For box expressions, use NZ drop instead of a free block

This falls naturally out of making drop elaboration work with `box`
expressions, which is probably required for sane MIR borrow-checking.
This is a pure refactoring with no intentional functional effects.

Author: arielb1

src/librustc/mir/tcx.rs

@@ -135,6 +135,11 @@ impl<'tcx> Lvalue<'tcx> {
     }
 }
 
+pub enum RvalueInitializationState {
+    Shallow,
+    Deep
+}
+
 impl<'tcx> Rvalue<'tcx> {
     pub fn ty<'a, 'gcx, D>(&self, local_decls: &D, tcx: TyCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx>
         where D: HasLocalDecls<'tcx>
@@ -206,6 +211,16 @@ impl<'tcx> Rvalue<'tcx> {
             }
         }
     }
+
+    #[inline]
+    /// Returns whether this rvalue is deeply initialized (most rvalues) or
+    /// whether its only shallowly initialized (`Rvalue::Box`).
+    pub fn initialization_state(&self) -> RvalueInitializationState {
+        match *self {
+            Rvalue::NullaryOp(NullOp::Box, _) => RvalueInitializationState::Shallow,
+            _ => RvalueInitializationState::Deep
+        }
+    }
 }
 
 impl<'tcx> Operand<'tcx> {

src/librustc_mir/build/expr/as_rvalue.rs

@@ -94,19 +94,19 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 }
                 block.and(Rvalue::UnaryOp(op, arg))
             }
-            ExprKind::Box { value, value_extents } => {
+            ExprKind::Box { value } => {
                 let value = this.hir.mirror(value);
                 let result = this.temp(expr.ty, expr_span);
                 // to start, malloc some memory of suitable type (thus far, uninitialized):
                 let box_ = Rvalue::NullaryOp(NullOp::Box, value.ty);
                 this.cfg.push_assign(block, source_info, &result, box_);
-                this.in_scope((value_extents, source_info), block, |this| {
+                if let Some(scope) = scope {
                     // schedule a shallow free of that memory, lest we unwind:
-                    this.schedule_box_free(expr_span, value_extents, &result, value.ty);
-                    // initialize the box contents:
-                    unpack!(block = this.into(&result.clone().deref(), block, value));
-                    block.and(Rvalue::Use(Operand::Consume(result)))
-                })
+                    this.schedule_drop(expr_span, scope, &result, value.ty);
+                }
+                // initialize the box contents:
+                unpack!(block = this.into(&result.clone().deref(), block, value));
+                block.and(Rvalue::Use(Operand::Consume(result)))
             }
             ExprKind::Cast { source } => {
                 let source = this.hir.mirror(source);

src/librustc_mir/build/scope.rs

@@ -89,9 +89,6 @@ should go to.
 
 use build::{BlockAnd, BlockAndExtension, Builder, CFG};
 use rustc::middle::region::CodeExtent;
-use rustc::middle::lang_items;
-use rustc::middle::const_val::ConstVal;
-use rustc::ty::subst::{Kind, Subst};
 use rustc::ty::{Ty, TyCtxt};
 use rustc::mir::*;
 use rustc::mir::transform::MirSource;
@@ -127,21 +124,6 @@ pub struct Scope<'tcx> {
     /// end of the vector (top of the stack) first.
     drops: Vec<DropData<'tcx>>,
 
-    /// A scope may only have one associated free, because:
-    ///
-    /// 1. We require a `free` to only be scheduled in the scope of
-    ///    `EXPR` in `box EXPR`;
-    /// 2. It only makes sense to have it translated into the diverge-path.
-    ///
-    /// This kind of drop will be run *after* all the regular drops
-    /// scheduled onto this scope, because drops may have dependencies
-    /// on the allocated memory.
-    ///
-    /// This is expected to go away once `box EXPR` becomes a sugar
-    /// for placement protocol and gets desugared in some earlier
-    /// stage.
-    free: Option<FreeData<'tcx>>,
-
     /// The cache for drop chain on “normal” exit into a particular BasicBlock.
     cached_exits: FxHashMap<(BasicBlock, CodeExtent), BasicBlock>,
 }
@@ -170,22 +152,6 @@ enum DropKind {
     Storage
 }
 
