Auto merge of #45384 - mikhail-m1:mir_add_false_edges_terminator_kind, r=arielb1

add TerminatorKind::FalseEdges and use it in matches

impl #45184 and fixes #45043 right way.

False edges unexpectedly affects uninitialized variables analysis in MIR borrowck.

Author: bors

src/librustc/ich/impls_mir.rs

@@ -62,7 +62,8 @@ for mir::Terminator<'gcx> {
             mir::TerminatorKind::Drop { .. } |
             mir::TerminatorKind::DropAndReplace { .. } |
             mir::TerminatorKind::Yield { .. } |
-            mir::TerminatorKind::Call { .. } => false,
+            mir::TerminatorKind::Call { .. } |
+            mir::TerminatorKind::FalseEdges { .. } => false,
         };
 
         if hash_spans_unconditionally {
@@ -210,6 +211,12 @@ for mir::TerminatorKind<'gcx> {
                 target.hash_stable(hcx, hasher);
                 cleanup.hash_stable(hcx, hasher);
             }
+            mir::TerminatorKind::FalseEdges { ref real_target, ref imaginary_targets } => {
+                real_target.hash_stable(hcx, hasher);
+                for target in imaginary_targets {
+                    target.hash_stable(hcx, hasher);
+                }
+            }
         }
     }
 }

src/librustc/mir/mod.rs

@@ -682,6 +682,11 @@ pub enum TerminatorKind<'tcx> {
 
     /// Indicates the end of the dropping of a generator
     GeneratorDrop,
+
+    FalseEdges {
+        real_target: BasicBlock,
+        imaginary_targets: Vec<BasicBlock>
+    },
 }
 
 impl<'tcx> Terminator<'tcx> {
@@ -731,6 +736,11 @@ impl<'tcx> TerminatorKind<'tcx> {
             }
             Assert { target, cleanup: Some(unwind), .. } => vec![target, unwind].into_cow(),
             Assert { ref target, .. } => slice::ref_slice(target).into_cow(),
+            FalseEdges { ref real_target, ref imaginary_targets } => {
+                let mut s = vec![*real_target];
+                s.extend_from_slice(imaginary_targets);
+                s.into_cow()
+            }
         }
     }
 
@@ -757,7 +767,12 @@ impl<'tcx> TerminatorKind<'tcx> {
                 vec![target]
             }
             Assert { ref mut target, cleanup: Some(ref mut unwind), .. } => vec![target, unwind],
-            Assert { ref mut target, .. } => vec![target]
+            Assert { ref mut target, .. } => vec![target],
+            FalseEdges { ref mut real_target, ref mut imaginary_targets } => {
+                let mut s = vec![real_target];
+                s.extend(imaginary_targets.iter_mut());
+                s
+            }
         }
     }
 }
@@ -874,7 +889,8 @@ impl<'tcx> TerminatorKind<'tcx> {
                 }
 
                 write!(fmt, ")")
-            }
+            },
+            FalseEdges { .. } => write!(fmt, "falseEdges")
         }
     }
 
@@ -910,7 +926,12 @@ impl<'tcx> TerminatorKind<'tcx> {
             }
             Assert { cleanup: None, .. } => vec!["".into()],
             Assert { .. } =>
-                vec!["success".into_cow(), "unwind".into_cow()]
+                vec!["success".into_cow(), "unwind".into_cow()],
+            FalseEdges { ref imaginary_targets, .. } => {
+                let mut l = vec!["real".into()];
+                l.resize(imaginary_targets.len() + 1, "imaginary".into());
+                l
+            }
         }
     }
 }
@@ -1878,6 +1899,8 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
             Resume => Resume,
             Return => Return,
             Unreachable => Unreachable,
+            FalseEdges { real_target, ref imaginary_targets } =>
+                FalseEdges { real_target, imaginary_targets: imaginary_targets.clone() }
         };
         Terminator {
             source_info: self.source_info,
@@ -1917,7 +1940,8 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
             Resume |
             Return |
             GeneratorDrop |
-            Unreachable => false
+            Unreachable |
+            FalseEdges { .. } => false
         }
     }
 }

src/librustc/mir/visit.rs

@@ -486,8 +486,15 @@ macro_rules! make_mir_visitor {
                         self.visit_operand(value, source_location);
                         self.visit_branch(block, resume);
                         drop.map(|t| self.visit_branch(block, t));
+
                     }
 
