Auto merge of #66703 - matthewjasper:drop-trees, r=<try>

[PERF] further drop changes

r? @ghost

src/librustc/mir/mod.rs

@@ -1088,8 +1088,18 @@ pub enum TerminatorKind<'tcx> {
     /// Indicates a terminator that can never be reached.
     Unreachable,
 
-    /// Drop the `Place`.
-    Drop { location: Place<'tcx>, target: BasicBlock, unwind: Option<BasicBlock> },
+    /// Drop the `Place`, possibly conditioned on a flag being true.
+    Drop {
+        location: Place<'tcx>,
+        /// Whether to drop the value.
+        ///
+        /// Before drop elaboration this is always `None. After drop elaboration
+        /// If this is `None` then the drop is unconditional, otherwise the drop
+        /// is only evaluated when the flag is true.
+        flag: Option<Place<'tcx>>,
+        target: BasicBlock,
+        unwind: Option<BasicBlock>,
+    },
 
     /// Drop the `Place` and assign the new value over it. This ensures
     /// that the assignment to `P` occurs *even if* the destructor for
@@ -1464,7 +1474,10 @@ impl<'tcx> TerminatorKind<'tcx> {
             Abort => write!(fmt, "abort"),
             Yield { ref value, .. } => write!(fmt, "_1 = suspend({:?})", value),
             Unreachable => write!(fmt, "unreachable"),
-            Drop { ref location, .. } => write!(fmt, "drop({:?})", location),
+            Drop { ref location, flag: Some(ref flag), .. } => {
+                write!(fmt, "if {:?} drop({:?})", flag, location)
+            }
+            Drop { ref location, flag: None, .. } => write!(fmt, "drop({:?})", location),
             DropAndReplace { ref location, ref value, .. } => {
                 write!(fmt, "replace({:?} <- {:?})", location, value)
             }
@@ -2967,8 +2980,13 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
                 values: values.clone(),
                 targets: targets.clone(),
             },
-            Drop { ref location, target, unwind } => {
-                Drop { location: location.fold_with(folder), target, unwind }
+            Drop { ref location, ref flag, target, unwind } => {
+                Drop {
+                    location: location.fold_with(folder),
+                    flag: flag.fold_with(folder),
+                    target,
+                    unwind,
+                }
             }
             DropAndReplace { ref location, ref value, target, unwind } => DropAndReplace {
                 location: location.fold_with(folder),
@@ -3025,7 +3043,9 @@ impl<'tcx> TypeFoldable<'tcx> for Terminator<'tcx> {
             SwitchInt { ref discr, switch_ty, .. } => {
                 discr.visit_with(visitor) || switch_ty.visit_with(visitor)
             }
-            Drop { ref location, .. } => location.visit_with(visitor),
+            Drop { ref location, ref flag, .. } => {
+                location.visit_with(visitor) || flag.visit_with(visitor)
+            },
             DropAndReplace { ref location, ref value, .. } => {
                 location.visit_with(visitor) || value.visit_with(visitor)
             }

src/librustc/mir/visit.rs

@@ -432,14 +432,22 @@ macro_rules! make_mir_visitor {
 
                     TerminatorKind::Drop {
                         location,
+                        flag,
                         target: _,
                         unwind: _,
                     } => {
                         self.visit_place(
                             location,
                             PlaceContext::MutatingUse(MutatingUseContext::Drop),
-                            source_location
+                            source_location,
                         );
+                        if let Some(flag) = flag {
+                            self.visit_place(
+                                flag,
+                                PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                                source_location,
+                            );
+                        }
                     }
 
