range = you must surround the range in parentheses to call its `{$func_name}` function
+
+hir_analysis_const_impl_for_non_const_trait =
+    const `impl` for trait `{$trait_name}` which is not marked with `#[const_trait]`
+    .suggestion = mark `{$trait_name}` as const
+    .note = marking a trait with `#[const_trait]` ensures all default method bodies are `const`
+    .adding = adding a non-const method body in the future would be a breaking change
+
+hir_analysis_const_bound_for_non_const_trait =
+    ~const can only be applied to `#[const_trait]` traits

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -539,10 +539,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if let Some(ty::BoundConstness::ConstIfConst) = constness
             && generics.has_self && !tcx.has_attr(def_id, sym::const_trait)
         {
-            tcx.sess.span_err(
-                span,
-                "~const can only be applied to `#[const_trait]` traits",
-            );
+            tcx.sess.emit_err(crate::errors::ConstBoundForNonConstTrait { span } );
         }
 
         (substs, arg_count)

compiler/rustc_hir_analysis/src/collect.rs

@@ -1294,34 +1294,38 @@ fn impl_trait_ref(tcx: TyCtxt<'_>, def_id: DefId) -> Option<ty::TraitRef<'_>> {
                 &icx,
                 ast_trait_ref,
                 selfty,
-                match impl_.constness {
-                    hir::Constness::Const => {
-                        if let Some(trait_def_id) = ast_trait_ref.trait_def_id() && !tcx.has_attr(trait_def_id, sym::const_trait) {
-                            let trait_name = tcx.item_name(trait_def_id);
-                            let mut err = tcx.sess.struct_span_err(
-                                ast_trait_ref.path.span,
-                                &format!("const `impl` for trait `{trait_name}` which is not marked with `#[const_trait]`"),
-                            );
-                            if trait_def_id.is_local() {
-                                let sp = tcx.def_span(trait_def_id).shrink_to_lo();
-                                err.span_suggestion(sp, &format!("mark `{trait_name}` as const"), "#[const_trait]", rustc_errors::Applicability::MachineApplicable);
-                            }
-                            err.note("marking a trait with `#[const_trait]` ensures all default method bodies are `const`");
-                            err.note("adding a non-const method body in the future would be a breaking change");
-                            err.emit();
-                            ty::BoundConstness::NotConst
-                        } else {
-                            ty::BoundConstness::ConstIfConst
-                        }
-                    },
-                    hir::Constness::NotConst => ty::BoundConstness::NotConst,
-                },
+                check_impl_constness(tcx, impl_.constness, ast_trait_ref),
             )
         }),
         _ => bug!(),
     }
 }
 
+fn check_impl_constness(
+    tcx: TyCtxt<'_>,
+    constness: hir::Constness,
+    ast_trait_ref: &hir::TraitRef<'_>,
+) -> ty::BoundConstness {
+    match constness {
+        hir::Constness::Const => {
+            if let Some(trait_def_id) = ast_trait_ref.trait_def_id() && !tcx.has_attr(trait_def_id, sym::const_trait) {
+                let trait_name = tcx.item_name(trait_def_id).to_string();
+                tcx.sess.emit_err(errors::ConstImplForNonConstTrait {
+                    trait_ref_span: ast_trait_ref.path.span,
+                    trait_name,
+                    local_trait_span: trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+                    marking: (),
+                    adding: (),
+                });
+                ty::BoundConstness::NotConst
+            } else {
+                ty::BoundConstness::ConstIfConst
+            }
+        },
+        hir::Constness::NotConst => ty::BoundConstness::NotConst,
+    }
+}
+
 fn impl_polarity(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ImplPolarity {
     let is_rustc_reservation = tcx.has_attr(def_id, sym::rustc_reservation_impl);
     let item = tcx.hir().expect_item(def_id.expect_local());

compiler/rustc_hir_analysis/src/errors.rs

@@ -249,3 +249,24 @@ pub struct ExpectedUsedSymbol {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_const_impl_for_non_const_trait)]
+pub struct ConstImplForNonConstTrait {
+    #[primary_span]
+    pub trait_ref_span: Span,
+    pub trait_name: String,
+    #[suggestion(applicability = "machine-applicable", code = "#[const_trait]")]
+    pub local_trait_span: Option<Span>,
+    #[note]
+    pub marking: (),
+    #[note(adding)]
+    pub adding: (),
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_const_bound_for_non_const_trait)]
+pub struct ConstBoundForNonConstTrait {
+    #[primary_span]
+    pub span: Span,
+}