Tweak output of some const pattern errors

- Add primary span labels.
- Point at const generic parameter used as pattern.
- Point at statics used as pattern.
- Point at let bindings used in const pattern.

compiler/rustc_mir_build/messages.ftl

@@ -86,7 +86,9 @@ mir_build_confused = missing patterns are not covered because `{$variable}` is i
 
 mir_build_const_defined_here = constant defined here
 
-mir_build_const_param_in_pattern = const parameters cannot be referenced in patterns
+mir_build_const_param_in_pattern = constant parameters cannot be referenced in patterns
+    .label = can't be used in patterns
+mir_build_const_param_in_pattern_def = constant defined here
 
 mir_build_const_pattern_depends_on_generic_parameter = constant pattern depends on a generic parameter, which is not allowed
     .label = `const` depends on a generic parameter
@@ -247,10 +249,12 @@ mir_build_mutation_of_layout_constrained_field_requires_unsafe_unsafe_op_in_unsa
     .label = mutation of layout constrained field
 
 mir_build_nan_pattern = cannot use NaN in patterns
+    .label = evaluates to `NaN`, which is not allowed in patterns
     .note = NaNs compare inequal to everything, even themselves, so this pattern would never match
     .help = try using the `is_nan` method instead
 
 mir_build_non_const_path = runtime values cannot be referenced in patterns
+    .label = references a runtime value
 
 mir_build_non_empty_never_pattern =
     mismatched types
@@ -270,6 +274,7 @@ mir_build_non_exhaustive_patterns_type_not_empty = non-exhaustive patterns: type
 
 mir_build_non_partial_eq_match =
     to use a constant of type `{$non_peq_ty}` in a pattern, the type must implement `PartialEq`
+    .label = constant of non-structural type
 
 mir_build_pattern_not_covered = refutable pattern in {$origin}
     .pattern_ty = the matched value is of type `{$pattern_ty}`
@@ -288,6 +293,8 @@ mir_build_rustc_box_attribute_error = `#[rustc_box]` attribute used incorrectly
     .missing_box = `#[rustc_box]` requires the `owned_box` lang item
 
 mir_build_static_in_pattern = statics cannot be referenced in patterns
+    .label = can't be used in patterns
+mir_build_static_in_pattern_def = `static` defined here
 
 mir_build_suggest_attempted_int_lit = alternatively, you could prepend the pattern with an underscore to define a new named variable; identifiers cannot begin with digits
 
@@ -339,6 +346,7 @@ mir_build_union_field_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
     .label = access to union field
 
 mir_build_union_pattern = cannot use unions in constant patterns
+    .label = can't use a `union` here
 
 mir_build_unreachable_making_this_unreachable = collectively making this unreachable
 

compiler/rustc_mir_build/src/errors.rs

@@ -631,20 +631,27 @@ pub(crate) struct NonExhaustiveMatchAllArmsGuarded;
 #[diag(mir_build_static_in_pattern, code = E0158)]
 pub(crate) struct StaticInPattern {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
+    #[label(mir_build_static_in_pattern_def)]
+    pub(crate) static_span: Span,
 }
 
 #[derive(Diagnostic)]
 #[diag(mir_build_const_param_in_pattern, code = E0158)]
 pub(crate) struct ConstParamInPattern {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
+    #[label(mir_build_const_param_in_pattern_def)]
+    pub(crate) const_span: Span,
 }
 
 #[derive(Diagnostic)]
 #[diag(mir_build_non_const_path, code = E0080)]
 pub(crate) struct NonConstPath {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
 }
 
@@ -869,6 +876,7 @@ pub(crate) enum Conflict {
 #[diag(mir_build_union_pattern)]
 pub(crate) struct UnionPattern {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
 }
 
@@ -886,6 +894,7 @@ pub(crate) struct TypeNotStructural<'tcx> {
 #[diag(mir_build_non_partial_eq_match)]
 pub(crate) struct TypeNotPartialEq<'tcx> {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
     pub(crate) non_peq_ty: Ty<'tcx>,
 }
@@ -912,6 +921,7 @@ pub(crate) struct UnsizedPattern<'tcx> {
 #[help]
 pub(crate) struct NaNPattern {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
 }
 

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

@@ -528,11 +528,17 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
             | Res::SelfCtor(..) => PatKind::Leaf { subpatterns },
             _ => {
                 let e = match res {
-                    Res::Def(DefKind::ConstParam, _) => {
-                        self.tcx.dcx().emit_err(ConstParamInPattern { span })
+                    Res::Def(DefKind::ConstParam, def_id) => {
+                        self.tcx.dcx().emit_err(ConstParamInPattern {
+                            span,
+                            const_span: self.tcx().def_span(def_id),
+                        })
                     }
-                    Res::Def(DefKind::Static { .. }, _) => {
-                        self.tcx.dcx().emit_err(StaticInPattern { span })
+                    Res::Def(DefKind::Static { .. }, def_id) => {
+                        self.tcx.dcx().emit_err(StaticInPattern {
+                            span,
+                            static_span: self.tcx().def_span(def_id),
+                        })
                     }
                     _ => self.tcx.dcx().emit_err(NonConstPath { span }),
                 };