Explain why we require _ for empty patterns

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, RevealedTy, RustcPatCtxt as PatCtxt, Usefulness,
+    UsefulnessReport, WitnessPat,
 };
 use rustc_session::lint::builtin::{
     BINDINGS_WITH_VARIANT_NAME, IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS,
@@ -998,27 +998,31 @@ fn report_non_exhaustive_match<'p, 'tcx>(
     err.note(format!("the matched value is of type `{}`", scrut_ty));
 
     if !is_empty_match {
-        let mut non_exhaustive_tys = FxIndexSet::default();
+        let mut special_tys = FxIndexSet::default();
         // Look at the first witness.
-        collect_non_exhaustive_tys(cx, &witnesses[0], &mut non_exhaustive_tys);
+        collect_special_tys(cx, &witnesses[0], &mut special_tys);
 
-        for ty in non_exhaustive_tys {
+        for ty in special_tys {
             if ty.is_ptr_sized_integral() {
-                if ty == cx.tcx.types.usize {
+                if ty.inner() == cx.tcx.types.usize {
                     err.note(format!(
                         "`{ty}` does not have a fixed maximum value, so half-open ranges are necessary to match \
                              exhaustively",
                     ));
-                } else if ty == cx.tcx.types.isize {
+                } else if ty.inner() == cx.tcx.types.isize {
                     err.note(format!(
                         "`{ty}` does not have fixed minimum and maximum values, so half-open ranges are necessary to match \
                              exhaustively",
                     ));
                 }
-            } else if ty == cx.tcx.types.str_ {
+            } else if ty.inner() == cx.tcx.types.str_ {
                 err.note("`&str` cannot be matched exhaustively, so a wildcard `_` is necessary");
-            } else if cx.is_foreign_non_exhaustive_enum(cx.reveal_opaque_ty(ty)) {
+            } else if cx.is_foreign_non_exhaustive_enum(ty) {
                 err.note(format!("`{ty}` is marked as non-exhaustive, so a wildcard `_` is necessary to match exhaustively"));
+            } else if cx.is_uninhabited(ty.inner()) && cx.tcx.features().min_exhaustive_patterns {
+                // The type is uninhabited yet there is a witness: we must be in the `MaybeInvalid`
+                // case.
+                err.note(format!("`{ty}` is uninhabited but is not being matched by value, so a wildcard `_` is required"));
             }
         }
     }
@@ -1168,22 +1172,22 @@ fn joined_uncovered_patterns<'p, 'tcx>(
     }
 }
 
-fn collect_non_exhaustive_tys<'tcx>(
+/// Collect types that require specific explanations when they show up in witnesses.
+fn collect_special_tys<'tcx>(
     cx: &PatCtxt<'_, 'tcx>,
     pat: &WitnessPat<'_, 'tcx>,
-    non_exhaustive_tys: &mut FxIndexSet<Ty<'tcx>>,
+    special_tys: &mut FxIndexSet<RevealedTy<'tcx>>,
 ) {
-    if matches!(pat.ctor(), Constructor::NonExhaustive) {
-        non_exhaustive_tys.insert(pat.ty().inner());
+    if matches!(pat.ctor(), Constructor::NonExhaustive | Constructor::Never) {
+        special_tys.insert(*pat.ty());
     }
     if let Constructor::IntRange(range) = pat.ctor() {
         if cx.is_range_beyond_boundaries(range, *pat.ty()) {
             // The range denotes the values before `isize::MIN` or the values after `usize::MAX`/`isize::MAX`.
-            non_exhaustive_tys.insert(pat.ty().inner());
+            special_tys.insert(*pat.ty());
         }
     }
-    pat.iter_fields()
-        .for_each(|field_pat| collect_non_exhaustive_tys(cx, field_pat, non_exhaustive_tys))
+    pat.iter_fields().for_each(|field_pat| collect_special_tys(cx, field_pat, special_tys))
 }
 
 fn report_adt_defined_here<'tcx>(

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -40,9 +40,15 @@ pub type WitnessPat<'p, 'tcx> = crate::pat::WitnessPat<RustcPatCtxt<'p, 'tcx>>;
 ///
 /// Use `.inner()` or deref to get to the `Ty<'tcx>`.
 #[repr(transparent)]
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
 pub struct RevealedTy<'tcx>(Ty<'tcx>);
 
+impl<'tcx> fmt::Display for RevealedTy<'tcx> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(fmt)
+    }
+}
+
 impl<'tcx> fmt::Debug for RevealedTy<'tcx> {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.fmt(fmt)

tests/ui/pattern/usefulness/empty-types.min_exh_pats.stderr

@@ -204,6 +204,7 @@ note: `Option<Void>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<Void>`
+   = note: `Void` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~             None => {},
@@ -349,6 +350,7 @@ LL |     match slice_never {
    |           ^^^^^^^^^^^ pattern `&[_, ..]` not covered
    |
    = note: the matched value is of type `&[!]`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         [] => {},
@@ -484,6 +486,7 @@ note: `Option<!>` defined here
    |
    = note: not covered
    = note: the matched value is of type `&Option<!>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &None => {},
@@ -502,6 +505,7 @@ note: `Option<!>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<!>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
@@ -520,6 +524,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_) => {},
@@ -538,6 +543,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_a) => {},
@@ -589,6 +595,7 @@ LL |     match ref_never {
    |           ^^^^^^^^^ pattern `&_` not covered
    |
    = note: the matched value is of type `&!`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
    = note: references are always considered inhabited
    = note: match arms with guards don't count towards exhaustivity
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
@@ -609,6 +616,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Err(_) => {},
@@ -627,6 +635,7 @@ note: `Option<Result<!, !>>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<Result<!, !>>`
+   = note: `Result<!, !>` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},

tests/ui/pattern/usefulness/slice_of_empty.min_exhaustive_patterns.stderr

@@ -5,6 +5,7 @@ LL |     match nevers {
    |           ^^^^^^ pattern `&[_, ..]` not covered
    |
    = note: the matched value is of type `&[!]`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &[] => (),