2 Managing the scope stack. The scopes are tied to lexical scopes, so as
3 we descend the THIR, we push a scope on the stack, build its
4 contents, and then pop it off. Every scope is named by a
9 When pushing a new [Scope], we record the current point in the graph (a
10 basic block); this marks the entry to the scope. We then generate more
11 stuff in the control-flow graph. Whenever the scope is exited, either
12 via a `break` or `return` or just by fallthrough, that marks an exit
13 from the scope. Each lexical scope thus corresponds to a single-entry,
14 multiple-exit (SEME) region in the control-flow graph.
16 For now, we record the `region::Scope` to each SEME region for later reference
17 (see caveat in next paragraph). This is because destruction scopes are tied to
18 them. This may change in the future so that MIR lowering determines its own
21 ### Not so SEME Regions
23 In the course of building matches, it sometimes happens that certain code
24 (namely guards) gets executed multiple times. This means that the scope lexical
25 scope may in fact correspond to multiple, disjoint SEME regions. So in fact our
26 mapping is from one scope to a vector of SEME regions. Since the SEME regions
27 are disjoint, the mapping is still one-to-one for the set of SEME regions that
30 Also in matches, the scopes assigned to arms are not always even SEME regions!
31 Each arm has a single region with one entry for each pattern. We manually
32 manipulate the scheduled drops in this scope to avoid dropping things multiple
37 The primary purpose for scopes is to insert drops: while building
38 the contents, we also accumulate places that need to be dropped upon
39 exit from each scope. This is done by calling `schedule_drop`. Once a
40 drop is scheduled, whenever we branch out we will insert drops of all
41 those places onto the outgoing edge. Note that we don't know the full
42 set of scheduled drops up front, and so whenever we exit from the
43 scope we only drop the values scheduled thus far. For example, consider
44 the scope S corresponding to this loop:
55 When processing the `let x`, we will add one drop to the scope for
56 `x`. The break will then insert a drop for `x`. When we process `let
57 y`, we will add another drop (in fact, to a subscope, but let's ignore
58 that for now); any later drops would also drop `y`.
62 There are numerous "normal" ways to early exit a scope: `break`,
63 `continue`, `return` (panics are handled separately). Whenever an
64 early exit occurs, the method `break_scope` is called. It is given the
65 current point in execution where the early exit occurs, as well as the
66 scope you want to branch to (note that all early exits from to some
67 other enclosing scope). `break_scope` will record the set of drops currently
68 scheduled in a [DropTree]. Later, before `in_breakable_scope` exits, the drops
69 will be added to the CFG.
71 Panics are handled in a similar fashion, except that the drops are added to the
72 MIR once the rest of the function has finished being lowered. If a terminator
73 can panic, call `diverge_from(block)` with the block containing the terminator
78 In addition to the normal scope stack, we track a loop scope stack
79 that contains only loops and breakable blocks. It tracks where a `break`,
80 `continue` or `return` should go to.
86 use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder, CFG};
87 use rustc_data_structures::fx::FxHashMap;
88 use rustc_index::vec::IndexVec;
89 use rustc_middle::middle::region;
90 use rustc_middle::mir::*;
91 use rustc_middle::thir::{Expr, LintLevel};
93 use rustc_span::{Span, DUMMY_SP};
96 pub struct Scopes<'tcx> {
99 /// The current set of breakable scopes. See module comment for more details.
100 breakable_scopes: Vec<BreakableScope<'tcx>>,
102 /// The scope of the innermost if-then currently being lowered.
103 if_then_scope: Option<IfThenScope>,
105 /// Drops that need to be done on unwind paths. See the comment on
106 /// [DropTree] for more details.
107 unwind_drops: DropTree,
109 /// Drops that need to be done on paths to the `GeneratorDrop` terminator.
110 generator_drops: DropTree,
115 /// The source scope this scope was created in.
116 source_scope: SourceScope,
118 /// the region span of this scope within source code.
119 region_scope: region::Scope,
121 /// set of places to drop when exiting this scope. This starts
122 /// out empty but grows as variables are declared during the
123 /// building process. This is a stack, so we always drop from the
124 /// end of the vector (top of the stack) first.
125 drops: Vec<DropData>,
127 moved_locals: Vec<Local>,
129 /// The drop index that will drop everything in and below this scope on an
131 cached_unwind_block: Option<DropIdx>,
133 /// The drop index that will drop everything in and below this scope on a
134 /// generator drop path.
135 cached_generator_drop_block: Option<DropIdx>,
138 #[derive(Clone, Copy, Debug)]
140 /// The `Span` where drop obligation was incurred (typically where place was
142 source_info: SourceInfo,
147 /// Whether this is a value Drop or a StorageDead.
151 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
152 pub(crate) enum DropKind {
158 struct BreakableScope<'tcx> {
159 /// Region scope of the loop
160 region_scope: region::Scope,
161 /// The destination of the loop/block expression itself (i.e., where to put
162 /// the result of a `break` or `return` expression)
163 break_destination: Place<'tcx>,
164 /// Drops that happen on the `break`/`return` path.
165 break_drops: DropTree,
166 /// Drops that happen on the `continue` path.
167 continue_drops: Option<DropTree>,
172 /// The if-then scope or arm scope
173 region_scope: region::Scope,
174 /// Drops that happen on the `else` path.
