Tweak wording of non-const traits used as const bounds

Use verbose suggestions and add additional labels/notes.

Add more test cases for stable/nightly and feature enabled/disabled.

compiler/rustc_hir_analysis/messages.ftl

@@ -96,12 +96,14 @@ hir_analysis_coercion_between_struct_same_note = expected coercion between the s
 
 hir_analysis_coercion_between_struct_single_note = expected a single field to be coerced, none found
 
-hir_analysis_const_bound_for_non_const_trait =
-    `{$modifier}` can only be applied to `#[const_trait]` traits
+hir_analysis_const_bound_for_non_const_trait = `{$modifier}` can only be applied to `#[const_trait]` traits
+    .label = can't be applied to `{$trait_name}`
+    .note = `{$trait_name}` can't be used with `{$modifier}` because it isn't annotated with `#[const_trait]`
+    .suggestion = {$suggestion_pre}mark `{$trait_name}` as `#[const_trait]` to allow it to have `const` implementations
 
-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
+hir_analysis_const_impl_for_non_const_trait = const `impl` for trait `{$trait_name}` which is not marked with `#[const_trait]`
+    .label = this trait is not `const`
+    .suggestion = {$suggestion_pre}mark `{$trait_name}` as `#[const_trait]` to allow it to have `const` implementations
     .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
 

compiler/rustc_hir_analysis/src/collect.rs

@@ -1637,11 +1637,23 @@ fn check_impl_constness(
     }
 
     let trait_name = tcx.item_name(trait_def_id).to_string();
+    let (local_trait_span, suggestion_pre) =
+        match (trait_def_id.is_local(), tcx.sess.is_nightly_build()) {
+            (true, true) => (
+                Some(tcx.def_span(trait_def_id).shrink_to_lo()),
+                if tcx.features().const_trait_impl() {
+                    ""
+                } else {
+                    "enable `#![feature(const_trait_impl)]` in your crate and "
+                },
+            ),
+            (false, _) | (_, false) => (None, ""),
+        };
     Some(tcx.dcx().emit_err(errors::ConstImplForNonConstTrait {
         trait_ref_span: hir_trait_ref.path.span,
         trait_name,
-        local_trait_span:
-            trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+        local_trait_span,
+        suggestion_pre,
         marking: (),
         adding: (),
     }))

compiler/rustc_hir_analysis/src/errors.rs

@@ -530,10 +530,16 @@ pub(crate) struct GenericArgsOnOverriddenImpl {
 #[diag(hir_analysis_const_impl_for_non_const_trait)]
 pub(crate) struct ConstImplForNonConstTrait {
     #[primary_span]
+    #[label]
     pub trait_ref_span: Span,
     pub trait_name: String,
-    #[suggestion(applicability = "machine-applicable", code = "#[const_trait]")]
+    #[suggestion(
+        applicability = "machine-applicable",
+        code = "#[const_trait] ",
+        style = "verbose"
+    )]
     pub local_trait_span: Option<Span>,
+    pub suggestion_pre: &'static str,
     #[note]
     pub marking: (),
     #[note(hir_analysis_adding)]
@@ -544,8 +550,19 @@ pub(crate) struct ConstImplForNonConstTrait {
 #[diag(hir_analysis_const_bound_for_non_const_trait)]
 pub(crate) struct ConstBoundForNonConstTrait {
     #[primary_span]
+    #[label]
     pub span: Span,
     pub modifier: &'static str,
+    #[note]
+    pub def_span: Option<Span>,
+    pub suggestion_pre: &'static str,
+    #[suggestion(
+        applicability = "machine-applicable",
+        code = "#[const_trait] ",
+        style = "verbose"
+    )]
+    pub suggestion: Option<Span>,
+    pub trait_name: String,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -737,9 +737,26 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         if let hir::BoundConstness::Always(span) | hir::BoundConstness::Maybe(span) = constness
             && !self.tcx().is_const_trait(trait_def_id)
         {
+            let (def_span, suggestion, suggestion_pre) =
+                match (trait_def_id.is_local(), self.tcx().sess.is_nightly_build()) {
+                    (true, true) => (
+                        None,
+                        Some(tcx.def_span(trait_def_id).shrink_to_lo()),
+                        if self.tcx().features().const_trait_impl() {
+                            ""
+                        } else {
+                            "enable `#![feature(const_trait_impl)]` in your crate and "
+                        },
+                    ),
+                    (false, _) | (_, false) => (Some(tcx.def_span(trait_def_id)), None, ""),
+                };
             self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
                 span,
                 modifier: constness.as_str(),
+                def_span,
+                trait_name: self.tcx().def_path_str(trait_def_id),
+                suggestion_pre,
+                suggestion,
             });
         }