Rollup merge of rust-lang#71840 - matthewjasper:drop-trees, r=oli-obk

Rework MIR drop tree lowering

This PR changes how drops are generated in MIR construction. This is the first half of the fix for rust-lang#47949.

Rather than generating the drops for a given unwind/break/continue/return/generator drop path as soon as they are needed, the required drops are recorded and get generated later.

The motivation for this is
* It simplifies the caching scheme, because it's now possible to walk up the currently scheduled drop tree to recover state.
* The basic block order for MIR more closely resembles execution order.

This PR also:
* Highlights cleanup blocks in the graphviz MIR output.
* Removes some unnecessary drop flag assignments.

src/librustc_mir/dataflow/move_paths/builder.rs

@@ -361,17 +361,18 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
     fn gather_terminator(&mut self, term: &Terminator<'tcx>) {
         match term.kind {
             TerminatorKind::Goto { target: _ }
+            | TerminatorKind::FalseEdges { .. }
+            | TerminatorKind::FalseUnwind { .. }
+            // In some sense returning moves the return place into the current
+            // call's destination, however, since there are no statements after
+            // this that could possibly access the return place, this doesn't
+            // need recording.
+            | TerminatorKind::Return
             | TerminatorKind::Resume
             | TerminatorKind::Abort
             | TerminatorKind::GeneratorDrop
-            | TerminatorKind::FalseEdges { .. }
-            | TerminatorKind::FalseUnwind { .. }
             | TerminatorKind::Unreachable => {}
 
-            TerminatorKind::Return => {
-                self.gather_move(Place::return_place());
-            }
-
             TerminatorKind::Assert { ref cond, .. } => {
                 self.gather_operand(cond);
             }

src/librustc_mir/util/elaborate_drops.rs

@@ -163,8 +163,6 @@ where
                     .patch_terminator(bb, TerminatorKind::Goto { target: self.succ });
             }
             DropStyle::Static => {
-                let loc = self.terminator_loc(bb);
-                self.elaborator.clear_drop_flag(loc, self.path, DropFlagMode::Deep);
                 self.elaborator.patch().patch_terminator(
                     bb,
                     TerminatorKind::Drop {
@@ -175,9 +173,7 @@ where
                 );
             }
             DropStyle::Conditional => {
-                let unwind = self.unwind; // FIXME(#43234)
-                let succ = self.succ;
-                let drop_bb = self.complete_drop(Some(DropFlagMode::Deep), succ, unwind);
+                let drop_bb = self.complete_drop(self.succ, self.unwind);
                 self.elaborator
                     .patch()
                     .patch_terminator(bb, TerminatorKind::Goto { target: drop_bb });
@@ -249,7 +245,7 @@ where
                 // our own drop flag.
                 path: self.path,
             }
-            .complete_drop(None, succ, unwind)
+            .complete_drop(succ, unwind)
         }
     }
 
@@ -278,13 +274,7 @@ where
         // Clear the "master" drop flag at the end. This is needed
         // because the "master" drop protects the ADT's discriminant,
         // which is invalidated after the ADT is dropped.
-        let (succ, unwind) = (self.succ, self.unwind); // FIXME(#43234)
-        (
-            self.drop_flag_reset_block(DropFlagMode::Shallow, succ, unwind),
-            unwind.map(|unwind| {
-                self.drop_flag_reset_block(DropFlagMode::Shallow, unwind, Unwind::InCleanup)
-            }),
-        )
+        (self.drop_flag_reset_block(DropFlagMode::Shallow, self.succ, self.unwind), self.unwind)
     }
 
     /// Creates a full drop ladder, consisting of 2 connected half-drop-ladders
@@ -820,11 +810,7 @@ where
                     self.open_drop_for_adt(def, substs)
                 }
             }
-            ty::Dynamic(..) => {
-                let unwind = self.unwind; // FIXME(#43234)
-                let succ = self.succ;
-                self.complete_drop(Some(DropFlagMode::Deep), succ, unwind)
-            }
+            ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
             ty::Array(ety, size) => {
                 let size = size.try_eval_usize(self.tcx(), self.elaborator.param_env());
                 self.open_drop_for_array(ety, size)
@@ -835,20 +821,10 @@ where
         }
     }
 