+                    TerminatorKind::FalseEdges { real_target, ref imaginary_targets } => {
+                        self.visit_branch(block, real_target);
+                        for target in imaginary_targets {
+                            self.visit_branch(block, *target);
+                        }
+                    }
                 }
             }
 

src/librustc_mir/borrow_check.rs

@@ -364,7 +364,8 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> DataflowResultsConsumer<'b, 'tcx>
             TerminatorKind::Resume |
             TerminatorKind::Return |
             TerminatorKind::GeneratorDrop |
-            TerminatorKind::Unreachable => {
+            TerminatorKind::Unreachable |
+            TerminatorKind::FalseEdges { .. } => {
                 // no data used, thus irrelevant to borrowck
             }
         }

src/librustc_mir/build/matches/mod.rs

@@ -21,7 +21,7 @@ use rustc::mir::*;
 use rustc::hir;
 use hair::*;
 use syntax::ast::{Name, NodeId};
-use syntax_pos::{DUMMY_SP, Span};
+use syntax_pos::Span;
 
 // helper functions, broken out by category:
 mod simplify;
@@ -54,29 +54,43 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             (body, scope.unwrap_or(self.visibility_scope))
         }).collect();
 
+        // create binding start block for link them by false edges
+        let candidate_count = arms.iter().fold(0, |ac, c| ac + c.patterns.len());
+        let pre_binding_blocks: Vec<_> = (0..candidate_count + 1)
+            .map(|_| self.cfg.start_new_block()).collect();
+
         // assemble a list of candidates: there is one candidate per
         // pattern, which means there may be more than one candidate
         // *per arm*. These candidates are kept sorted such that the
         // highest priority candidate comes first in the list.
         // (i.e. same order as in source)
+
         let candidates: Vec<_> =
             arms.iter()
                 .enumerate()
                 .flat_map(|(arm_index, arm)| {
                     arm.patterns.iter()
                                 .map(move |pat| (arm_index, pat, arm.guard.clone()))
                 })
-                .map(|(arm_index, pattern, guard)| {
+                .zip(pre_binding_blocks.iter().zip(pre_binding_blocks.iter().skip(1)))
+                .map(|((arm_index, pattern, guard),
+                       (pre_binding_block, next_candidate_pre_binding_block))| {
                     Candidate {
                         span: pattern.span,
                         match_pairs: vec![MatchPair::new(discriminant_lvalue.clone(), pattern)],
                         bindings: vec![],
                         guard,
                         arm_index,
+                        pre_binding_block: *pre_binding_block,
+                        next_candidate_pre_binding_block: *next_candidate_pre_binding_block,
                     }
                 })
                 .collect();
 
+        let outer_source_info = self.source_info(span);
+        self.cfg.terminate(*pre_binding_blocks.last().unwrap(),
+                           outer_source_info, TerminatorKind::Unreachable);
+
         // this will generate code to test discriminant_lvalue and
         // branch to the appropriate arm block
         let otherwise = self.match_candidates(span, &mut arm_blocks, candidates, block);
@@ -148,7 +162,11 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
             bindings: vec![],
             guard: None,
-            arm_index: 0, // since we don't call `match_candidates`, this field is unused
+
+            // since we don't call `match_candidates`, next fields is unused
+            arm_index: 0,
+            pre_binding_block: block,
+            next_candidate_pre_binding_block: block
         };
 
         // Simplify the candidate. Since the pattern is irrefutable, this should
@@ -278,6 +296,10 @@ pub struct Candidate<'pat, 'tcx:'pat> {
 
