mir: Add TerminatorKind::FalseUnwind

Sometimes a simple goto misses the cleanup/unwind edges. Specifically, in the
case of infinite loops such as those introduced by a loop statement without any
other out edges. Analogous to TerminatorKind::FalseEdges; this new terminator
kind is used when we want borrowck to consider an unwind path, but real control
flow should never actually take it.

src/librustc/ich/impls_mir.rs

@@ -201,6 +201,10 @@ for mir::TerminatorKind<'gcx> {
                     target.hash_stable(hcx, hasher);
                 }
             }
+            mir::TerminatorKind::FalseUnwind { ref real_target, ref unwind } => {
+                real_target.hash_stable(hcx, hasher);
+                unwind.hash_stable(hcx, hasher);
+            }
         }
     }
 }

src/librustc/mir/mod.rs

@@ -803,9 +803,28 @@ pub enum TerminatorKind<'tcx> {
     /// Indicates the end of the dropping of a generator
     GeneratorDrop,
 
+    /// A block where control flow only ever takes one real path, but borrowck
+    /// needs to be more conservative.
     FalseEdges {
+        /// The target normal control flow will take
         real_target: BasicBlock,
-        imaginary_targets: Vec<BasicBlock>
+        /// The list of blocks control flow could conceptually take, but won't
+        /// in practice
+        imaginary_targets: Vec<BasicBlock>,
+    },
+    /// A terminator for blocks that only take one path in reality, but where we
+    /// reserve the right to unwind in borrowck, even if it won't happen in practice.
+    /// This can arise in infinite loops with no function calls for example.
+    FalseUnwind {
+        /// The target normal control flow will take
+        real_target: BasicBlock,
+        /// The imaginary cleanup block link. This particular path will never be taken
+        /// in practice, but in order to avoid fragility we want to always
+        /// consider it in borrowck. We don't want to accept programs which
+        /// pass borrowck only when panic=abort or some assertions are disabled
+        /// due to release vs. debug mode builds. This needs to be an Option because
+        /// of the remove_noop_landing_pads and no_landing_pads passes
+        unwind: Option<BasicBlock>,
     },
 }
 
@@ -865,6 +884,8 @@ impl<'tcx> TerminatorKind<'tcx> {
                 s.extend_from_slice(imaginary_targets);
                 s.into_cow()
             }
+            FalseUnwind { real_target: t, unwind: Some(u) } => vec![t, u].into_cow(),
+            FalseUnwind { real_target: ref t, unwind: None } => slice::from_ref(t).into_cow(),
         }
     }
 
@@ -897,6 +918,8 @@ impl<'tcx> TerminatorKind<'tcx> {
                 s.extend(imaginary_targets.iter_mut());
                 s
             }
+            FalseUnwind { real_target: ref mut t, unwind: Some(ref mut u) } => vec![t, u],
+            FalseUnwind { ref mut real_target, unwind: None } => vec![real_target],
         }
     }
 
@@ -916,7 +939,8 @@ impl<'tcx> TerminatorKind<'tcx> {
             TerminatorKind::Call { cleanup: ref mut unwind, .. } |
             TerminatorKind::Assert { cleanup: ref mut unwind, .. } |
             TerminatorKind::DropAndReplace { ref mut unwind, .. } |
-            TerminatorKind::Drop { ref mut unwind, .. } => {
+            TerminatorKind::Drop { ref mut unwind, .. } |
+            TerminatorKind::FalseUnwind { ref mut unwind, .. } => {
                 Some(unwind)
             }
         }
@@ -1045,7 +1069,8 @@ impl<'tcx> TerminatorKind<'tcx> {
 
                 write!(fmt, ")")
             },
-            FalseEdges { .. } => write!(fmt, "falseEdges")
+            FalseEdges { .. } => write!(fmt, "falseEdges"),
+            FalseUnwind { .. } => write!(fmt, "falseUnwind"),
         }
     }
 
@@ -1087,6 +1112,8 @@ impl<'tcx> TerminatorKind<'tcx> {
                 l.resize(imaginary_targets.len() + 1, "imaginary".into());
                 l
             }
+            FalseUnwind { unwind: Some(_), .. } => vec!["real".into(), "cleanup".into()],
+            FalseUnwind { unwind: None, .. } => vec!["real".into()],
         }
     }
 }
@@ -2189,7 +2216,8 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
             Return => Return,
             Unreachable => Unreachable,
             FalseEdges { real_target, ref imaginary_targets } =>
-                FalseEdges { real_target, imaginary_targets: imaginary_targets.clone() }
+                FalseEdges { real_target, imaginary_targets: imaginary_targets.clone() },
+            FalseUnwind { real_target, unwind } => FalseUnwind { real_target, unwind },
         };
         Terminator {
             source_info: self.source_info,
@@ -2231,7 +2259,8 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
             Return |
             GeneratorDrop |
             Unreachable |
-            FalseEdges { .. } => false
+            FalseEdges { .. } |
+            FalseUnwind { .. } => false
         }
     }
 }