-    fn complete_drop(
-        &mut self,
-        drop_mode: Option<DropFlagMode>,
-        succ: BasicBlock,
-        unwind: Unwind,
-    ) -> BasicBlock {
-        debug!("complete_drop({:?},{:?})", self, drop_mode);
+    fn complete_drop(&mut self, succ: BasicBlock, unwind: Unwind) -> BasicBlock {
+        debug!("complete_drop(succ={:?}, unwind={:?})", succ, unwind);
 
         let drop_block = self.drop_block(succ, unwind);
-        let drop_block = if let Some(mode) = drop_mode {
-            self.drop_flag_reset_block(mode, drop_block, unwind)
-        } else {
-            drop_block
-        };
 
         self.drop_flag_test_block(drop_block, succ, unwind)
     }
@@ -861,6 +837,11 @@ where
     ) -> BasicBlock {
         debug!("drop_flag_reset_block({:?},{:?})", self, mode);
 
+        if unwind.is_cleanup() {
+            // The drop flag isn't read again on the unwind path, so don't
+            // bother setting it.
+            return succ;
+        }
         let block = self.new_block(unwind, TerminatorKind::Goto { target: succ });
         let block_start = Location { block, statement_index: 0 };
         self.elaborator.clear_drop_flag(block_start, self.path, mode);
@@ -969,11 +950,6 @@ where
         self.elaborator.patch().new_temp(ty, self.source_info.span)
     }
 
-    fn terminator_loc(&mut self, bb: BasicBlock) -> Location {
-        let body = self.elaborator.body();
-        self.elaborator.patch().terminator_loc(body, bb)
-    }
-
     fn constant_usize(&self, val: u16) -> Operand<'tcx> {
         Operand::Constant(box Constant {
             span: self.source_info.span,

src/librustc_mir/util/graphviz.rs

@@ -97,12 +97,17 @@ where
     write!(w, r#"<table border="0" cellborder="1" cellspacing="0">"#)?;
 
     // Basic block number at the top.
+    let (blk, color) = if data.is_cleanup {
+        (format!("{} (cleanup)", block.index()), "lightblue")
+    } else {
+        (format!("{}", block.index()), "gray")
+    };
     write!(
         w,
-        r#"<tr><td {attrs} colspan="{colspan}">{blk}</td></tr>"#,
-        attrs = r#"bgcolor="gray" align="center""#,
+        r#"<tr><td bgcolor="{color}" align="center" colspan="{colspan}">{blk}</td></tr>"#,
         colspan = num_cols,
-        blk = block.index()
+        blk = blk,
+        color = color
     )?;
 
     init(w)?;

src/librustc_mir_build/build/block.rs

@@ -26,14 +26,16 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         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, |this| {
-                            this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
-                        });
-                    this.cfg.goto(unpack!(block_exit), source_info, exit_block);
-                    exit_block.unit()
+                    this.in_breakable_scope(None, destination, span, |this| {
+                        Some(this.ast_block_stmts(
+                            destination,
+                            block,
+                            span,
+                            stmts,
+                            expr,
+                            safety_mode,
+                        ))
+                    })
                 } else {
                     this.ast_block_stmts(destination, block, span, stmts, expr, safety_mode)
                 }

src/librustc_mir_build/build/expr/into.rs

@@ -134,32 +134,30 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // 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();
 
                 // Start the loop.
                 this.cfg.goto(block, source_info, loop_block);
 
-                this.in_breakable_scope(Some(loop_block), exit_block, destination, move |this| {
+                this.in_breakable_scope(Some(loop_block), destination, expr_span, move |this| {
                     // conduct the test, if necessary
                     let 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),
-                        },
+                        TerminatorKind::FalseUnwind { real_target: body_block, unwind: None },
                     );
+                    this.diverge_from(loop_block);
 
                     // The “return” value of the loop body must always be an unit. We therefore
                     // introduce a unit temporary as the destination for the loop body.
                     let tmp = this.get_unit_temp();
                     // Execute the body, branching back to the test.
                     let body_block_end = unpack!(this.into(tmp, body_block, body));
                     this.cfg.goto(body_block_end, source_info, loop_block);
-                });
-                exit_block.unit()
+
+                    // Loops are only exited by `break` expressions.
+                    None
+                })
             }
             ExprKind::Call { ty, fun, args, from_hir_call } => {
                 let intrinsic = match ty.kind {
@@ -201,7 +199,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         .collect();
 
                     let success = this.cfg.start_new_block();
-                    let cleanup = this.diverge_cleanup();
 
                     this.record_operands_moved(&args);
 