     // ...and then we branch to arm with this index.
     arm_index: usize,
+
+    // ...and the blocks for add false edges between candidates
+    pre_binding_block: BasicBlock,
+    next_candidate_pre_binding_block: BasicBlock,
 }
 
 #[derive(Clone, Debug)]
@@ -398,17 +420,43 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             candidates.iter().take_while(|c| c.match_pairs.is_empty()).count();
         debug!("match_candidates: {:?} candidates fully matched", fully_matched);
         let mut unmatched_candidates = candidates.split_off(fully_matched);
-        for (index, candidate) in candidates.into_iter().enumerate() {
+
+        let fully_matched_with_guard =
+            candidates.iter().take_while(|c| c.guard.is_some()).count();
+
+        let unreachable_candidates = if fully_matched_with_guard + 1 < candidates.len() {
+            candidates.split_off(fully_matched_with_guard + 1)
+        } else {
+            vec![]
+        };
+
+        for candidate in candidates {
             // If so, apply any bindings, test the guard (if any), and
             // branch to the arm.
-            let is_last = index == fully_matched - 1;
-            if let Some(b) = self.bind_and_guard_matched_candidate(block, arm_blocks,
-                                                                   candidate, is_last) {
+            if let Some(b) = self.bind_and_guard_matched_candidate(block, arm_blocks, candidate) {
                 block = b;
             } else {
                 // if None is returned, then any remaining candidates
                 // are unreachable (at least not through this path).
-                return vec![];
+                // Link them with false edges.
+                debug!("match_candidates: add false edges for unreachable {:?} and unmatched {:?}",
+                       unreachable_candidates, unmatched_candidates);
+                for candidate in unreachable_candidates {
+                    let source_info = self.source_info(candidate.span);
+                    let target = self.cfg.start_new_block();
+                    if let Some(otherwise) = self.bind_and_guard_matched_candidate(target,
+                                                                                   arm_blocks,
+                                                                                   candidate) {
+                        self.cfg.terminate(otherwise, source_info, TerminatorKind::Unreachable);
+                    }
+                }
+
+                if unmatched_candidates.is_empty() {
+                    return vec![]
+                } else {
+                    let target = self.cfg.start_new_block();
+                    return self.match_candidates(span, arm_blocks, unmatched_candidates, target);
+                }
             }
         }
 
@@ -423,9 +471,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             self.test_candidates(span, arm_blocks, &unmatched_candidates, block);
 
         // If the target candidates were exhaustive, then we are done.
-        if otherwise.is_empty() {
-            return vec![];
-        }
+        // But for borrowck continue build decision tree.
 
         // If all candidates were sorted into `target_candidates` somewhere, then
         // the initial set was inexhaustive.
@@ -666,48 +712,50 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
     fn bind_and_guard_matched_candidate<'pat>(&mut self,
                                               mut block: BasicBlock,
                                               arm_blocks: &mut ArmBlocks,
-                                              candidate: Candidate<'pat, 'tcx>,
-                                              is_last_arm: bool)
+                                              candidate: Candidate<'pat, 'tcx>)
                                               -> Option<BasicBlock> {
         debug!("bind_and_guard_matched_candidate(block={:?}, candidate={:?})",
                block, candidate);
 
         debug_assert!(candidate.match_pairs.is_empty());
 
-        self.bind_matched_candidate(block, candidate.bindings);
-
         let arm_block = arm_blocks.blocks[candidate.arm_index];
+        let candidate_source_info = self.source_info(candidate.span);
+
+        self.cfg.terminate(block, candidate_source_info,
+                               TerminatorKind::Goto { target: candidate.pre_binding_block });
+
+        block = self.cfg.start_new_block();
+        self.cfg.terminate(candidate.pre_binding_block, candidate_source_info,
+                               TerminatorKind::FalseEdges {
+                                   real_target: block,
+                                   imaginary_targets:
+                                       vec![candidate.next_candidate_pre_binding_block]});
+
+        self.bind_matched_candidate(block, candidate.bindings);
 
