Reveal opaque types in exhaustiveness checking

Author: Nadrieril

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

@@ -28,13 +28,15 @@ use rustc_span::hygiene::DesugaringKind;
 use rustc_span::Span;
 
 pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGuaranteed> {
+    let typeck_results = tcx.typeck(def_id);
     let (thir, expr) = tcx.thir_body(def_id)?;
     let thir = thir.borrow();
     let pattern_arena = TypedArena::default();
     let dropless_arena = DroplessArena::default();
     let mut visitor = MatchVisitor {
         tcx,
         thir: &*thir,
+        typeck_results,
         param_env: tcx.param_env(def_id),
         lint_level: tcx.local_def_id_to_hir_id(def_id),
         let_source: LetSource::None,
@@ -80,6 +82,7 @@ enum LetSource {
 struct MatchVisitor<'thir, 'p, 'tcx> {
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
+    typeck_results: &'tcx ty::TypeckResults<'tcx>,
     thir: &'thir Thir<'tcx>,
     lint_level: HirId,
     let_source: LetSource,
@@ -382,6 +385,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             scrutinee.map(|scrut| self.is_known_valid_scrutinee(scrut)).unwrap_or(true);
         MatchCheckCtxt {
             tcx: self.tcx,
+            typeck_results: self.typeck_results,
             param_env: self.param_env,
             module: self.tcx.parent_module(self.lint_level).to_def_id(),
             pattern_arena: self.pattern_arena,

compiler/rustc_pattern_analysis/src/lib.rs

@@ -62,7 +62,8 @@ pub trait TypeCx: Sized + Clone + fmt::Debug {
     /// patterns during analysis.
     type PatData: Clone + Default;
 
-    fn is_opaque_ty(ty: Self::Ty) -> bool;
+    /// FIXME(Nadrieril): `Cx` should only give us revealed types.
+    fn reveal_opaque_ty(&self, ty: Self::Ty) -> Self::Ty;
     fn is_exhaustive_patterns_feature_on(&self) -> bool;
 
     /// The number of fields for this constructor.

compiler/rustc_pattern_analysis/src/lints.rs

@@ -48,22 +48,14 @@ impl<'a, 'p, 'tcx> PatternColumn<'a, 'p, 'tcx> {
     fn is_empty(&self) -> bool {
         self.patterns.is_empty()
     }
-    fn head_ty(&self) -> Option<Ty<'tcx>> {
+    fn head_ty(&self, cx: MatchCtxt<'a, 'p, 'tcx>) -> Option<Ty<'tcx>> {
         if self.patterns.len() == 0 {
             return None;
         }
-        // If the type is opaque and it is revealed anywhere in the column, we take the revealed
-        // version. Otherwise we could encounter constructors for the revealed type and crash.
-        let first_ty = self.patterns[0].ty();
-        if RustcMatchCheckCtxt::is_opaque_ty(first_ty) {
-            for pat in &self.patterns {
-                let ty = pat.ty();
-                if !RustcMatchCheckCtxt::is_opaque_ty(ty) {
-                    return Some(ty);
-                }
-            }
-        }
-        Some(first_ty)
+
+        let ty = self.patterns[0].ty();
+        // FIXME(Nadrieril): `Cx` should only give us revealed types.
+        Some(cx.tycx.reveal_opaque_ty(ty))
     }
 
     /// Do constructor splitting on the constructors of the column.
@@ -125,7 +117,7 @@ fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
     cx: MatchCtxt<'a, 'p, 'tcx>,
     column: &PatternColumn<'a, 'p, 'tcx>,
 ) -> Vec<WitnessPat<'p, 'tcx>> {
-    let Some(ty) = column.head_ty() else {
+    let Some(ty) = column.head_ty(cx) else {
         return Vec::new();
     };
     let pcx = &PlaceCtxt::new_dummy(cx, ty);
@@ -226,7 +218,7 @@ pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
     cx: MatchCtxt<'a, 'p, 'tcx>,
     column: &PatternColumn<'a, 'p, 'tcx>,
 ) {
-    let Some(ty) = column.head_ty() else {
+    let Some(ty) = column.head_ty(cx) else {
         return;
     };
     let pcx = &PlaceCtxt::new_dummy(cx, ty);

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -44,6 +44,7 @@ pub type WitnessPat<'p, 'tcx> = crate::pat::WitnessPat<RustcMatchCheckCtxt<'p, '
 #[derive(Clone)]
 pub struct RustcMatchCheckCtxt<'p, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
+    pub typeck_results: &'tcx ty::TypeckResults<'tcx>,
     /// The module in which the match occurs. This is necessary for
     /// checking inhabited-ness of types because whether a type is (visibly)
     /// inhabited can depend on whether it was defined in the current module or
@@ -101,6 +102,21 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
         }
     }
 