                     TerminatorKind::DropAndReplace {

src/librustc/ty/instance.rs

@@ -166,10 +166,14 @@ impl<'tcx> InstanceDef<'tcx> {
             return true
         }
         if let ty::InstanceDef::DropGlue(..) = *self {
-            // Drop glue wants to be instantiated at every codegen
+            // Drop glue generally wants to be instantiated at every codegen
             // unit, but without an #[inline] hint. We should make this
             // available to normal end-users.
-            return true
+            //
+            // When compiling with incremental, we can generate a lot of
+            // codegen units. Including drop glue into all of them has a
+            // considerable compile time cost.
+            return tcx.sess.opts.incremental.is_none();
         }
         tcx.codegen_fn_attrs(self.def_id()).requests_inline()
     }

src/librustc_codegen_ssa/mir/block.rs

@@ -321,8 +321,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         helper: TerminatorCodegenHelper<'b, 'tcx>,
         mut bx: Bx,
         location: &mir::Place<'tcx>,
+        flag: &Option<mir::Place<'tcx>>,
         target: mir::BasicBlock,
         unwind: Option<mir::BasicBlock>,
+        source_info: mir::SourceInfo,
     ) {
         let ty = location.ty(self.mir, bx.tcx()).ty;
         let ty = self.monomorphize(&ty);
@@ -335,6 +337,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             return
         }
 
+        if let Some(flag) = flag {
+            let flag = self.codegen_consume(&mut bx, &flag.as_ref()).immediate();
+            let lltarget = helper.llblock(self, target);
+            let drop_block = self.new_block("drop");
+            helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
+            bx.cond_br(flag, drop_block.llbb(), lltarget);
+            bx = drop_block;
+            self.set_debug_loc(&mut bx, source_info);
+        }
+
         let place = self.codegen_place(&mut bx, &location.as_ref());
         let (args1, args2);
         let mut args = if let Some(llextra) = place.llextra {
@@ -854,8 +866,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 bx.unreachable();
             }
 
-            mir::TerminatorKind::Drop { ref location, target, unwind } => {
-                self.codegen_drop_terminator(helper, bx, location, target, unwind);
+            mir::TerminatorKind::Drop { ref location, ref flag, target, unwind } => {
+                self.codegen_drop_terminator(
+                    helper,
+                    bx,
+                    location,
+                    flag,
+                    target,
+                    unwind,
+                    terminator.source_info,
+                );
             }
 
             mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, cleanup } => {

src/librustc_mir/borrow_check/mod.rs

@@ -618,6 +618,7 @@ impl<'cx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tcx
             }
             TerminatorKind::Drop {
                 location: ref drop_place,
+                flag: _,
                 target: _,
                 unwind: _,
             } => {

src/librustc_mir/borrow_check/nll/invalidation.rs

@@ -155,6 +155,7 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
             }
             TerminatorKind::Drop {
                 location: ref drop_place,
+                flag: _,
                 target: _,
                 unwind: _,
             } => {

src/librustc_mir/build/block.rs

@@ -2,17 +2,20 @@ use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
 use crate::build::ForGuard::OutsideGuard;
 use crate::build::matches::ArmHasGuard;
 use crate::hair::*;
+use rustc::middle::region;
 use rustc::mir::*;
 use rustc::hir;
 use syntax_pos::Span;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
-    pub fn ast_block(&mut self,
-                     destination: &Place<'tcx>,
-                     block: BasicBlock,
-                     ast_block: &'tcx hir::Block,
-                     source_info: SourceInfo)
-                     -> BlockAnd<()> {
+    pub fn ast_block(
+        &mut self,
+        destination: &Place<'tcx>,
+        scope: Option<region::Scope>,
+        block: BasicBlock,
+        ast_block: &'tcx hir::Block,
+        source_info: SourceInfo,
+    ) -> BlockAnd<()> {
         let Block {
             region_scope,
             opt_destruction_scope,
@@ -21,37 +24,50 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             expr,
             targeted_by_break,
             safety_mode
-        } =
-            self.hir.mirror(ast_block);
+        } = self.hir.mirror(ast_block);
         self.in_opt_scope(opt_destruction_scope.map(|de|(de, source_info)), move |this| {
             this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
                 if targeted_by_break {
-                    // This is a `break`-able block
-                    let exit_block = this.cfg.start_new_block();
-                    let block_exit = this.in_breakable_scope(
-                        None, exit_block, destination.clone(), |this| {
-                            this.ast_block_stmts(destination, block, span, stmts, expr,
-                                                 safety_mode)
-                        });
-                    this.cfg.terminate(unpack!(block_exit), source_info,
-                                       TerminatorKind::Goto { target: exit_block });
-                    exit_block.unit()
+                    this.in_breakable_scope(
+                        None,
+                        destination.clone(),
+                        scope,
+                        span,
+                        |this| Some(this.ast_block_stmts(
+                            destination,
+                            scope,
+                            block,
+                            span,
+                            stmts,
+                            expr,
+                            safety_mode,
+                        )),
+                    )
                 } else {
-                    this.ast_block_stmts(destination, block, span, stmts, expr,
-                                         safety_mode)
+                    this.ast_block_stmts(
+                        destination,
+                        scope,
+                        block,
+                        span,
+                        stmts,
+                        expr,
+                        safety_mode,
+                    )
                 }
             })
         })
     }
 