175 else_drops: DropTree,
178 /// The target of an expression that breaks out of a scope
179 #[derive(Clone, Copy, Debug)]
180 crate enum BreakableTarget {
181 Continue(region::Scope),
182 Break(region::Scope),
186 rustc_index::newtype_index! {
187 struct DropIdx { .. }
190 const ROOT_NODE: DropIdx = DropIdx::from_u32(0);
192 /// A tree of drops that we have deferred lowering. It's used for:
194 /// * Drops on unwind paths
195 /// * Drops on generator drop paths (when a suspended generator is dropped)
196 /// * Drops on return and loop exit paths
197 /// * Drops on the else path in an `if let` chain
199 /// Once no more nodes could be added to the tree, we lower it to MIR in one go
203 /// Drops in the tree.
204 drops: IndexVec<DropIdx, (DropData, DropIdx)>,
205 /// Map for finding the inverse of the `next_drop` relation:
207 /// `previous_drops[(drops[i].1, drops[i].0.local, drops[i].0.kind)] == i`
208 previous_drops: FxHashMap<(DropIdx, Local, DropKind), DropIdx>,
209 /// Edges into the `DropTree` that need to be added once it's lowered.
210 entry_points: Vec<(DropIdx, BasicBlock)>,
214 /// Whether there's anything to do for the cleanup path, that is,
215 /// when unwinding through this scope. This includes destructors,
216 /// but not StorageDead statements, which don't get emitted at all
217 /// for unwinding, for several reasons:
218 /// * clang doesn't emit llvm.lifetime.end for C++ unwinding
219 /// * LLVM's memory dependency analysis can't handle it atm
220 /// * polluting the cleanup MIR with StorageDead creates
221 /// landing pads even though there's no actual destructors
222 /// * freeing up stack space has no effect during unwinding
223 /// Note that for generators we do emit StorageDeads, for the
224 /// use of optimizations in the MIR generator transform.
225 fn needs_cleanup(&self) -> bool {
226 self.drops.iter().any(|drop| match drop.kind {
227 DropKind::Value => true,
228 DropKind::Storage => false,
232 fn invalidate_cache(&mut self) {
233 self.cached_unwind_block = None;
234 self.cached_generator_drop_block = None;
238 /// A trait that determined how [DropTree] creates its blocks and
239 /// links to any entry nodes.
240 trait DropTreeBuilder<'tcx> {
241 /// Create a new block for the tree. This should call either
242 /// `cfg.start_new_block()` or `cfg.start_new_cleanup_block()`.
243 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock;
245 /// Links a block outside the drop tree, `from`, to the block `to` inside
247 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock);
252 // The root node of the tree doesn't represent a drop, but instead
253 // represents the block in the tree that should be jumped to once all
254 // of the required drops have been performed.
255 let fake_source_info = SourceInfo::outermost(DUMMY_SP);
257 DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
258 let drop_idx = DropIdx::MAX;
259 let drops = IndexVec::from_elem_n((fake_data, drop_idx), 1);
260 Self { drops, entry_points: Vec::new(), previous_drops: FxHashMap::default() }
263 fn add_drop(&mut self, drop: DropData, next: DropIdx) -> DropIdx {
264 let drops = &mut self.drops;
267 .entry((next, drop.local, drop.kind))
268 .or_insert_with(|| drops.push((drop, next)))
271 fn add_entry(&mut self, from: BasicBlock, to: DropIdx) {
272 debug_assert!(to < self.drops.next_index());
273 self.entry_points.push((to, from));
276 /// Builds the MIR for a given drop tree.
278 /// `blocks` should have the same length as `self.drops`, and may have its
279 /// first value set to some already existing block.
280 fn build_mir<'tcx, T: DropTreeBuilder<'tcx>>(
283 blocks: &mut IndexVec<DropIdx, Option<BasicBlock>>,
285 debug!("DropTree::build_mir(drops = {:#?})", self);
286 assert_eq!(blocks.len(), self.drops.len());
288 self.assign_blocks::<T>(cfg, blocks);
289 self.link_blocks(cfg, blocks)
292 /// Assign blocks for all of the drops in the drop tree that need them.
293 fn assign_blocks<'tcx, T: DropTreeBuilder<'tcx>>(
296 blocks: &mut IndexVec<DropIdx, Option<BasicBlock>>,
298 // StorageDead statements can share blocks with each other and also with
299 // a Drop terminator. We iterate through the drops to find which drops
300 // need their own block.
301 #[derive(Clone, Copy)]
303 // This drop is unreachable
305 // This drop is only reachable through the `StorageDead` with the
308 // This drop has more than one way of being reached, or it is
309 // branched to from outside the tree, or its predecessor is a
314 let mut needs_block = IndexVec::from_elem(Block::None, &self.drops);
315 if blocks[ROOT_NODE].is_some() {
316 // In some cases (such as drops for `continue`) the root node
317 // already has a block. In this case, make sure that we don't
319 needs_block[ROOT_NODE] = Block::Own;
322 // Sort so that we only need to check the last value.
