Auto merge of #127159 - Nadrieril:hide-candidate, r=<try>

match lowering: Hide `Candidate` from outside the lowering algorithm

The internals of `Candidate` are tricky and a source of confusion. This PR makes it so we don't expose `Candidate`s outside the lowering algorithm. Now:
- false edges are handled in `lower_match_tree`;
- `lower_match_tree` takes a list of patterns as input;
- `lower_match_tree` returns a flat datastructure that contains only the necessary information.

r? `@matthewjasper`

try-job: x86_64-rust-for-linux

Author: bors

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

@@ -1,5 +1,3 @@
-use std::marker::PhantomData;
-
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_middle::mir::*;
 use rustc_middle::ty::Ty;
@@ -18,18 +16,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         from_block: BasicBlock,
         real_target: BasicBlock,
-        imaginary_target: Option<BasicBlock>,
+        imaginary_target: BasicBlock,
         source_info: SourceInfo,
     ) {
-        match imaginary_target {
-            Some(target) if target != real_target => {
-                self.cfg.terminate(
-                    from_block,
-                    source_info,
-                    TerminatorKind::FalseEdge { real_target, imaginary_target: target },
-                );
-            }
-            _ => self.cfg.goto(from_block, source_info, real_target),
+        if imaginary_target != real_target {
+            self.cfg.terminate(
+                from_block,
+                source_info,
+                TerminatorKind::FalseEdge { real_target, imaginary_target },
+            );
+        } else {
+            self.cfg.goto(from_block, source_info, real_target)
         }
     }
 }
@@ -71,10 +68,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 ///    a MIR pass run after borrow checking.
 pub(super) fn collect_fake_borrows<'tcx>(
     cx: &mut Builder<'_, 'tcx>,
-    candidates: &[&mut Candidate<'_, 'tcx>],
+    candidates: &[Candidate<'_, 'tcx>],
     temp_span: Span,
     scrutinee_base: PlaceBase,
 ) -> Vec<(Place<'tcx>, Local, FakeBorrowKind)> {
+    if candidates.iter().all(|candidate| !candidate.has_guard) {
+        // Fake borrows are only used when there is a guard.
+        return Vec::new();
+    }
     let mut collector =
         FakeBorrowCollector { cx, scrutinee_base, fake_borrows: FxIndexMap::default() };
     for candidate in candidates.iter() {
@@ -222,57 +223,6 @@ impl<'a, 'b, 'tcx> FakeBorrowCollector<'a, 'b, 'tcx> {
     }
 }
 
-/// Visit all the bindings of these candidates. Because or-alternatives bind the same variables, we
-/// only explore the first one of each or-pattern.
-pub(super) fn visit_bindings<'tcx>(
-    candidates: &[&mut Candidate<'_, 'tcx>],
-    f: impl FnMut(&Binding<'tcx>),
-) {
-    let mut visitor = BindingsVisitor { f, phantom: PhantomData };
-    for candidate in candidates.iter() {
-        visitor.visit_candidate(candidate);
-    }
-}
-
-pub(super) struct BindingsVisitor<'tcx, F> {
-    f: F,
-    phantom: PhantomData<&'tcx ()>,
-}
-
-impl<'tcx, F> BindingsVisitor<'tcx, F>
-where
-    F: FnMut(&Binding<'tcx>),
-{
-    fn visit_candidate(&mut self, candidate: &Candidate<'_, 'tcx>) {
-        for binding in &candidate.extra_data.bindings {
-            (self.f)(binding)
-        }
-        for match_pair in &candidate.match_pairs {
-            self.visit_match_pair(match_pair);
-        }
-    }
-
-    fn visit_flat_pat(&mut self, flat_pat: &FlatPat<'_, 'tcx>) {
-        for binding in &flat_pat.extra_data.bindings {
-            (self.f)(binding)
-        }
-        for match_pair in &flat_pat.match_pairs {
-            self.visit_match_pair(match_pair);
-        }
-    }
-
-    fn visit_match_pair(&mut self, match_pair: &MatchPairTree<'_, 'tcx>) {
-        if let TestCase::Or { pats, .. } = &match_pair.test_case {
-            // All the or-alternatives should bind the same locals, so we only visit the first one.
-            self.visit_flat_pat(&pats[0])
-        } else {
-            for subpair in &match_pair.subpairs {
-                self.visit_match_pair(subpair);
-            }
-        }
-    }
-}
-
 #[must_use]
 pub(crate) fn ref_pat_borrow_kind(ref_mutability: Mutability) -> BorrowKind {
     match ref_mutability {

