Auto merge of #47802 - bobtwinkles:loop_false_edge, r=nikomatsakis

[NLL] Add false edges out of infinite loops

Resolves #46036 by adding a `cleanup` member to the `FalseEdges` terminator kind. There's also a small doc fix to one of the other comments in `into.rs` which I can pull out in to another PR if desired =)

This PR should pass CI but the test suite has been relatively unstable on my system so I'm not 100% sure.

r? @nikomatsakis

Author: bors

src/librustc/ich/impls_mir.rs

@@ -219,6 +219,10 @@ 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

@@ -816,9 +816,28 @@ 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>,
     },
 }
 
@@ -878,6 +897,8 @@ 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(),
         }
     }
 
@@ -910,6 +931,8 @@ 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],
         }
     }
 
@@ -929,7 +952,8 @@ 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)
             }
         }
@@ -1058,7 +1082,8 @@ impl<'tcx> TerminatorKind<'tcx> {
 
                 write!(fmt, ")")
             },
-            FalseEdges { .. } => write!(fmt, "falseEdges")
+            FalseEdges { .. } => write!(fmt, "falseEdges"),
+            FalseUnwind { .. } => write!(fmt, "falseUnwind"),
         }
     }
 
@@ -1100,6 +1125,8 @@ 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()],
         }
     }
 }
@@ -2202,7 +2229,8 @@ 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,
@@ -2244,7 +2272,8 @@ 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

@@ -575,7 +575,8 @@ 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/expr/into.rs

@@ -104,8 +104,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 // Or:
                 //
                 // [block: If(lhs)] -false-> [else_block: If(rhs)] -true-> [true_block]
-                //        |                          | (false)
-                //        +----------true------------+-------------------> [false_block]
+                //        | (true)                   | (false)
+                //  [true_block]               [false_block]
 
                 let (true_block, false_block, mut else_block, join_block) =
                     (this.cfg.start_new_block(), this.cfg.start_new_block(),
@@ -147,20 +147,24 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 join_block.unit()
             }
             ExprKind::Loop { condition: opt_cond_expr, body } => {
-                // [block] --> [loop_block] ~~> [loop_block_end] -1-> [exit_block]
-                //                  ^                  |
-                //                  |                  0
-                //                  |                  |
-                //                  |                  v
-                //           [body_block_end] <~~~ [body_block]
+                // [block] --> [loop_block] -/eval. cond./-> [loop_block_end] -1-> [exit_block]
+                //                  ^                               |
+                //                  |                               0
+                //                  |                               |
+                //                  |                               v
+                //           [body_block_end] <-/eval. body/-- [body_block]
                 //
                 // If `opt_cond_expr` is `None`, then the graph is somewhat simplified:
                 //
-                // [block] --> [loop_block / body_block ] ~~> [body_block_end]    [exit_block]
-                //                         ^                          |
-                //                         |                          |
-                //                         +--------------------------+
-                //
+                // [block]
+                //    |
+                //   [loop_block] -> [body_block] -/eval. body/-> [body_block_end]
+                //    |        ^                                         |
+                // false link  |                                         |
+                //    |        +-----------------------------------------+
+                //    +-> [diverge_cleanup]
+                // The false link is required to make sure borrowck considers unwinds through the
+                // body, even when the exact code in the body cannot unwind
 
                 let loop_block = this.cfg.start_new_block();
                 let exit_block = this.cfg.start_new_block();
@@ -188,7 +192,13 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             // always `()` anyway
                             this.cfg.push_assign_unit(exit_block, source_info, destination);
                         } else {
-                            body_block = loop_block;
+                            body_block = this.cfg.start_new_block();
+                            let diverge_cleanup = this.diverge_cleanup();
+                            this.cfg.terminate(loop_block, source_info,
+                                               TerminatorKind::FalseUnwind {
+                                                   real_target: body_block,
+                                                   unwind: Some(diverge_cleanup)
+                                               })
                         }
 
                         // The “return” value of the loop body must always be an unit. We therefore

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 => {