323 let entry_points = &mut self.entry_points;
326 for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
327 if entry_points.last().map_or(false, |entry_point| entry_point.0 == drop_idx) {
328 let block = *blocks[drop_idx].get_or_insert_with(|| T::make_block(cfg));
329 needs_block[drop_idx] = Block::Own;
330 while entry_points.last().map_or(false, |entry_point| entry_point.0 == drop_idx) {
331 let entry_block = entry_points.pop().unwrap().1;
332 T::add_entry(cfg, entry_block, block);
335 match needs_block[drop_idx] {
336 Block::None => continue,
338 blocks[drop_idx].get_or_insert_with(|| T::make_block(cfg));
340 Block::Shares(pred) => {
341 blocks[drop_idx] = blocks[pred];
344 if let DropKind::Value = drop_data.0.kind {
345 needs_block[drop_data.1] = Block::Own;
346 } else if drop_idx != ROOT_NODE {
347 match &mut needs_block[drop_data.1] {
348 pred @ Block::None => *pred = Block::Shares(drop_idx),
349 pred @ Block::Shares(_) => *pred = Block::Own,
355 debug!("assign_blocks: blocks = {:#?}", blocks);
356 assert!(entry_points.is_empty());
359 fn link_blocks<'tcx>(
362 blocks: &IndexVec<DropIdx, Option<BasicBlock>>,
364 for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
365 let Some(block) = blocks[drop_idx] else { continue };
366 match drop_data.0.kind {
368 let terminator = TerminatorKind::Drop {
369 target: blocks[drop_data.1].unwrap(),
370 // The caller will handle this if needed.
372 place: drop_data.0.local.into(),
374 cfg.terminate(block, drop_data.0.source_info, terminator);
376 // Root nodes don't correspond to a drop.
377 DropKind::Storage if drop_idx == ROOT_NODE => {}
378 DropKind::Storage => {
379 let stmt = Statement {
380 source_info: drop_data.0.source_info,
381 kind: StatementKind::StorageDead(drop_data.0.local),
383 cfg.push(block, stmt);
384 let target = blocks[drop_data.1].unwrap();
386 // Diagnostics don't use this `Span` but debuginfo
387 // might. Since we don't want breakpoints to be placed
388 // here, especially when this is on an unwind path, we
390 let source_info = SourceInfo { span: DUMMY_SP, ..drop_data.0.source_info };
391 let terminator = TerminatorKind::Goto { target };
392 cfg.terminate(block, source_info, terminator);
400 impl<'tcx> Scopes<'tcx> {
401 pub(crate) fn new() -> Self {
404 breakable_scopes: Vec::new(),
406 unwind_drops: DropTree::new(),
407 generator_drops: DropTree::new(),
411 fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo), vis_scope: SourceScope) {
412 debug!("push_scope({:?})", region_scope);
413 self.scopes.push(Scope {
414 source_scope: vis_scope,
415 region_scope: region_scope.0,
417 moved_locals: vec![],
418 cached_unwind_block: None,
419 cached_generator_drop_block: None,
423 fn pop_scope(&mut self, region_scope: (region::Scope, SourceInfo)) -> Scope {
424 let scope = self.scopes.pop().unwrap();
425 assert_eq!(scope.region_scope, region_scope.0);
429 fn scope_index(&self, region_scope: region::Scope, span: Span) -> usize {
432 .rposition(|scope| scope.region_scope == region_scope)
433 .unwrap_or_else(|| span_bug!(span, "region_scope {:?} does not enclose", region_scope))
436 /// Returns the topmost active scope, which is known to be alive until
437 /// the next scope expression.
438 fn topmost(&self) -> region::Scope {
439 self.scopes.last().expect("topmost_scope: no scopes present").region_scope
443 impl<'a, 'tcx> Builder<'a, 'tcx> {
444 // Adding and removing scopes
445 // ==========================
446 // Start a breakable scope, which tracks where `continue`, `break` and
447 // `return` should branch to.
448 crate fn in_breakable_scope<F>(
450 loop_block: Option<BasicBlock>,
451 break_destination: Place<'tcx>,
456 F: FnOnce(&mut Builder<'a, 'tcx>) -> Option<BlockAnd<()>>,
458 let region_scope = self.scopes.topmost();
459 let scope = BreakableScope {
462 break_drops: DropTree::new(),
463 continue_drops: loop_block.map(|_| DropTree::new()),
465 self.scopes.breakable_scopes.push(scope);
466 let normal_exit_block = f(self);
467 let breakable_scope = self.scopes.breakable_scopes.pop().unwrap();
468 assert!(breakable_scope.region_scope == region_scope);
469 let break_block = self.build_exit_tree(breakable_scope.break_drops, None);
470 if let Some(drops) = breakable_scope.continue_drops {
471 self.build_exit_tree(drops, loop_block);
473 match (normal_exit_block, break_block) {
474 (Some(block), None) | (None, Some(block)) => block,
475 (None, None) => self.cfg.start_new_block().unit(),
476 (Some(normal_block), Some(exit_block)) => {
477 let target = self.cfg.start_new_block();
478 let source_info = self.source_info(span);
480 unpack!(normal_block),
482 TerminatorKind::Goto { target },
487 TerminatorKind::Goto { target },
494 /// Start an if-then scope which tracks drop for `if` expressions and `if`
497 /// For an if-let chain:
499 /// if let Some(x) = a && let Some(y) = b && let Some(z) = c { ... }
501 /// There are three possible ways the condition can be false and we may have
502 /// to drop `x`, `x` and `y`, or neither depending on which binding fails.
503 /// To handle this correctly we use a `DropTree` in a similar way to a
504 /// `loop` expression and 'break' out on all of the 'else' paths.
507 /// - We don't need to keep a stack of scopes in the `Builder` because the
508 /// 'else' paths will only leave the innermost scope.
509 /// - This is also used for match guards.
