Rollup merge of #113019 - ericmarkmartin:warning-for-guard-non-exhaustion, r=fee1-dead

add note for non-exhaustive matches with guards

Associated issue: #92197

When a match statement includes guards on every match arm (and is therefore necessarily non-exhaustive), add a note to the error E0004 diagnostic noting this.

compiler/rustc_mir_build/messages.ftl

@@ -214,6 +214,9 @@ mir_build_mutation_of_layout_constrained_field_requires_unsafe_unsafe_op_in_unsa
 
 mir_build_non_const_path = runtime values cannot be referenced in patterns
 
+mir_build_non_exhaustive_match_all_arms_guarded =
+    match arms with guards don't count towards exhaustivity
+
 mir_build_non_exhaustive_omitted_pattern = some variants are not matched explicitly
     .help = ensure that all variants are matched explicitly by adding the suggested match arms
     .note = the matched value is of type `{$scrut_ty}` and the `non_exhaustive_omitted_patterns` attribute was found

compiler/rustc_mir_build/src/errors.rs

@@ -432,6 +432,10 @@ impl<'a> IntoDiagnostic<'a> for NonExhaustivePatternsTypeNotEmpty<'_, '_, '_> {
     }
 }
 
+#[derive(Subdiagnostic)]
+#[note(mir_build_non_exhaustive_match_all_arms_guarded)]
+pub struct NonExhaustiveMatchAllArmsGuarded;
+
 #[derive(Diagnostic)]
 #[diag(mir_build_static_in_pattern, code = "E0158")]
 pub struct StaticInPattern {

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

@@ -830,6 +830,11 @@ fn non_exhaustive_match<'p, 'tcx>(
             _ => " or multiple match arms",
         },
     );
+
+    let all_arms_have_guards = arms.iter().all(|arm_id| thir[*arm_id].guard.is_some());
+    if !is_empty_match && all_arms_have_guards {
+        err.subdiagnostic(NonExhaustiveMatchAllArmsGuarded);
+    }
     if let Some((span, sugg)) = suggestion {
         err.span_suggestion_verbose(span, msg, sugg, Applicability::HasPlaceholders);
     } else {