Auto merge of #128034 - Nadrieril:explain-unreachable, r=<try>

exhaustiveness: Explain why a given pattern is considered unreachable

This PR tells the user why a given pattern is considered unreachable. I reused the intersection information we were already computing; even though it's incomplete I convinced myself that it is sufficient to always get a set of patterns that cover the unreachable one.

I'm not a fan of the diagnostic messages I came up with, I'm open to suggestions.

Fixes #127870. This is also the other one of the two diagnostic improvements I wanted to do before #122792.

Note: the first commit is #128015, the second is an unrelated drive-by tweak.

r? `@compiler-errors`

Author: bors

compiler/rustc_mir_build/messages.ftl

@@ -327,7 +327,10 @@ mir_build_union_pattern = cannot use unions in constant patterns
 
 mir_build_unreachable_pattern = unreachable pattern
     .label = unreachable pattern
-    .catchall_label = matches any value
+    .unreachable_matches_no_values = this pattern matches no values because `{$ty}` is uninhabited
+    .unreachable_covered_by_catchall = matches any value
+    .unreachable_covered_by_one = matches all the values already
+    .unreachable_covered_by_many = matches some of the same values
 
 mir_build_unsafe_fn_safe_body = an unsafe function restricts its caller, but its body is safe by default
 mir_build_unsafe_not_inherited = items do not inherit unsafety from separate enclosing items

compiler/rustc_mir_build/src/errors.rs

@@ -582,11 +582,37 @@ pub(crate) struct NonConstPath {
 
 #[derive(LintDiagnostic)]
 #[diag(mir_build_unreachable_pattern)]
-pub(crate) struct UnreachablePattern {
+pub(crate) struct UnreachablePattern<'tcx> {
     #[label]
     pub(crate) span: Option<Span>,
-    #[label(mir_build_catchall_label)]
-    pub(crate) catchall: Option<Span>,
+    #[subdiagnostic]
+    pub(crate) matches_no_values: Option<UnreachableMatchesNoValues<'tcx>>,
+    #[label(mir_build_unreachable_covered_by_catchall)]
+    pub(crate) covered_by_catchall: Option<Span>,
+    #[label(mir_build_unreachable_covered_by_one)]
+    pub(crate) covered_by_one: Option<Span>,
+    #[subdiagnostic]
+    pub(crate) covered_by_many: Option<UnreachableCoveredByMany>,
+}
+
+#[derive(Subdiagnostic)]
+#[note(mir_build_unreachable_matches_no_values)]
+pub(crate) struct UnreachableMatchesNoValues<'tcx> {
+    pub(crate) ty: Ty<'tcx>,
+}
+
+pub(crate) struct UnreachableCoveredByMany(pub(crate) Vec<Span>);
+
+impl Subdiagnostic for UnreachableCoveredByMany {
+    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
+        self,
+        diag: &mut Diag<'_, G>,
+        _f: &F,
+    ) {
+        for span in self.0 {
+            diag.span_label(span, fluent::mir_build_unreachable_covered_by_many);
+        }
+    }
 }
 
 #[derive(Diagnostic)]