510 crate fn in_if_then_scope<F>(
512 region_scope: region::Scope,
514 ) -> (BasicBlock, BasicBlock)
516 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<()>,
518 let scope = IfThenScope { region_scope, else_drops: DropTree::new() };
519 let previous_scope = mem::replace(&mut self.scopes.if_then_scope, Some(scope));
521 let then_block = unpack!(f(self));
523 let if_then_scope = mem::replace(&mut self.scopes.if_then_scope, previous_scope).unwrap();
524 assert!(if_then_scope.region_scope == region_scope);
526 let else_block = self
527 .build_exit_tree(if_then_scope.else_drops, None)
528 .map_or_else(|| self.cfg.start_new_block(), |else_block_and| unpack!(else_block_and));
530 (then_block, else_block)
533 crate fn in_opt_scope<F, R>(
535 opt_scope: Option<(region::Scope, SourceInfo)>,
539 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>,
541 debug!("in_opt_scope(opt_scope={:?})", opt_scope);
542 if let Some(region_scope) = opt_scope {
543 self.push_scope(region_scope);
546 let rv = unpack!(block = f(self));
547 if let Some(region_scope) = opt_scope {
548 unpack!(block = self.pop_scope(region_scope, block));
550 debug!("in_scope: exiting opt_scope={:?} block={:?}", opt_scope, block);
554 /// Convenience wrapper that pushes a scope and then executes `f`
555 /// to build its contents, popping the scope afterwards.
556 crate fn in_scope<F, R>(
558 region_scope: (region::Scope, SourceInfo),
559 lint_level: LintLevel,
563 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>,
565 debug!("in_scope(region_scope={:?})", region_scope);
566 let source_scope = self.source_scope;
568 if let LintLevel::Explicit(current_hir_id) = lint_level {
569 // Use `maybe_lint_level_root_bounded` with `root_lint_level` as a bound
570 // to avoid adding Hir dependencies on our parents.
571 // We estimate the true lint roots here to avoid creating a lot of source scopes.
573 let parent_root = tcx.maybe_lint_level_root_bounded(
574 self.source_scopes[source_scope].local_data.as_ref().assert_crate_local().lint_root,
577 let current_root = tcx.maybe_lint_level_root_bounded(current_hir_id, self.hir_id);
579 if parent_root != current_root {
580 self.source_scope = self.new_source_scope(
582 LintLevel::Explicit(current_root),
587 self.push_scope(region_scope);
589 let rv = unpack!(block = f(self));
590 unpack!(block = self.pop_scope(region_scope, block));
591 self.source_scope = source_scope;
592 debug!("in_scope: exiting region_scope={:?} block={:?}", region_scope, block);
596 /// Push a scope onto the stack. You can then build code in this
597 /// scope and call `pop_scope` afterwards. Note that these two
598 /// calls must be paired; using `in_scope` as a convenience
599 /// wrapper maybe preferable.
600 crate fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo)) {
601 self.scopes.push_scope(region_scope, self.source_scope);
604 /// Pops a scope, which should have region scope `region_scope`,
605 /// adding any drops onto the end of `block` that are needed.
606 /// This must match 1-to-1 with `push_scope`.
609 region_scope: (region::Scope, SourceInfo),
610 mut block: BasicBlock,
612 debug!("pop_scope({:?}, {:?})", region_scope, block);
614 block = self.leave_top_scope(block);
616 self.scopes.pop_scope(region_scope);
621 /// Sets up the drops for breaking from `block` to `target`.
622 crate fn break_scope(
624 mut block: BasicBlock,
625 value: Option<&Expr<'tcx>>,
626 target: BreakableTarget,
627 source_info: SourceInfo,
629 let span = source_info.span;
631 let get_scope_index = |scope: region::Scope| {
632 // find the loop-scope by its `region::Scope`.
636 .rposition(|breakable_scope| breakable_scope.region_scope == scope)
637 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
639 let (break_index, destination) = match target {
640 BreakableTarget::Return => {
641 let scope = &self.scopes.breakable_scopes[0];
642 if scope.break_destination != Place::return_place() {
643 span_bug!(span, "`return` in item with no return scope");
645 (0, Some(scope.break_destination))
647 BreakableTarget::Break(scope) => {
648 let break_index = get_scope_index(scope);
649 let scope = &self.scopes.breakable_scopes[break_index];
650 (break_index, Some(scope.break_destination))
652 BreakableTarget::Continue(scope) => {
653 let break_index = get_scope_index(scope);
658 if let Some(destination) = destination {
659 if let Some(value) = value {
660 debug!("stmt_expr Break val block_context.push(SubExpr)");
661 self.block_context.push(BlockFrame::SubExpr);
662 unpack!(block = self.expr_into_dest(destination, block, value));
663 self.block_context.pop();
665 self.cfg.push_assign_unit(block, source_info, destination, self.tcx)
668 assert!(value.is_none(), "`return` and `break` should have a destination");
669 if self.tcx.sess.instrument_coverage() {
670 // Unlike `break` and `return`, which push an `Assign` statement to MIR, from which
671 // a Coverage code region can be generated, `continue` needs no `Assign`; but
672 // without one, the `InstrumentCoverage` MIR pass cannot generate a code region for
673 // `continue`. Coverage will be missing unless we add a dummy `Assign` to MIR.