src/librustc/mir/visit.rs

@@ -495,15 +495,21 @@ 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 } => {
+                    TerminatorKind::FalseEdges { real_target, ref imaginary_targets} => {
                         self.visit_branch(block, real_target);
                         for target in imaginary_targets {
                             self.visit_branch(block, *target);
                         }
                     }
+
+                    TerminatorKind::FalseUnwind { real_target, unwind } => {
+                        self.visit_branch(block, real_target);
+                        if let Some(unwind) = unwind {
+                            self.visit_branch(block, unwind);
+                        }
+                    }
                 }
             }
 

src/librustc_mir/borrow_check/mod.rs

@@ -576,7 +576,8 @@ impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx
             TerminatorKind::Goto { target: _ }
             | TerminatorKind::Abort
             | TerminatorKind::Unreachable
-            | TerminatorKind::FalseEdges { .. } => {
+            | TerminatorKind::FalseEdges { real_target: _, imaginary_targets: _ }
+            | TerminatorKind::FalseUnwind { real_target: _, unwind: _ } => {
                 // no data used, thus irrelevant to borrowck
             }
         }

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -796,7 +796,8 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
             | TerminatorKind::GeneratorDrop
             | TerminatorKind::Unreachable
             | TerminatorKind::Drop { .. }
-            | TerminatorKind::FalseEdges { .. } => {
+            | TerminatorKind::FalseEdges { .. }
+            | TerminatorKind::FalseUnwind { .. } => {
                 // no checks needed for these
             }
 
@@ -1152,6 +1153,18 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     self.assert_iscleanup(mir, block_data, *target, is_cleanup);
                 }
             }
+            TerminatorKind::FalseUnwind {
+                real_target,
+                unwind
+            } => {
+                self.assert_iscleanup(mir, block_data, real_target, is_cleanup);
+                if let Some(unwind) = unwind {
+                    if is_cleanup {
+                        span_mirbug!(self, block_data, "cleanup in cleanup block via false unwind");
+                    }
+                    self.assert_iscleanup(mir, block_data, unwind, true);
+                }
+            }
         }
     }
 

src/librustc_mir/build/matches/mod.rs

@@ -728,7 +728,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                                TerminatorKind::FalseEdges {
                                    real_target: block,
                                    imaginary_targets:
-                                       vec![candidate.next_candidate_pre_binding_block]});
+                                       vec![candidate.next_candidate_pre_binding_block],
+                               });
 
         self.bind_matched_candidate(block, candidate.bindings);
 
@@ -749,7 +750,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                                TerminatorKind::FalseEdges {
                                    real_target: otherwise,
                                    imaginary_targets:
-                                       vec![candidate.next_candidate_pre_binding_block] });
+                                       vec![candidate.next_candidate_pre_binding_block],
+                               });
             Some(otherwise)
         } else {
             self.cfg.terminate(block, candidate_source_info,

src/librustc_mir/dataflow/impls/borrows.rs

@@ -517,6 +517,7 @@ impl<'a, 'gcx, 'tcx> Borrows<'a, 'gcx, 'tcx> {
             mir::TerminatorKind::Yield {..} |
             mir::TerminatorKind::Goto {..} |
             mir::TerminatorKind::FalseEdges {..} |
+            mir::TerminatorKind::FalseUnwind {..} |
             mir::TerminatorKind::Unreachable => {}
         }
     }

src/librustc_mir/dataflow/mod.rs

@@ -864,6 +864,14 @@ impl<'a, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D> where D: BitDenotation
                     self.propagate_bits_into_entry_set_for(in_out, changed, target);
                 }
             }
+            mir::TerminatorKind::FalseUnwind { ref real_target, unwind } => {
+                self.propagate_bits_into_entry_set_for(in_out, changed, real_target);
+                if let Some(ref unwind) = unwind {
+                    if !self.dead_unwinds.contains(&bb) {
+                        self.propagate_bits_into_entry_set_for(in_out, changed, unwind);
+                    }
+                }
+            }
         }
     }
 

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -346,6 +346,7 @@ impl<'b, 'a, 'gcx, 'tcx> Gatherer<'b, 'a, 'gcx, 'tcx> {
             TerminatorKind::Abort |
             TerminatorKind::GeneratorDrop |
             TerminatorKind::FalseEdges { .. } |
+            TerminatorKind::FalseUnwind { .. } |
             TerminatorKind::Unreachable => { }
 