-    fn ast_block_stmts(&mut self,
-                       destination: &Place<'tcx>,
-                       mut block: BasicBlock,
-                       span: Span,
-                       stmts: Vec<StmtRef<'tcx>>,
-                       expr: Option<ExprRef<'tcx>>,
-                       safety_mode: BlockSafety)
-                       -> BlockAnd<()> {
+    fn ast_block_stmts(
+        &mut self,
+        destination: &Place<'tcx>,
+        scope: Option<region::Scope>,
+        mut block: BasicBlock,
+        span: Span,
+        stmts: Vec<StmtRef<'tcx>>,
+        expr: Option<ExprRef<'tcx>>,
+        safety_mode: BlockSafety,
+    ) -> BlockAnd<()> {
         let this = self;
 
         // This convoluted structure is to avoid using recursion as we walk down a list
@@ -177,7 +193,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 this.block_context.currently_ignores_tail_results();
             this.block_context.push(BlockFrame::TailExpr { tail_result_is_ignored });
 
-            unpack!(block = this.into(destination, block, expr));
+            unpack!(block = this.into(destination, scope, block, expr));
             let popped = this.block_context.pop();
 
             assert!(popped.map_or(false, |bf|bf.is_tail_expr()));

src/librustc_mir/build/expr/as_rvalue.rs

@@ -130,11 +130,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 this.cfg
                     .push_assign(block, source_info, &Place::from(result), box_);
 
-                // initialize the box contents:
+                // Initialize the box contents. No scope is needed since the
+                // `Box` is already scheduled to be dropped.
                 unpack!(
                     block = this.into(
                         &this.hir.tcx().mk_place_deref(Place::from(result)),
-                        block, value
+                        None,
+                        block,
+                        value,
                     )
                 );
                 block.and(Rvalue::Use(Operand::Move(Place::from(result))))
@@ -257,14 +260,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ExprKind::Yield { value } => {
                 let value = unpack!(block = this.as_operand(block, scope, value));
                 let resume = this.cfg.start_new_block();
-                let cleanup = this.generator_drop_cleanup();
+                this.generator_drop_cleanup(block);
                 this.cfg.terminate(
                     block,
                     source_info,
                     TerminatorKind::Yield {
                         value: value,
                         resume: resume,
-                        drop: cleanup,
+                        drop: None,
                     },
                 );
                 resume.and(this.unit_rvalue())

src/librustc_mir/build/expr/as_temp.rs

@@ -114,16 +114,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
-        unpack!(block = this.into(temp_place, block, expr));
-
-        if let Some(temp_lifetime) = temp_lifetime {
-            this.schedule_drop(
-                expr_span,
-                temp_lifetime,
-                temp,
-                DropKind::Value,
-            );
-        }
+        unpack!(block = this.into(temp_place, temp_lifetime, block, expr));
 
         block.and(temp)
     }