674 self.add_dummy_assignment(span, block, source_info);
678 let region_scope = self.scopes.breakable_scopes[break_index].region_scope;
679 let scope_index = self.scopes.scope_index(region_scope, span);
680 let drops = if destination.is_some() {
681 &mut self.scopes.breakable_scopes[break_index].break_drops
683 self.scopes.breakable_scopes[break_index].continue_drops.as_mut().unwrap()
685 let mut drop_idx = ROOT_NODE;
686 for scope in &self.scopes.scopes[scope_index + 1..] {
687 for drop in &scope.drops {
688 drop_idx = drops.add_drop(*drop, drop_idx);
691 drops.add_entry(block, drop_idx);
693 // `build_drop_tree` doesn't have access to our source_info, so we
694 // create a dummy terminator now. `TerminatorKind::Resume` is used
695 // because MIR type checking will panic if it hasn't been overwritten.
696 self.cfg.terminate(block, source_info, TerminatorKind::Resume);
698 self.cfg.start_new_block().unit()
701 crate fn break_for_else(
704 target: region::Scope,
705 source_info: SourceInfo,
707 let scope_index = self.scopes.scope_index(target, source_info.span);
708 let if_then_scope = self
712 .unwrap_or_else(|| span_bug!(source_info.span, "no if-then scope found"));
714 assert_eq!(if_then_scope.region_scope, target, "breaking to incorrect scope");
716 let mut drop_idx = ROOT_NODE;
717 let drops = &mut if_then_scope.else_drops;
718 for scope in &self.scopes.scopes[scope_index + 1..] {
719 for drop in &scope.drops {
720 drop_idx = drops.add_drop(*drop, drop_idx);
723 drops.add_entry(block, drop_idx);
725 // `build_drop_tree` doesn't have access to our source_info, so we
726 // create a dummy terminator now. `TerminatorKind::Resume` is used
727 // because MIR type checking will panic if it hasn't been overwritten.
728 self.cfg.terminate(block, source_info, TerminatorKind::Resume);
731 // Add a dummy `Assign` statement to the CFG, with the span for the source code's `continue`
733 fn add_dummy_assignment(&mut self, span: Span, block: BasicBlock, source_info: SourceInfo) {
734 let local_decl = LocalDecl::new(self.tcx.mk_unit(), span).internal();
735 let temp_place = Place::from(self.local_decls.push(local_decl));
736 self.cfg.push_assign_unit(block, source_info, temp_place, self.tcx);
739 fn leave_top_scope(&mut self, block: BasicBlock) -> BasicBlock {
740 // If we are emitting a `drop` statement, we need to have the cached
741 // diverge cleanup pads ready in case that drop panics.
742 let needs_cleanup = self.scopes.scopes.last().map_or(false, |scope| scope.needs_cleanup());
743 let is_generator = self.generator_kind.is_some();
744 let unwind_to = if needs_cleanup { self.diverge_cleanup() } else { DropIdx::MAX };
746 let scope = self.scopes.scopes.last().expect("leave_top_scope called with no scopes");
747 unpack!(build_scope_drops(
749 &mut self.scopes.unwind_drops,
753 is_generator && needs_cleanup,
758 /// Creates a new source scope, nested in the current one.
759 crate fn new_source_scope(
762 lint_level: LintLevel,
763 safety: Option<Safety>,
765 let parent = self.source_scope;
767 "new_source_scope({:?}, {:?}, {:?}) - parent({:?})={:?}",
772 self.source_scopes.get(parent)
774 let scope_local_data = SourceScopeLocalData {
775 lint_root: if let LintLevel::Explicit(lint_root) = lint_level {
778 self.source_scopes[parent].local_data.as_ref().assert_crate_local().lint_root
780 safety: safety.unwrap_or_else(|| {
781 self.source_scopes[parent].local_data.as_ref().assert_crate_local().safety
784 self.source_scopes.push(SourceScopeData {
786 parent_scope: Some(parent),
788 inlined_parent_scope: None,
789 local_data: ClearCrossCrate::Set(scope_local_data),
793 /// Given a span and the current source scope, make a SourceInfo.
794 crate fn source_info(&self, span: Span) -> SourceInfo {
795 SourceInfo { span, scope: self.source_scope }
800 /// Returns the scope that we should use as the lifetime of an
801 /// operand. Basically, an operand must live until it is consumed.
802 /// This is similar to, but not quite the same as, the temporary
803 /// scope (which can be larger or smaller).
807 /// let x = foo(bar(X, Y));
809 /// We wish to pop the storage for X and Y after `bar()` is
810 /// called, not after the whole `let` is completed.
812 /// As another example, if the second argument diverges:
814 /// foo(Box::new(2), panic!())
816 /// We would allocate the box but then free it on the unwinding
817 /// path; we would also emit a free on the 'success' path from
818 /// panic, but that will turn out to be removed as dead-code.
819 crate fn local_scope(&self) -> region::Scope {
820 self.scopes.topmost()
825 crate fn schedule_drop_storage_and_value(
828 region_scope: region::Scope,
831 self.schedule_drop(span, region_scope, local, DropKind::Storage);
832 self.schedule_drop(span, region_scope, local, DropKind::Value);
835 /// Indicates that `place` should be dropped on exit from `region_scope`.
837 /// When called with `DropKind::Storage`, `place` shouldn't be the return
838 /// place, or a function parameter.
