Explain why a given pattern is considered unreachable

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)]

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

@@ -16,8 +16,8 @@ use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
 use rustc_pattern_analysis::errors::Uncovered;
 use rustc_pattern_analysis::rustc::{
-    Constructor, DeconstructedPat, MatchArm, RevealedTy, RustcPatCtxt as PatCtxt, Usefulness,
-    UsefulnessReport, WitnessPat,
+    Constructor, DeconstructedPat, MatchArm, RedundancyExplanation, RevealedTy,
+    RustcPatCtxt as PatCtxt, Usefulness, UsefulnessReport, WitnessPat,
 };
 use rustc_session::lint::builtin::{
     BINDINGS_WITH_VARIANT_NAME, IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS,
@@ -409,9 +409,9 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             {
                 let mut redundant_subpats = redundant_subpats.clone();
                 // Emit lints in the order in which they occur in the file.
-                redundant_subpats.sort_unstable_by_key(|pat| pat.data().span);
-                for pat in redundant_subpats {
-                    report_unreachable_pattern(cx, arm.arm_data, pat.data().span, None)
+                redundant_subpats.sort_unstable_by_key(|(pat, _)| pat.data().span);
+                for (pat, explanation) in redundant_subpats {
+                    report_unreachable_pattern(cx, arm.arm_data, pat, &explanation)
                 }
             }
         }
@@ -910,26 +910,52 @@ fn report_irrefutable_let_patterns(
 fn report_unreachable_pattern<'p, 'tcx>(
     cx: &PatCtxt<'p, 'tcx>,
     hir_id: HirId,
-    span: Span,
-    catchall: Option<Span>,
+    pat: &DeconstructedPat<'p, 'tcx>,
+    explanation: &RedundancyExplanation<'p, 'tcx>,
 ) {
-    cx.tcx.emit_node_span_lint(
-        UNREACHABLE_PATTERNS,
-        hir_id,
-        span,
-        UnreachablePattern { span: if catchall.is_some() { Some(span) } else { None }, catchall },
-    );
+    let pat_span = pat.data().span;
+    let mut lint = UnreachablePattern {
+        span: Some(pat_span),
+        matches_no_values: None,
+        covered_by_catchall: None,
+        covered_by_one: None,
+        covered_by_many: None,
+    };
+    match explanation.covered_by.as_slice() {
+        [] => {
+            // Empty pattern; we report the uninhabited type that caused the emptiness.
+            lint.span = None; // Don't label the pattern itself
+            pat.walk(&mut |subpat| {
+                let ty = **subpat.ty();
+                if cx.is_uninhabited(ty) {
+                    lint.matches_no_values = Some(UnreachableMatchesNoValues { ty });
+                    false // No need to dig further.
+                } else if matches!(subpat.ctor(), Constructor::Ref | Constructor::UnionField) {
+                    false // Don't explore further since they are not by-value.
+                } else {
+                    true
+                }
+            });
+        }
+        [covering_pat] if pat_is_catchall(covering_pat) => {
+            lint.covered_by_catchall = Some(covering_pat.data().span);
+        }
+        [covering_pat] => {
+            lint.covered_by_one = Some(covering_pat.data().span);
+        }
+        covering_pats => {
+            let covering_spans = covering_pats.iter().map(|p| p.data().span).collect();
+            lint.covered_by_many = Some(UnreachableCoveredByMany(covering_spans));
+        }
+    }
+    cx.tcx.emit_node_span_lint(UNREACHABLE_PATTERNS, hir_id, pat_span, lint);
 }
 
 /// Report unreachable arms, if any.
 fn report_arm_reachability<'p, 'tcx>(cx: &PatCtxt<'p, 'tcx>, report: &UsefulnessReport<'p, 'tcx>) {
-    let mut catchall = None;
     for (arm, is_useful) in report.arm_usefulness.iter() {
-        if matches!(is_useful, Usefulness::Redundant) {
-            report_unreachable_pattern(cx, arm.arm_data, arm.pat.data().span, catchall)
-        }
-        if !arm.has_guard && catchall.is_none() && pat_is_catchall(arm.pat) {
-            catchall = Some(arm.pat.data().span);
+        if let Usefulness::Redundant(explanation) = is_useful {
+            report_unreachable_pattern(cx, arm.arm_data, arm.pat, explanation)
         }
     }
 }