never patterns: lower never patterns to Unreachable in MIR

compiler/rustc_ast_lowering/src/expr.rs

@@ -581,8 +581,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.dcx().emit_err(NeverPatternWithGuard { span: g.span });
             }
 
-            // We add a fake `loop {}` arm body so that it typecks to `!`.
-            // FIXME(never_patterns): Desugar into a call to `unreachable_unchecked`.
+            // We add a fake `loop {}` arm body so that it typecks to `!`. The mir lowering of never
+            // patterns ensures this loop is not reachable.
             let block = self.arena.alloc(hir::Block {
                 stmts: &[],
                 expr: None,

compiler/rustc_middle/src/thir.rs

@@ -682,6 +682,23 @@ impl<'tcx> Pat<'tcx> {
             true
         })
     }
+
+    /// Whether this a never pattern.
+    pub fn is_never_pattern(&self) -> bool {
+        let mut is_never_pattern = false;
+        self.walk(|pat| match &pat.kind {
+            PatKind::Never => {
+                is_never_pattern = true;
+                false
+            }
+            PatKind::Or { pats } => {
+                is_never_pattern = pats.iter().all(|p| p.is_never_pattern());
+                false
+            }
+            _ => true,
+        });
+        is_never_pattern
+    }
 }
 
 impl<'tcx> IntoDiagArg for Pat<'tcx> {

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -1016,6 +1016,9 @@ struct PatternExtraData<'tcx> {
 
     /// Types that must be asserted.
     ascriptions: Vec<Ascription<'tcx>>,
+
+    /// Whether this corresponds to a never pattern.
+    is_never: bool,
 }
 
 impl<'tcx> PatternExtraData<'tcx> {
@@ -1041,12 +1044,14 @@ impl<'tcx, 'pat> FlatPat<'pat, 'tcx> {
         pattern: &'pat Pat<'tcx>,
         cx: &mut Builder<'_, 'tcx>,
     ) -> Self {
+        let is_never = pattern.is_never_pattern();
         let mut flat_pat = FlatPat {
             match_pairs: vec![MatchPair::new(place, pattern, cx)],
             extra_data: PatternExtraData {
                 span: pattern.span,
                 bindings: Vec::new(),
                 ascriptions: Vec::new(),
+                is_never,
             },
         };
         cx.simplify_match_pairs(&mut flat_pat.match_pairs, &mut flat_pat.extra_data);
@@ -1062,6 +1067,8 @@ struct Candidate<'pat, 'tcx> {
     match_pairs: Vec<MatchPair<'pat, 'tcx>>,
 
     /// ...and if this is non-empty, one of these subcandidates also has to match...
+    // Invariant: at the end of the algorithm, this must never contain a `is_never` candidate
+    // because that would break binding consistency.
     subcandidates: Vec<Candidate<'pat, 'tcx>>,
 
     /// ...and the guard must be evaluated if there is one.
@@ -1172,6 +1179,7 @@ enum TestCase<'pat, 'tcx> {
     Range(&'pat PatRange<'tcx>),
     Slice { len: usize, variable_length: bool },
     Deref { temp: Place<'tcx>, mutability: Mutability },
+    Never,
     Or { pats: Box<[FlatPat<'pat, 'tcx>]> },
 }
 
@@ -1238,6 +1246,9 @@ enum TestKind<'tcx> {
         temp: Place<'tcx>,
         mutability: Mutability,
     },
+
+    /// Assert unreachability of never patterns.
+    Never,
 }
 
 /// A test to perform to determine which [`Candidate`] matches a value.
@@ -1662,6 +1673,27 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.cfg.goto(or_block, source_info, any_matches);
             }
             candidate.pre_binding_block = Some(any_matches);
+        } else {
+            // Never subcandidates may have a set of bindings inconsistent with their siblings,
+            // which would break later code. So we filter them out. Note that we can't filter out
+            // top-level candidates this way.
+            candidate.subcandidates.retain_mut(|candidate| {
+                if candidate.extra_data.is_never {
+                    candidate.visit_leaves(|subcandidate| {
+                        let block = subcandidate.pre_binding_block.unwrap();
+                        // That block is already unreachable but needs a terminator to make the MIR well-formed.
+                        let source_info = self.source_info(subcandidate.extra_data.span);
+                        self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+                    });
+                    false
+                } else {
+                    true
+                }
+            });
+            if candidate.subcandidates.is_empty() {
+                // If `candidate` has become a leaf candidate, ensure it has a `pre_binding_block`.
+                candidate.pre_binding_block = Some(self.cfg.start_new_block());
+            }
         }
     }
 