839 crate fn schedule_drop(
842 region_scope: region::Scope,
846 let needs_drop = match drop_kind {
848 if !self.local_decls[local].ty.needs_drop(self.tcx, self.param_env) {
853 DropKind::Storage => {
854 if local.index() <= self.arg_count {
857 "`schedule_drop` called with local {:?} and arg_count {}",
866 // When building drops, we try to cache chains of drops to reduce the
867 // number of `DropTree::add_drop` calls. This, however, means that
868 // whenever we add a drop into a scope which already had some entries
869 // in the drop tree built (and thus, cached) for it, we must invalidate
870 // all caches which might branch into the scope which had a drop just
871 // added to it. This is necessary, because otherwise some other code
872 // might use the cache to branch into already built chain of drops,
873 // essentially ignoring the newly added drop.
875 // For example consider there’s two scopes with a drop in each. These
876 // are built and thus the caches are filled:
878 // +--------------------------------------------------------+
879 // | +---------------------------------+ |
880 // | | +--------+ +-------------+ | +---------------+ |
881 // | | | return | <-+ | drop(outer) | <-+ | drop(middle) | |
882 // | | +--------+ +-------------+ | +---------------+ |
883 // | +------------|outer_scope cache|--+ |
884 // +------------------------------|middle_scope cache|------+
886 // Now, a new, inner-most scope is added along with a new drop into
887 // both inner-most and outer-most scopes:
889 // +------------------------------------------------------------+
890 // | +----------------------------------+ |
891 // | | +--------+ +-------------+ | +---------------+ | +-------------+
892 // | | | return | <+ | drop(new) | <-+ | drop(middle) | <--+| drop(inner) |
893 // | | +--------+ | | drop(outer) | | +---------------+ | +-------------+
894 // | | +-+ +-------------+ | |
895 // | +---|invalid outer_scope cache|----+ |
896 // +----=----------------|invalid middle_scope cache|-----------+
898 // If, when adding `drop(new)` we do not invalidate the cached blocks for both
899 // outer_scope and middle_scope, then, when building drops for the inner (right-most)
900 // scope, the old, cached blocks, without `drop(new)` will get used, producing the
903 // Note that this code iterates scopes from the inner-most to the outer-most,
904 // invalidating caches of each scope visited. This way bare minimum of the
905 // caches gets invalidated. i.e., if a new drop is added into the middle scope, the
906 // cache of outer scope stays intact.
908 // Since we only cache drops for the unwind path and the generator drop
909 // path, we only need to invalidate the cache for drops that happen on
910 // the unwind or generator drop paths. This means that for
911 // non-generators we don't need to invalidate caches for `DropKind::Storage`.
912 let invalidate_caches = needs_drop || self.generator_kind.is_some();
913 for scope in self.scopes.scopes.iter_mut().rev() {
914 if invalidate_caches {
915 scope.invalidate_cache();
918 if scope.region_scope == region_scope {
919 let region_scope_span = region_scope.span(self.tcx, &self.region_scope_tree);
920 // Attribute scope exit drops to scope's closing brace.
921 let scope_end = self.tcx.sess.source_map().end_point(region_scope_span);
923 scope.drops.push(DropData {
924 source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
933 span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, local);
936 /// Indicates that the "local operand" stored in `local` is
937 /// *moved* at some point during execution (see `local_scope` for
938 /// more information about what a "local operand" is -- in short,
939 /// it's an intermediate operand created as part of preparing some
940 /// MIR instruction). We use this information to suppress
941 /// redundant drops on the non-unwind paths. This results in less
942 /// MIR, but also avoids spurious borrow check errors
945 /// Example: when compiling the call to `foo` here:
951 /// we would evaluate `bar()` to an operand `_X`. We would also
952 /// schedule `_X` to be dropped when the expression scope for
953 /// `foo(bar())` is exited. This is relevant, for example, if the
954 /// later arguments should unwind (it would ensure that `_X` gets
955 /// dropped). However, if no unwind occurs, then `_X` will be
956 /// unconditionally consumed by the `call`:
961 /// _R = CALL(foo, _X, ...)
965 /// However, `_X` is still registered to be dropped, and so if we
966 /// do nothing else, we would generate a `DROP(_X)` that occurs
967 /// after the call. This will later be optimized out by the
968 /// drop-elaboration code, but in the meantime it can lead to
969 /// spurious borrow-check errors -- the problem, ironically, is
970 /// not the `DROP(_X)` itself, but the (spurious) unwind pathways
971 /// that it creates. See #64391 for an example.
972 crate fn record_operands_moved(&mut self, operands: &[Operand<'tcx>]) {
973 let local_scope = self.local_scope();
974 let scope = self.scopes.scopes.last_mut().unwrap();
976 assert_eq!(scope.region_scope, local_scope, "local scope is not the topmost scope!",);
978 // look for moves of a local variable, like `MOVE(_X)`
979 let locals_moved = operands.iter().flat_map(|operand| match operand {
980 Operand::Copy(_) | Operand::Constant(_) => None,
981 Operand::Move(place) => place.as_local(),
984 for local in locals_moved {
985 // check if we have a Drop for this operand and -- if so
986 // -- add it to the list of moved operands. Note that this
987 // local might not have been an operand created for this
988 // call, it could come from other places too.
989 if scope.drops.iter().any(|drop| drop.local == local && drop.kind == DropKind::Value) {
990 scope.moved_locals.push(local);
997 /// Returns the [DropIdx] for the innermost drop if the function unwound at
998 /// this point. The `DropIdx` will be created if it doesn't already exist.