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

@@ -37,11 +37,11 @@ fn full_tested_match() -> () {
     }
 
     bb2: {
-        falseEdge -> [real: bb7, imaginary: bb3];
+        falseEdge -> [real: bb8, imaginary: bb3];
     }
 
     bb3: {
-        falseEdge -> [real: bb12, imaginary: bb5];
+        falseEdge -> [real: bb7, imaginary: bb5];
     }
 
     bb4: {
@@ -50,26 +50,41 @@ fn full_tested_match() -> () {
 
     bb5: {
         _1 = (const 3_i32, const 3_i32);
-        goto -> bb13;
+        goto -> bb14;
     }
 
     bb6: {
         goto -> bb1;
     }
 
     bb7: {
+        StorageLive(_9);
+        _9 = ((_2 as Some).0: i32);
+        StorageLive(_10);
+        _10 = _9;
+        _1 = (const 2_i32, move _10);
+        StorageDead(_10);
+        StorageDead(_9);
+        goto -> bb14;
+    }
+
+    bb8: {
         StorageLive(_6);
         _6 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_7);
-        _7 = guard() -> [return: bb8, unwind: bb15];
+        _7 = guard() -> [return: bb10, unwind: bb16];
     }
 
-    bb8: {
-        switchInt(move _7) -> [0: bb10, otherwise: bb9];
+    bb9: {
+        goto -> bb3;
     }
 
-    bb9: {
+    bb10: {
+        switchInt(move _7) -> [0: bb12, otherwise: bb11];
+    }
+
+    bb11: {
         StorageDead(_7);
         FakeRead(ForMatchGuard, _3);
         FakeRead(ForGuardBinding, _6);
@@ -81,44 +96,33 @@ fn full_tested_match() -> () {
         StorageDead(_8);
         StorageDead(_5);
         StorageDead(_6);
-        goto -> bb13;
+        goto -> bb14;
     }
 
-    bb10: {
-        goto -> bb11;
+    bb12: {
+        goto -> bb13;
     }
 
-    bb11: {
+    bb13: {
         StorageDead(_7);
         StorageDead(_6);
-        goto -> bb3;
-    }
-
-    bb12: {
-        StorageLive(_9);
-        _9 = ((_2 as Some).0: i32);
-        StorageLive(_10);
-        _10 = _9;
-        _1 = (const 2_i32, move _10);
-        StorageDead(_10);
-        StorageDead(_9);
-        goto -> bb13;
+        goto -> bb9;
     }
 
-    bb13: {
+    bb14: {
         PlaceMention(_1);
         StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;
     }
 
-    bb14: {
+    bb15: {
         FakeRead(ForMatchedPlace(None), _1);
         unreachable;
     }
 
-    bb15 (cleanup): {
+    bb16 (cleanup): {
         resume;
     }
 }

tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir

@@ -37,7 +37,7 @@ fn full_tested_match2() -> () {
     }
 
     bb2: {
-        falseEdge -> [real: bb7, imaginary: bb5];
+        falseEdge -> [real: bb8, imaginary: bb5];
     }
 
     bb3: {
@@ -48,34 +48,43 @@ fn full_tested_match2() -> () {
         _1 = (const 2_i32, move _10);
         StorageDead(_10);
         StorageDead(_9);
-        goto -> bb13;
+        goto -> bb14;
     }
 
     bb4: {
         goto -> bb1;
     }
 
     bb5: {
-        falseEdge -> [real: bb12, imaginary: bb3];
+        falseEdge -> [real: bb7, imaginary: bb3];
     }
 
     bb6: {
         goto -> bb1;
     }
 
     bb7: {
+        _1 = (const 3_i32, const 3_i32);
+        goto -> bb14;
+    }
+
+    bb8: {
         StorageLive(_6);
         _6 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_7);
-        _7 = guard() -> [return: bb8, unwind: bb15];
+        _7 = guard() -> [return: bb10, unwind: bb16];
     }
 
-    bb8: {
-        switchInt(move _7) -> [0: bb10, otherwise: bb9];
+    bb9: {
+        falseEdge -> [real: bb3, imaginary: bb5];
     }
 
-    bb9: {
+    bb10: {
+        switchInt(move _7) -> [0: bb12, otherwise: bb11];
+    }
+
+    bb11: {
         StorageDead(_7);
         FakeRead(ForMatchGuard, _3);
         FakeRead(ForGuardBinding, _6);
@@ -87,38 +96,33 @@ fn full_tested_match2() -> () {
         StorageDead(_8);
         StorageDead(_5);
         StorageDead(_6);
-        goto -> bb13;
+        goto -> bb14;
     }
 
-    bb10: {
-        goto -> bb11;
+    bb12: {
+        goto -> bb13;
     }
 
-    bb11: {
+    bb13: {
         StorageDead(_7);
         StorageDead(_6);
-        falseEdge -> [real: bb3, imaginary: bb5];
-    }
-
-    bb12: {
-        _1 = (const 3_i32, const 3_i32);
-        goto -> bb13;
+        goto -> bb9;
     }
 
-    bb13: {
+    bb14: {
         PlaceMention(_1);
         StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;
     }
 
-    bb14: {
+    bb15: {
         FakeRead(ForMatchedPlace(None), _1);
         unreachable;
     }
 
-    bb15 (cleanup): {
+    bb16 (cleanup): {
         resume;
     }
 }