@@ -2008,6 +2040,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             block = fresh_block;
         }
 
+        if candidate.extra_data.is_never {
+            // This arm has a dummy body, we don't need to generate code for it. `block` is already
+            // unreachable (except via false edge).
+            let source_info = self.source_info(candidate.extra_data.span);
+            self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+            return self.cfg.start_new_block();
+        }
+
         self.ascribe_types(
             block,
             parent_data

compiler/rustc_mir_build/src/build/matches/test.rs

@@ -44,6 +44,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
             TestCase::Deref { temp, mutability } => TestKind::Deref { temp, mutability },
 
+            TestCase::Never => TestKind::Never,
+
             TestCase::Or { .. } => bug!("or-patterns should have already been handled"),
 
             TestCase::Irrefutable { .. } => span_bug!(
@@ -262,6 +264,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let target = target_block(TestBranch::Success);
                 self.call_deref(block, target, place, mutability, ty, temp, test.span);
             }
+
+            TestKind::Never => {
+                // Check that the place is initialized.
+                // FIXME(never_patterns): Also assert validity of the data at `place`.
+                self.cfg.push_fake_read(
+                    block,
+                    source_info,
+                    FakeReadCause::ForMatchedPlace(None),
+                    place,
+                );
+                // A never pattern is only allowed on an uninhabited type, so validity of the data
+                // implies unreachability.
+                self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+            }
         }
     }
 
@@ -710,6 +726,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Some(TestBranch::Success)
             }
 
+            (TestKind::Never, _) => {
+                fully_matched = true;
+                Some(TestBranch::Success)
+            }
+
             (
                 TestKind::Switch { .. }
                 | TestKind::SwitchInt { .. }

compiler/rustc_mir_build/src/build/matches/util.rs

@@ -124,7 +124,8 @@ impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
         let default_irrefutable = || TestCase::Irrefutable { binding: None, ascription: None };
         let mut subpairs = Vec::new();
         let test_case = match pattern.kind {
-            PatKind::Never | PatKind::Wild | PatKind::Error(_) => default_irrefutable(),
+            PatKind::Wild | PatKind::Error(_) => default_irrefutable(),
+
             PatKind::Or { ref pats } => TestCase::Or {
                 pats: pats.iter().map(|pat| FlatPat::new(place_builder.clone(), pat, cx)).collect(),
             },
@@ -260,6 +261,8 @@ impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
                 subpairs.push(MatchPair::new(PlaceBuilder::from(temp).deref(), subpattern, cx));
                 TestCase::Deref { temp, mutability }
             }
+
+            PatKind::Never => TestCase::Never,
         };
 
         MatchPair { place, test_case, subpairs, pattern }

tests/mir-opt/building/match/never_patterns.opt1.SimplifyCfg-initial.after.mir

@@ -0,0 +1,41 @@
+// MIR for `opt1` after SimplifyCfg-initial
+
+fn opt1(_1: &Result<u32, Void>) -> &u32 {
+    debug res => _1;
+    let mut _0: &u32;
+    let mut _2: isize;
+    let _3: &u32;
+    let mut _4: !;
+    let mut _5: ();
+    scope 1 {
+        debug x => _3;
+    }
+
+    bb0: {
+        PlaceMention(_1);
+        _2 = discriminant((*_1));
+        switchInt(move _2) -> [0: bb2, 1: bb3, otherwise: bb1];
+    }
+
+    bb1: {
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
+    }
+
+    bb2: {
+        falseEdge -> [real: bb4, imaginary: bb3];
+    }
+
+    bb3: {
+        FakeRead(ForMatchedPlace(None), (((*_1) as Err).0: Void));
+        unreachable;
+    }
+
+    bb4: {
+        StorageLive(_3);
+        _3 = &(((*_1) as Ok).0: u32);
+        _0 = &(*_3);
+        StorageDead(_3);
+        return;
+    }
+}