         if let Some(guard) = candidate.guard {
             // the block to branch to if the guard fails; if there is no
             // guard, this block is simply unreachable
             let guard = self.hir.mirror(guard);
             let source_info = self.source_info(guard.span);
             let cond = unpack!(block = self.as_local_operand(block, guard));
-            let otherwise = self.cfg.start_new_block();
+
+            let false_edge_block = self.cfg.start_new_block();
             self.cfg.terminate(block, source_info,
-                               TerminatorKind::if_(self.hir.tcx(), cond, arm_block, otherwise));
-            Some(otherwise)
-        } else if !is_last_arm {
-            // Add always true guard in case of more than one arm
-            // it creates false edges and allow MIR borrowck detects errors
-            // FIXME(#45184) -- permit "false edges"
-            let source_info = self.source_info(candidate.span);
-            let true_expr = Expr {
-                temp_lifetime: None,
-                ty: self.hir.tcx().types.bool,
-                span: DUMMY_SP,
-                kind: ExprKind::Literal{literal: self.hir.true_literal()},
-            };
-            let cond = unpack!(block = self.as_local_operand(block, true_expr));
+                               TerminatorKind::if_(self.hir.tcx(), cond, arm_block,
+                                   false_edge_block));
+
             let otherwise = self.cfg.start_new_block();
-            self.cfg.terminate(block, source_info,
-                               TerminatorKind::if_(self.hir.tcx(), cond, arm_block, otherwise));
+            self.cfg.terminate(false_edge_block, source_info,
+                               TerminatorKind::FalseEdges {
+                                   real_target: otherwise,
+                                   imaginary_targets:
+                                       vec![candidate.next_candidate_pre_binding_block] });
             Some(otherwise)
         } else {
-            let source_info = self.source_info(candidate.span);
-            self.cfg.terminate(block, source_info,
-                               TerminatorKind::Goto { target: arm_block  });
+            self.cfg.terminate(block, candidate_source_info,
+                               TerminatorKind::Goto { target: arm_block });
             None
         }
     }

src/librustc_mir/build/matches/test.rs

@@ -598,6 +598,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             bindings: candidate.bindings.clone(),
             guard: candidate.guard.clone(),
             arm_index: candidate.arm_index,
+            pre_binding_block: candidate.pre_binding_block,
+            next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
         }
     }
 
@@ -659,6 +661,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             bindings: candidate.bindings.clone(),
             guard: candidate.guard.clone(),
             arm_index: candidate.arm_index,
+            pre_binding_block: candidate.pre_binding_block,
+            next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
         }
     }
 

src/librustc_mir/dataflow/mod.rs

@@ -721,6 +721,12 @@ impl<'a, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D> where D: BitDenotation
                     self.propagate_bits_into_entry_set_for(in_out, changed, dest_bb);
                 }
             }
+            mir::TerminatorKind::FalseEdges { ref real_target, ref imaginary_targets } => {
+                self.propagate_bits_into_entry_set_for(in_out, changed, real_target);
+                for target in imaginary_targets {
+                    self.propagate_bits_into_entry_set_for(in_out, changed, target);
+                }
+            }
         }
     }
 

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -305,6 +305,7 @@ impl<'b, 'a, 'gcx, 'tcx> Gatherer<'b, 'a, 'gcx, 'tcx> {
             TerminatorKind::Goto { target: _ } |
             TerminatorKind::Resume |
             TerminatorKind::GeneratorDrop |
+            TerminatorKind::FalseEdges { .. } |
             TerminatorKind::Unreachable => { }
 
             TerminatorKind::Return => {

src/librustc_mir/transform/check_unsafety.rs

@@ -73,7 +73,8 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
             TerminatorKind::GeneratorDrop |
             TerminatorKind::Resume |
             TerminatorKind::Return |
-            TerminatorKind::Unreachable => {
+            TerminatorKind::Unreachable |
+            TerminatorKind::FalseEdges { .. } => {
                                 // safe (at least as emitted during MIR construction)
             }