+    /// Type inference occasionally gives us opaque types in places where corresponding patterns
+    /// have more specific types. To avoid inconsistencies as well as detect opaque uninhabited
+    /// types, we use the corresponding concrete type if possible.
+    fn reveal_opaque_ty(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if let ty::Alias(ty::Opaque, alias_ty) = ty.kind() {
+            if let Some(local_def_id) = alias_ty.def_id.as_local() {
+                let key = ty::OpaqueTypeKey { def_id: local_def_id, args: alias_ty.args };
+                if let Some(real_ty) = self.typeck_results.concrete_opaque_types.get(&key) {
+                    return real_ty.ty;
+                }
+            }
+        }
+        ty
+    }
+
     // In the cases of either a `#[non_exhaustive]` field list or a non-public field, we hide
     // uninhabited fields in order not to reveal the uninhabitedness of the whole variant.
     // This lists the fields we keep along with their types.
@@ -873,8 +889,9 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
     fn is_exhaustive_patterns_feature_on(&self) -> bool {
         self.tcx.features().exhaustive_patterns
     }
-    fn is_opaque_ty(ty: Self::Ty) -> bool {
-        matches!(ty.kind(), ty::Alias(ty::Opaque, ..))
+
+    fn reveal_opaque_ty(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        self.reveal_opaque_ty(ty)
     }
 
     fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: Self::Ty) -> usize {

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -865,24 +865,14 @@ impl<'a, 'p, Cx: TypeCx> Matrix<'a, 'p, Cx> {
         matrix
     }
 
-    fn head_ty(&self) -> Option<Cx::Ty> {
+    fn head_ty(&self, mcx: MatchCtxt<'a, 'p, Cx>) -> Option<Cx::Ty> {
         if self.column_count() == 0 {
             return None;
         }
 
-        let mut ty = self.wildcard_row.head().ty();
-        // If the type is opaque and it is revealed anywhere in the column, we take the revealed
-        // version. Otherwise we could encounter constructors for the revealed type and crash.
-        if Cx::is_opaque_ty(ty) {
-            for pat in self.heads() {
-                let pat_ty = pat.ty();
-                if !Cx::is_opaque_ty(pat_ty) {
-                    ty = pat_ty;
-                    break;
-                }
-            }
-        }
-        Some(ty)
+        let ty = self.wildcard_row.head().ty();
+        // FIXME(Nadrieril): `Cx` should only give us revealed types.
+        Some(mcx.tycx.reveal_opaque_ty(ty))
     }
     fn column_count(&self) -> usize {
         self.wildcard_row.len()
@@ -1181,7 +1171,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
 ) -> WitnessMatrix<Cx> {
     debug_assert!(matrix.rows().all(|r| r.len() == matrix.column_count()));
 
-    let Some(ty) = matrix.head_ty() else {
+    let Some(ty) = matrix.head_ty(mcx) else {
         // The base case: there are no columns in the matrix. We are morally pattern-matching on ().
         // A row is useful iff it has no (unguarded) rows above it.
         for row in matrix.rows_mut() {

tests/ui/pattern/usefulness/impl-trait.rs

@@ -14,7 +14,7 @@ enum Void {}
 fn return_never_rpit(x: Void) -> impl Copy {
     if false {
         match return_never_rpit(x) {
-            _ => {}
+            _ => {} //~ ERROR unreachable
         }
     }
     x
@@ -28,7 +28,7 @@ type T = impl Copy;
 fn return_never_tait(x: Void) -> T {
     if false {
         match return_never_tait(x) {
-            _ => {}
+            _ => {} //~ ERROR unreachable
         }
     }
     x
@@ -42,7 +42,7 @@ fn option_never(x: Void) -> Option<impl Copy> {
     if false {
         match option_never(x) {
             None => {}
-            Some(_) => {}
+            Some(_) => {} //~ ERROR unreachable
         }
         match option_never(x) {
             None => {}
@@ -75,7 +75,7 @@ fn inner_never(x: Void) {
     type T = impl Copy;
     let y: T = x;
     match y {
-        _ => {}
+        _ => {} //~ ERROR unreachable
     }
 }
 
@@ -93,7 +93,7 @@ type U = impl Copy;
 fn unify_never(x: Void, u: U) -> U {
     if false {
         match u {
-            _ => {}
+            _ => {} //~ ERROR unreachable
         }
     }
     x

tests/ui/pattern/usefulness/impl-trait.stderr

@@ -1,21 +1,45 @@
 error: unreachable pattern
-  --> $DIR/impl-trait.rs:61:13
+  --> $DIR/impl-trait.rs:17:13
    |
-LL |             Some(_) => {}
-   |             ^^^^^^^
+LL |             _ => {}
+   |             ^
    |
 note: the lint level is defined here
   --> $DIR/impl-trait.rs:5:9
    |
 LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
+error: unreachable pattern
+  --> $DIR/impl-trait.rs:31:13
+   |
+LL |             _ => {}
+   |             ^
+
+error: unreachable pattern
+  --> $DIR/impl-trait.rs:45:13
+   |
+LL |             Some(_) => {}
+   |             ^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/impl-trait.rs:61:13
+   |
+LL |             Some(_) => {}
+   |             ^^^^^^^
+
 error: unreachable pattern
   --> $DIR/impl-trait.rs:65:13
    |
 LL |             _ => {}
    |             ^
 
+error: unreachable pattern
+  --> $DIR/impl-trait.rs:78:9
+   |
+LL |         _ => {}
+   |         ^
+
 error: unreachable pattern
   --> $DIR/impl-trait.rs:88:9
    |
@@ -24,6 +48,12 @@ LL |         _ => {}
 LL |         Some((a, b)) => {}
    |         ^^^^^^^^^^^^ unreachable pattern
 
+error: unreachable pattern
+  --> $DIR/impl-trait.rs:96:13
+   |
+LL |             _ => {}
+   |             ^
+
 error: unreachable pattern
   --> $DIR/impl-trait.rs:107:9
    |
@@ -66,6 +96,6 @@ LL +         _ => todo!(),
 LL +     }
    |
 
-error: aborting due to 7 previous errors
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0004`.