-#[derive(Debug)]
-struct FreeData<'tcx> {
-    /// span where free obligation was incurred
-    span: Span,
-
-    /// Lvalue containing the allocated box.
-    value: Lvalue<'tcx>,
-
-    /// type of item for which the box was allocated for (i.e. the T in Box<T>).
-    item_ty: Ty<'tcx>,
-
-    /// The cached block containing code to run the free. The block will also execute all the drops
-    /// in the scope.
-    cached_block: Option<BasicBlock>
-}
-
 #[derive(Clone, Debug)]
 pub struct BreakableScope<'tcx> {
     /// Extent of the loop
@@ -224,9 +190,6 @@ impl<'tcx> Scope<'tcx> {
                 *cached_block = None;
             }
         }
-        if let Some(ref mut freedata) = self.free {
-            freedata.cached_block = None;
-        }
     }
 
     /// Returns the cached entrypoint for diverging exit from this scope.
@@ -242,8 +205,6 @@ impl<'tcx> Scope<'tcx> {
         });
         if let Some(cached_block) = drops.next() {
             Some(cached_block.expect("drop cache is not filled"))
-        } else if let Some(ref data) = self.free {
-            Some(data.cached_block.expect("free cache is not filled"))
         } else {
             None
         }
@@ -333,7 +294,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             extent_span: extent.1.span,
             needs_cleanup: false,
             drops: vec![],
-            free: None,
             cached_exits: FxHashMap()
         });
     }
@@ -382,7 +342,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         });
         let len = self.scopes.len();
         assert!(scope_count < len, "should not use `exit_scope` to pop ALL scopes");
-        let tmp = self.get_unit_temp();
 
         // If we are emitting a `drop` statement, we need to have the cached
         // diverge cleanup pads ready in case that drop panics.
@@ -415,13 +374,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
             // End all regions for scopes out of which we are breaking.
             self.cfg.push_end_region(block, extent.1, scope.extent);
-
-            if let Some(ref free_data) = scope.free {
-                let next = self.cfg.start_new_block();
-                let free = build_free(self.hir.tcx(), &tmp, free_data, next);
-                self.cfg.terminate(block, scope.source_info(span), free);
-                block = next;
-            }
         }
         }
         let scope = &self.scopes[len - scope_count];
@@ -607,36 +559,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         span_bug!(span, "extent {:?} not in scope to drop {:?}", extent, lvalue);
     }
 
-    /// Schedule dropping of a not-yet-fully-initialised box.
-    ///
-    /// This cleanup will only be translated into unwind branch.
-    /// The extent should be for the `EXPR` inside `box EXPR`.
-    /// There may only be one “free” scheduled in any given scope.
-    pub fn schedule_box_free(&mut self,
-                             span: Span,
-                             extent: CodeExtent,
-                             value: &Lvalue<'tcx>,
-                             item_ty: Ty<'tcx>) {
-        for scope in self.scopes.iter_mut().rev() {
-            // See the comment in schedule_drop above. The primary difference is that we invalidate
-            // the unwind blocks unconditionally. That’s because the box free may be considered
-            // outer-most cleanup within the scope.
-            scope.invalidate_cache(true);
-            if scope.extent == extent {
-                assert!(scope.free.is_none(), "scope already has a scheduled free!");
-                scope.needs_cleanup = true;
-                scope.free = Some(FreeData {
-                    span: span,
-                    value: value.clone(),
-                    item_ty: item_ty,
-                    cached_block: None
-                });
-                return;
-            }
-        }
-        span_bug!(span, "extent {:?} not in scope to free {:?}", extent, value);
-    }
-
     // Other
     // =====
     /// Creates a path that performs all required cleanup for unwinding.
@@ -650,7 +572,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         }
         assert!(!self.scopes.is_empty()); // or `any` above would be false
 
-        let unit_temp = self.get_unit_temp();
         let Builder { ref mut hir, ref mut cfg, ref mut scopes,
                       ref mut cached_resume_block, .. } = *self;
 