999 fn diverge_cleanup(&mut self) -> DropIdx {
1000 let is_generator = self.generator_kind.is_some();
1001 let (uncached_scope, mut cached_drop) = self
1007 .find_map(|(scope_idx, scope)| {
1008 scope.cached_unwind_block.map(|cached_block| (scope_idx + 1, cached_block))
1010 .unwrap_or((0, ROOT_NODE));
1012 for scope in &mut self.scopes.scopes[uncached_scope..] {
1013 for drop in &scope.drops {
1014 if is_generator || drop.kind == DropKind::Value {
1015 cached_drop = self.scopes.unwind_drops.add_drop(*drop, cached_drop);
1018 scope.cached_unwind_block = Some(cached_drop);
1024 /// Prepares to create a path that performs all required cleanup for a
1025 /// terminator that can unwind at the given basic block.
1027 /// This path terminates in Resume. The path isn't created until after all
1028 /// of the non-unwind paths in this item have been lowered.
1029 crate fn diverge_from(&mut self, start: BasicBlock) {
1032 self.cfg.block_data(start).terminator().kind,
1033 TerminatorKind::Assert { .. }
1034 | TerminatorKind::Call { .. }
1035 | TerminatorKind::DropAndReplace { .. }
1036 | TerminatorKind::FalseUnwind { .. }
1037 | TerminatorKind::InlineAsm { .. }
1039 "diverge_from called on block with terminator that cannot unwind."
1042 let next_drop = self.diverge_cleanup();
1043 self.scopes.unwind_drops.add_entry(start, next_drop);
1046 /// Sets up a path that performs all required cleanup for dropping a
1047 /// generator, starting from the given block that ends in
1048 /// [TerminatorKind::Yield].
1050 /// This path terminates in GeneratorDrop.
1051 crate fn generator_drop_cleanup(&mut self, yield_block: BasicBlock) {
1054 self.cfg.block_data(yield_block).terminator().kind,
1055 TerminatorKind::Yield { .. }
1057 "generator_drop_cleanup called on block with non-yield terminator."
1059 let (uncached_scope, mut cached_drop) = self
1065 .find_map(|(scope_idx, scope)| {
1066 scope.cached_generator_drop_block.map(|cached_block| (scope_idx + 1, cached_block))
1068 .unwrap_or((0, ROOT_NODE));
1070 for scope in &mut self.scopes.scopes[uncached_scope..] {
1071 for drop in &scope.drops {
1072 cached_drop = self.scopes.generator_drops.add_drop(*drop, cached_drop);
1074 scope.cached_generator_drop_block = Some(cached_drop);
1077 self.scopes.generator_drops.add_entry(yield_block, cached_drop);
1080 /// Utility function for *non*-scope code to build their own drops
1081 crate fn build_drop_and_replace(
1086 value: Operand<'tcx>,
1088 let source_info = self.source_info(span);
1089 let next_target = self.cfg.start_new_block();
1094 TerminatorKind::DropAndReplace { place, value, target: next_target, unwind: None },
1096 self.diverge_from(block);
1101 /// Creates an `Assert` terminator and return the success block.
1102 /// If the boolean condition operand is not the expected value,
1103 /// a runtime panic will be caused with the given message.
1107 cond: Operand<'tcx>,
1109 msg: AssertMessage<'tcx>,
1112 let source_info = self.source_info(span);
1113 let success_block = self.cfg.start_new_block();
1118 TerminatorKind::Assert { cond, expected, msg, target: success_block, cleanup: None },
1120 self.diverge_from(block);
1125 /// Unschedules any drops in the top scope.
1127 /// This is only needed for `match` arm scopes, because they have one
1128 /// entrance per pattern, but only one exit.
1129 crate fn clear_top_scope(&mut self, region_scope: region::Scope) {
1130 let top_scope = self.scopes.scopes.last_mut().unwrap();
1132 assert_eq!(top_scope.region_scope, region_scope);
1134 top_scope.drops.clear();
1135 top_scope.invalidate_cache();
1139 /// Builds drops for `pop_scope` and `leave_top_scope`.
1140 fn build_scope_drops<'tcx>(
1141 cfg: &mut CFG<'tcx>,
1142 unwind_drops: &mut DropTree,
1144 mut block: BasicBlock,
1145 mut unwind_to: DropIdx,
1146 storage_dead_on_unwind: bool,
1149 debug!("build_scope_drops({:?} -> {:?})", block, scope);
1151 // Build up the drops in evaluation order. The end result will
1154 // [SDs, drops[n]] --..> [SDs, drop[1]] -> [SDs, drop[0]] -> [[SDs]]
1158 // [drop[n]] -...-> [drop[1]] ------> [drop[0]] ------> [last_unwind_to]
1160 // The horizontal arrows represent the execution path when the drops return
1161 // successfully. The downwards arrows represent the execution path when the
1162 // drops panic (panicking while unwinding will abort, so there's no need for
1163 // another set of arrows).
1165 // For generators, we unwind from a drop on a local to its StorageDead
1166 // statement. For other functions we don't worry about StorageDead. The
1167 // drops for the unwind path should have already been generated by
1168 // `diverge_cleanup_gen`.
1170 for drop_data in scope.drops.iter().rev() {
1171 let source_info = drop_data.source_info;
1172 let local = drop_data.local;
1174 match drop_data.kind {
1175 DropKind::Value => {
1176 // `unwind_to` should drop the value that we're about to
1177 // schedule. If dropping this value panics, then we continue
1178 // with the *next* value on the unwind path.
1179 debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
1180 debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
1181 unwind_to = unwind_drops.drops[unwind_to].1;
1183 // If the operand has been moved, and we are not on an unwind
1184 // path, then don't generate the drop. (We only take this into
1185 // account for non-unwind paths so as not to disturb the
1186 // caching mechanism.)