             TerminatorKind::Return => {

src/librustc_mir/interpret/terminator/mod.rs

@@ -165,6 +165,7 @@ impl<'a, 'tcx, M: Machine<'tcx>> EvalContext<'a, 'tcx, M> {
             Resume => unimplemented!(),
             Abort => unimplemented!(),
             FalseEdges { .. } => bug!("should have been eliminated by `simplify_branches` mir pass"),
+            FalseUnwind { .. } => bug!("should have been eliminated by `simplify_branches` mir pass"),
             Unreachable => return err!(Unreachable),
         }
 

src/librustc_mir/monomorphize/collector.rs

@@ -636,7 +636,8 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
             mir::TerminatorKind::Assert { .. } => {}
             mir::TerminatorKind::GeneratorDrop |
             mir::TerminatorKind::Yield { .. } |
-            mir::TerminatorKind::FalseEdges { .. } => bug!(),
+            mir::TerminatorKind::FalseEdges { .. } |
+            mir::TerminatorKind::FalseUnwind { .. } => bug!(),
         }
 
         self.super_terminator_kind(block, kind, location);

src/librustc_mir/transform/check_unsafety.rs

@@ -76,7 +76,8 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
             TerminatorKind::Abort |
             TerminatorKind::Return |
             TerminatorKind::Unreachable |
-            TerminatorKind::FalseEdges { .. } => {
+            TerminatorKind::FalseEdges { .. } |
+            TerminatorKind::FalseUnwind { .. } => {
                 // safe (at least as emitted during MIR construction)
             }
 

src/librustc_mir/transform/inline.rs

@@ -813,6 +813,9 @@ impl<'a, 'tcx> MutVisitor<'tcx> for Integrator<'a, 'tcx> {
                     *target = self.update_target(*target);
                 }
             }
+            TerminatorKind::FalseUnwind { real_target: _ , unwind: _ } =>
+                // see the ordering of passes in the optimized_mir query.
+                bug!("False unwinds should have been removed before inlining")
         }
     }
 

src/librustc_mir/transform/qualify_consts.rs

@@ -327,7 +327,8 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                 TerminatorKind::GeneratorDrop |
                 TerminatorKind::Yield { .. } |
                 TerminatorKind::Unreachable |
-                TerminatorKind::FalseEdges { .. } => None,
+                TerminatorKind::FalseEdges { .. } |
+                TerminatorKind::FalseUnwind { .. } => None,
 
                 TerminatorKind::Return => {
                     // Check for unused values. This usually means

src/librustc_mir/transform/remove_noop_landing_pads.rs

@@ -75,7 +75,8 @@ impl RemoveNoopLandingPads {
             TerminatorKind::Goto { .. } |
             TerminatorKind::Resume |
             TerminatorKind::SwitchInt { .. } |
-            TerminatorKind::FalseEdges { .. } => {
+            TerminatorKind::FalseEdges { .. } |
+            TerminatorKind::FalseUnwind { .. } => {
                 terminator.successors().iter().all(|succ| {
                     nop_landing_pads.contains(succ.index())
                 })

src/librustc_mir/transform/simplify_branches.rs

@@ -64,6 +64,9 @@ impl MirPass for SimplifyBranches {
                 TerminatorKind::FalseEdges { real_target, .. } => {
                     TerminatorKind::Goto { target: real_target }
                 },
+                TerminatorKind::FalseUnwind { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                },
                 _ => continue
             };
         }

src/librustc_passes/mir_stats.rs

@@ -123,6 +123,7 @@ impl<'a, 'tcx> mir_visit::Visitor<'tcx> for StatCollector<'a, 'tcx> {
             TerminatorKind::GeneratorDrop => "TerminatorKind::GeneratorDrop",
             TerminatorKind::Yield { .. } => "TerminatorKind::Yield",
             TerminatorKind::FalseEdges { .. } => "TerminatorKind::FalseEdges",
+            TerminatorKind::FalseUnwind { .. } => "TerminatorKind::FalseUnwind",
         }, kind);
         self.super_terminator_kind(block, kind, location);
     }

src/librustc_trans/mir/analyze.rs

@@ -242,7 +242,8 @@ pub fn cleanup_kinds<'a, 'tcx>(mir: &mir::Mir<'tcx>) -> IndexVec<mir::BasicBlock
                 TerminatorKind::Unreachable |
                 TerminatorKind::SwitchInt { .. } |
                 TerminatorKind::Yield { .. } |
-                TerminatorKind::FalseEdges { .. } => {
+                TerminatorKind::FalseEdges { .. } |
+                TerminatorKind::FalseUnwind { .. } => {
                     /* nothing to do */
                 }
                 TerminatorKind::Call { cleanup: unwind, .. } |

src/librustc_trans/mir/block.rs

@@ -608,8 +608,9 @@ impl<'a, 'tcx> FunctionCx<'a, 'tcx> {
                         cleanup);
             }
             mir::TerminatorKind::GeneratorDrop |
-            mir::TerminatorKind::Yield { .. } |
-            mir::TerminatorKind::FalseEdges { .. } => bug!("generator ops in trans"),
+            mir::TerminatorKind::Yield { .. } => bug!("generator ops in trans"),
+            mir::TerminatorKind::FalseEdges { .. } |
+            mir::TerminatorKind::FalseUnwind { .. } => bug!("borrowck false edges in trans"),
         }
     }