@@ -679,7 +600,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
         for scope in scopes.iter_mut() {
             target = build_diverge_scope(
-                hir.tcx(), cfg, &unit_temp, scope.extent_span, scope, target);
+                hir.tcx(), cfg, scope.extent_span, scope, target);
         }
         Some(target)
     }
@@ -805,9 +726,8 @@ fn build_scope_drops<'tcx>(cfg: &mut CFG<'tcx>,
     block.unit()
 }
 
-fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
+fn build_diverge_scope<'a, 'gcx, 'tcx>(_tcx: TyCtxt<'a, 'gcx, 'tcx>,
                                        cfg: &mut CFG<'tcx>,
-                                       unit_temp: &Lvalue<'tcx>,
                                        span: Span,
                                        scope: &mut Scope<'tcx>,
                                        mut target: BasicBlock)
@@ -832,19 +752,6 @@ fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
         scope: visibility_scope
     };
 
-    // Next, build up any free.
-    if let Some(ref mut free_data) = scope.free {
-        target = if let Some(cached_block) = free_data.cached_block {
-            cached_block
-        } else {
-            let into = cfg.start_new_cleanup_block();
-            cfg.terminate(into, source_info(free_data.span),
-                          build_free(tcx, unit_temp, free_data, target));
-            free_data.cached_block = Some(into);
-            into
-        };
-    }
-
     // Next, build up the drops. Here we iterate the vector in
     // *forward* order, so that we generate drops[0] first (right to
     // left in diagram above).
@@ -888,24 +795,3 @@ fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 
     target
 }
-
-fn build_free<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
-                              unit_temp: &Lvalue<'tcx>,
-                              data: &FreeData<'tcx>,
-                              target: BasicBlock)
-                              -> TerminatorKind<'tcx> {
-    let free_func = tcx.require_lang_item(lang_items::BoxFreeFnLangItem);
-    let substs = tcx.intern_substs(&[Kind::from(data.item_ty)]);
-    TerminatorKind::Call {
-        func: Operand::Constant(box Constant {
-            span: data.span,
-            ty: tcx.type_of(free_func).subst(tcx, substs),
-            literal: Literal::Value {
-                value: ConstVal::Function(free_func, substs),
-            }
-        }),
-        args: vec![Operand::Consume(data.value.clone())],
-        destination: Some((unit_temp.clone(), target)),
-        cleanup: None
-    }
-}

src/librustc_mir/dataflow/drop_flag_effects.rs

@@ -240,7 +240,7 @@ pub(crate) fn drop_flag_effects_for_function_entry<'a, 'tcx, F>(
         let lookup_result = move_data.rev_lookup.find(&lvalue);
         on_lookup_result_bits(tcx, mir, move_data,
                               lookup_result,
-                              |moi| callback(moi, DropFlagState::Present));
+                              |mpi| callback(mpi, DropFlagState::Present));
     }
 }
 
@@ -270,7 +270,7 @@ pub(crate) fn drop_flag_effects_for_location<'a, 'tcx, F>(
 
         on_all_children_bits(tcx, mir, move_data,
                              path,
-                             |moi| callback(moi, DropFlagState::Absent))
+                             |mpi| callback(mpi, DropFlagState::Absent))
     }
 
     let block = &mir[loc.block];
@@ -279,11 +279,21 @@ pub(crate) fn drop_flag_effects_for_location<'a, 'tcx, F>(
             mir::StatementKind::SetDiscriminant{ .. } => {
                 span_bug!(stmt.source_info.span, "SetDiscrimant should not exist during borrowck");
             }
-            mir::StatementKind::Assign(ref lvalue, _) => {
-                debug!("drop_flag_effects: assignment {:?}", stmt);
-                 on_lookup_result_bits(tcx, mir, move_data,
-                                       move_data.rev_lookup.find(lvalue),
-                                       |moi| callback(moi, DropFlagState::Present))
+            mir::StatementKind::Assign(ref lvalue, ref rvalue) => {
+                match rvalue.initialization_state() {
+                    mir::tcx::RvalueInitializationState::Shallow => {
+                        debug!("drop_flag_effects: box assignment {:?}", stmt);
+                        if let LookupResult::Exact(mpi) = move_data.rev_lookup.find(lvalue) {
+                            callback(mpi, DropFlagState::Present);
+                        }
+                    }
+                    mir::tcx::RvalueInitializationState::Deep => {
+                        debug!("drop_flag_effects: assignment {:?}", stmt);
+                        on_lookup_result_bits(tcx, mir, move_data,
+                                              move_data.rev_lookup.find(lvalue),
+                                              |mpi| callback(mpi, DropFlagState::Present))
+                    }
+                }
             }
             mir::StatementKind::StorageLive(_) |
             mir::StatementKind::StorageDead(_) |
@@ -298,7 +308,7 @@ pub(crate) fn drop_flag_effects_for_location<'a, 'tcx, F>(
                 mir::TerminatorKind::DropAndReplace { ref location, .. } => {
                     on_lookup_result_bits(tcx, mir, move_data,
                                           move_data.rev_lookup.find(location),
-                                          |moi| callback(moi, DropFlagState::Present))
+                                          |mpi| callback(mpi, DropFlagState::Present))
                 }
                 _ => {
                     // other terminators do not contain move-ins

