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_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)]
+ #[subdiagnostic]
+ pub(crate) matches_no_values: Option<UnreachableMatchesNoValues<'tcx>>,
+ #[label(mir_build_unreachable_catchall)]
  pub(crate) 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, RustcPatCtxt as PatCtxt, Usefulness, UsefulnessReport,
- WitnessPat,
+ Constructor, DeconstructedPat, MatchArm, RedundancyExplanation, 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 +910,53 @@ 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: None,
+ matches_no_values: None,
+ catchall: None,
+ covered_by_one: None,
+ covered_by_many: None,
+ };
+ match explanation {
+ RedundancyExplanation::Empty => {
+ pat.walk(&mut |subpat| {
+ let ty = **subpat.ty();
+ if cx.is_uninhabited(ty) {
+ lint.matches_no_values = Some(UnreachableMatchesNoValues { ty });
+ false
+ } else {
+ true
+ }
+ });
+ }
+ RedundancyExplanation::CoveredBy(covering_pats) => {
+ lint.span = Some(pat_span); // Add a label to the pattern itself
+ if covering_pats.len() == 1 {
+ let covering_pat = covering_pats[0];
+ let covering_span = covering_pat.data().span;
+ if pat_is_catchall(covering_pat) {
+ lint.catchall = Some(covering_span);
+ } else {
+ lint.covered_by_one = Some(covering_span);
+ }
+ } else {
+ 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)
  }
  }
 }

compiler/rustc_pattern_analysis/src/pat.rs

@@ -11,7 +11,7 @@ use crate::{PatCx, PrivateUninhabitedField};
 use self::Constructor::*;
 
 /// A globally unique id to distinguish patterns.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub(crate) struct PatId(u32);
 impl PatId {
  fn new() -> Self {
@@ -147,6 +147,21 @@ impl<Cx: PatCx> fmt::Debug for DeconstructedPat<Cx> {
  }
 }
 
+/// Delegate to `uid`.
+impl<Cx: PatCx> PartialEq for DeconstructedPat<Cx> {
+ fn eq(&self, other: &Self) -> bool {
+ self.uid == other.uid
+ }
+}
+/// Delegate to `uid`.
+impl<Cx: PatCx> Eq for DeconstructedPat<Cx> {}
+/// Delegate to `uid`.
+impl<Cx: PatCx> std::hash::Hash for DeconstructedPat<Cx> {
+ fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+ self.uid.hash(state);
+ }
+}
+
 /// Represents either a pattern obtained from user input or a wildcard constructed during the
 /// algorithm. Do not use `Wild` to represent a wildcard pattern comping from user input.
 ///

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -32,6 +32,8 @@ pub type Constructor<'p, 'tcx> = crate::constructor::Constructor<RustcPatCtxt<'p
 pub type ConstructorSet<'p, 'tcx> = crate::constructor::ConstructorSet<RustcPatCtxt<'p, 'tcx>>;
 pub type DeconstructedPat<'p, 'tcx> = crate::pat::DeconstructedPat<RustcPatCtxt<'p, 'tcx>>;
 pub type MatchArm<'p, 'tcx> = crate::MatchArm<'p, RustcPatCtxt<'p, 'tcx>>;
+pub type RedundancyExplanation<'p, 'tcx> =
+ crate::usefulness::RedundancyExplanation<'p, RustcPatCtxt<'p, 'tcx>>;
 pub type Usefulness<'p, 'tcx> = crate::usefulness::Usefulness<'p, RustcPatCtxt<'p, 'tcx>>;
 pub type UsefulnessReport<'p, 'tcx> =
  crate::usefulness::UsefulnessReport<'p, RustcPatCtxt<'p, 'tcx>>;