tests/mir-opt/building/match/never_patterns.opt2.SimplifyCfg-initial.after.mir

@@ -0,0 +1,35 @@
+// MIR for `opt2` after SimplifyCfg-initial
+
+fn opt2(_1: &Result<u32, Void>) -> &u32 {
+    debug res => _1;
+    let mut _0: &u32;
+    let mut _2: isize;
+    let _3: &u32;
+    scope 1 {
+        debug x => _3;
+    }
+
+    bb0: {
+        PlaceMention(_1);
+        _2 = discriminant((*_1));
+        switchInt(move _2) -> [0: bb2, 1: bb3, otherwise: bb1];
+    }
+
+    bb1: {
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
+    }
+
+    bb2: {
+        StorageLive(_3);
+        _3 = &(((*_1) as Ok).0: u32);
+        _0 = &(*_3);
+        StorageDead(_3);
+        return;
+    }
+
+    bb3: {
+        FakeRead(ForMatchedPlace(None), (((*_1) as Err).0: Void));
+        unreachable;
+    }
+}

tests/mir-opt/building/match/never_patterns.opt3.SimplifyCfg-initial.after.mir

@@ -0,0 +1,35 @@
+// MIR for `opt3` after SimplifyCfg-initial
+
+fn opt3(_1: &Result<u32, Void>) -> &u32 {
+    debug res => _1;
+    let mut _0: &u32;
+    let mut _2: isize;
+    let _3: &u32;
+    scope 1 {
+        debug x => _3;
+    }
+
+    bb0: {
+        PlaceMention(_1);
+        _2 = discriminant((*_1));
+        switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1];
+    }
+
+    bb1: {
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
+    }
+
+    bb2: {
+        FakeRead(ForMatchedPlace(None), (((*_1) as Err).0: Void));
+        unreachable;
+    }
+
+    bb3: {
+        StorageLive(_3);
+        _3 = &(((*_1) as Ok).0: u32);
+        _0 = &(*_3);
+        StorageDead(_3);
+        return;
+    }
+}

tests/mir-opt/building/match/never_patterns.rs

@@ -0,0 +1,44 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum Void {}
+
+// EMIT_MIR never_patterns.opt1.SimplifyCfg-initial.after.mir
+fn opt1(res: &Result<u32, Void>) -> &u32 {
+    // CHECK-LABEL: fn opt1(
+    // CHECK: bb0: {
+    // CHECK-NOT: {{bb.*}}: {
+    // CHECK: return;
+    match res {
+        Ok(x) => x,
+        Err(!),
+    }
+}
+
+// EMIT_MIR never_patterns.opt2.SimplifyCfg-initial.after.mir
+fn opt2(res: &Result<u32, Void>) -> &u32 {
+    // CHECK-LABEL: fn opt2(
+    // CHECK: bb0: {
+    // CHECK-NOT: {{bb.*}}: {
+    // CHECK: return;
+    match res {
+        Ok(x) | Err(!) => x,
+    }
+}
+
+// EMIT_MIR never_patterns.opt3.SimplifyCfg-initial.after.mir
+fn opt3(res: &Result<u32, Void>) -> &u32 {
+    // CHECK-LABEL: fn opt3(
+    // CHECK: bb0: {
+    // CHECK-NOT: {{bb.*}}: {
+    // CHECK: return;
+    match res {
+        Err(!) | Ok(x) => x,
+    }
+}
+
+fn main() {
+    assert_eq!(opt1(&Ok(0)), &0);
+    assert_eq!(opt2(&Ok(0)), &0);
+    assert_eq!(opt3(&Ok(0)), &0);
+}

tests/ui/rfcs/rfc-0000-never_patterns/check.rs

@@ -1,3 +1,4 @@
+// Check that never patterns can't have bodies or guards.
 #![feature(never_patterns)]
 #![allow(incomplete_features)]