src/librustc_mir/dataflow/impls/mod.rs

@@ -23,6 +23,7 @@ use super::MoveDataParamEnv;
 use util::elaborate_drops::DropFlagState;
 
 use super::move_paths::{HasMoveData, MoveData, MoveOutIndex, MovePathIndex};
+use super::move_paths::LookupResult;
 use super::{BitDenotation, BlockSets, DataflowOperator};
 
 use super::drop_flag_effects_for_function_entry;
@@ -469,18 +470,30 @@ impl<'a, 'tcx> BitDenotation for MovingOutStatements<'a, 'tcx> {
             mir::StatementKind::SetDiscriminant { .. } => {
                 span_bug!(stmt.source_info.span, "SetDiscriminant should not exist in borrowck");
             }
-            mir::StatementKind::Assign(ref lvalue, _) => {
+            mir::StatementKind::Assign(ref lvalue, ref rvalue) => {
                 // assigning into this `lvalue` kills all
                 // MoveOuts from it, and *also* all MoveOuts
                 // for children and associated fragment sets.
-                on_lookup_result_bits(tcx,
-                                     mir,
-                                     move_data,
-                                     rev_lookup.find(lvalue),
-                                     |mpi| for moi in &path_map[mpi] {
-                                         assert!(moi.index() < bits_per_block);
-                                         sets.kill_set.add(&moi);
-                                     });
+                match rvalue.initialization_state() {
+                    mir::tcx::RvalueInitializationState::Shallow => {
+                        if let LookupResult::Exact(mpi) = rev_lookup.find(lvalue) {
+                             for moi in &path_map[mpi] {
+                                 assert!(moi.index() < bits_per_block);
+                                 sets.kill_set.add(&moi);
+                             }
+                        }
+                    }
+                    mir::tcx::RvalueInitializationState::Deep => {
+                        on_lookup_result_bits(tcx,
+                                              mir,
+                                              move_data,
+                                              rev_lookup.find(lvalue),
+                                              |mpi| for moi in &path_map[mpi] {
+                                                  assert!(moi.index() < bits_per_block);
+                                                  sets.kill_set.add(&moi);
+                                              });
+                    }
+                }
             }
             mir::StatementKind::StorageLive(_) |
             mir::StatementKind::StorageDead(_) |

src/librustc_mir/dataflow/move_paths/mod.rs

@@ -11,6 +11,7 @@
 
 use rustc::ty::{self, TyCtxt};
 use rustc::mir::*;
+use rustc::mir::tcx::RvalueInitializationState;
 use rustc::util::nodemap::FxHashMap;
 use rustc_data_structures::indexed_vec::{IndexVec};
 
@@ -406,6 +407,12 @@ impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
         match stmt.kind {
             StatementKind::Assign(ref lval, ref rval) => {
                 self.create_move_path(lval);
+                if let RvalueInitializationState::Shallow = rval.initialization_state() {
+                    // Box starts out uninitialized - need to create a separate
+                    // move-path for the interior so it will be separate from
+                    // the exterior.
+                    self.create_move_path(&lval.clone().deref());
+                }
                 self.gather_rvalue(loc, rval);
             }
             StatementKind::StorageLive(_) |