@@ -211,7 +208,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         TerminatorKind::Call {
                             func: fun,
                             args,
-                            cleanup: Some(cleanup),
+                            cleanup: None,
                             // FIXME(varkor): replace this with an uninhabitedness-based check.
                             // This requires getting access to the current module to call
                             // `tcx.is_ty_uninhabited_from`, which is currently tricky to do.
@@ -223,6 +220,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             from_hir_call,
                         },
                     );
+                    this.diverge_from(block);
                     success.unit()
                 }
             }
@@ -358,12 +356,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let scope = this.local_scope();
                 let value = unpack!(block = this.as_operand(block, scope, value));
                 let resume = this.cfg.start_new_block();
-                let cleanup = this.generator_drop_cleanup();
                 this.cfg.terminate(
                     block,
                     source_info,
-                    TerminatorKind::Yield { value, resume, resume_arg: destination, drop: cleanup },
+                    TerminatorKind::Yield { value, resume, resume_arg: destination, drop: None },
                 );
+                this.generator_drop_cleanup(block);
                 resume.unit()
             }
 

src/librustc_mir_build/build/matches/mod.rs

@@ -225,8 +225,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         outer_source_info: SourceInfo,
         fake_borrow_temps: Vec<(Place<'tcx>, Local)>,
     ) -> BlockAnd<()> {
-        let match_scope = self.scopes.topmost();
-
         let arm_end_blocks: Vec<_> = arm_candidates
             .into_iter()
             .map(|(arm, candidate)| {
@@ -247,7 +245,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     let arm_block = this.bind_pattern(
                         outer_source_info,
                         candidate,
-                        arm.guard.as_ref().map(|g| (g, match_scope)),
+                        arm.guard.as_ref(),
                         &fake_borrow_temps,
                         scrutinee_span,
                         Some(arm.scope),
@@ -284,7 +282,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         outer_source_info: SourceInfo,
         candidate: Candidate<'_, 'tcx>,
-        guard: Option<(&Guard<'tcx>, region::Scope)>,
+        guard: Option<&Guard<'tcx>>,
         fake_borrow_temps: &Vec<(Place<'tcx>, Local)>,
         scrutinee_span: Span,
         arm_scope: Option<region::Scope>,
@@ -1590,7 +1588,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         candidate: Candidate<'pat, 'tcx>,
         parent_bindings: &[(Vec<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
-        guard: Option<(&Guard<'tcx>, region::Scope)>,
+        guard: Option<&Guard<'tcx>>,
         fake_borrows: &Vec<(Place<'tcx>, Local)>,
         scrutinee_span: Span,
         schedule_drops: bool,
@@ -1702,7 +1700,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         //      the reference that we create for the arm.
         //    * So we eagerly create the reference for the arm and then take a
         //      reference to that.
-        if let Some((guard, region_scope)) = guard {
+        if let Some(guard) = guard {
             let tcx = self.hir.tcx();
             let bindings = parent_bindings
                 .iter()
@@ -1746,12 +1744,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 unreachable
             });
             let outside_scope = self.cfg.start_new_block();
-            self.exit_scope(
-                source_info.span,
-                region_scope,
-                otherwise_post_guard_block,
-                outside_scope,
-            );
+            self.exit_top_scope(otherwise_post_guard_block, outside_scope, source_info);
             self.false_edges(
                 outside_scope,
                 otherwise_block,

src/librustc_mir_build/build/matches/test.rs

@@ -423,7 +423,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let bool_ty = self.hir.bool_ty();
         let eq_result = self.temp(bool_ty, source_info.span);
         let eq_block = self.cfg.start_new_block();
-        let cleanup = self.diverge_cleanup();
         self.cfg.terminate(
             block,
             source_info,
@@ -441,10 +440,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 }),
                 args: vec![val, expect],
                 destination: Some((eq_result, eq_block)),
-                cleanup: Some(cleanup),
+                cleanup: None,
                 from_hir_call: false,
             },
         );
+        self.diverge_from(block);
 
         if let [success_block, fail_block] = *make_target_blocks(self) {
             // check the result