1187 if scope.moved_locals.iter().any(|&o| o == local) {
1191 unwind_drops.add_entry(block, unwind_to);
1193 let next = cfg.start_new_block();
1197 TerminatorKind::Drop { place: local.into(), target: next, unwind: None },
1201 DropKind::Storage => {
1202 if storage_dead_on_unwind {
1203 debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
1204 debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
1205 unwind_to = unwind_drops.drops[unwind_to].1;
1207 // Only temps and vars need their storage dead.
1208 assert!(local.index() > arg_count);
1209 cfg.push(block, Statement { source_info, kind: StatementKind::StorageDead(local) });
1216 impl<'a, 'tcx: 'a> Builder<'a, 'tcx> {
1217 /// Build a drop tree for a breakable scope.
1219 /// If `continue_block` is `Some`, then the tree is for `continue` inside a
1220 /// loop. Otherwise this is for `break` or `return`.
1223 mut drops: DropTree,
1224 continue_block: Option<BasicBlock>,
1225 ) -> Option<BlockAnd<()>> {
1226 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1227 blocks[ROOT_NODE] = continue_block;
1229 drops.build_mir::<ExitScopes>(&mut self.cfg, &mut blocks);
1231 // Link the exit drop tree to unwind drop tree.
1232 if drops.drops.iter().any(|(drop, _)| drop.kind == DropKind::Value) {
1233 let unwind_target = self.diverge_cleanup();
1234 let mut unwind_indices = IndexVec::from_elem_n(unwind_target, 1);
1235 for (drop_idx, drop_data) in drops.drops.iter_enumerated().skip(1) {
1236 match drop_data.0.kind {
1237 DropKind::Storage => {
1238 if self.generator_kind.is_some() {
1239 let unwind_drop = self
1242 .add_drop(drop_data.0, unwind_indices[drop_data.1]);
1243 unwind_indices.push(unwind_drop);
1245 unwind_indices.push(unwind_indices[drop_data.1]);
1248 DropKind::Value => {
1249 let unwind_drop = self
1252 .add_drop(drop_data.0, unwind_indices[drop_data.1]);
1255 .add_entry(blocks[drop_idx].unwrap(), unwind_indices[drop_data.1]);
1256 unwind_indices.push(unwind_drop);
1261 blocks[ROOT_NODE].map(BasicBlock::unit)
1264 /// Build the unwind and generator drop trees.
1265 crate fn build_drop_trees(&mut self) {
1266 if self.generator_kind.is_some() {
1267 self.build_generator_drop_trees();
1269 Self::build_unwind_tree(
1271 &mut self.scopes.unwind_drops,
1278 fn build_generator_drop_trees(&mut self) {
1279 // Build the drop tree for dropping the generator while it's suspended.
1280 let drops = &mut self.scopes.generator_drops;
1281 let cfg = &mut self.cfg;
1282 let fn_span = self.fn_span;
1283 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1284 drops.build_mir::<GeneratorDrop>(cfg, &mut blocks);
1285 if let Some(root_block) = blocks[ROOT_NODE] {
1288 SourceInfo::outermost(fn_span),
1289 TerminatorKind::GeneratorDrop,
1293 // Build the drop tree for unwinding in the normal control flow paths.
1294 let resume_block = &mut None;
1295 let unwind_drops = &mut self.scopes.unwind_drops;
1296 Self::build_unwind_tree(cfg, unwind_drops, fn_span, resume_block);
1298 // Build the drop tree for unwinding when dropping a suspended
1301 // This is a different tree to the standard unwind paths here to
1302 // prevent drop elaboration from creating drop flags that would have
1303 // to be captured by the generator. I'm not sure how important this
1304 // optimization is, but it is here.
1305 for (drop_idx, drop_data) in drops.drops.iter_enumerated() {
1306 if let DropKind::Value = drop_data.0.kind {
1307 debug_assert!(drop_data.1 < drops.drops.next_index());
1308 drops.entry_points.push((drop_data.1, blocks[drop_idx].unwrap()));
1311 Self::build_unwind_tree(cfg, drops, fn_span, resume_block);
1314 fn build_unwind_tree(
1315 cfg: &mut CFG<'tcx>,
1316 drops: &mut DropTree,
1318 resume_block: &mut Option<BasicBlock>,
1320 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1321 blocks[ROOT_NODE] = *resume_block;
1322 drops.build_mir::<Unwind>(cfg, &mut blocks);
1323 if let (None, Some(resume)) = (*resume_block, blocks[ROOT_NODE]) {
1324 cfg.terminate(resume, SourceInfo::outermost(fn_span), TerminatorKind::Resume);
1326 *resume_block = blocks[ROOT_NODE];
1331 // DropTreeBuilder implementations.
1335 impl<'tcx> DropTreeBuilder<'tcx> for ExitScopes {
1336 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1337 cfg.start_new_block()
1339 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1340 cfg.block_data_mut(from).terminator_mut().kind = TerminatorKind::Goto { target: to };
1344 struct GeneratorDrop;
1346 impl<'tcx> DropTreeBuilder<'tcx> for GeneratorDrop {
1347 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1348 cfg.start_new_block()
1350 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1351 let term = cfg.block_data_mut(from).terminator_mut();
1352 if let TerminatorKind::Yield { ref mut drop, .. } = term.kind {
1356 term.source_info.span,
1357 "cannot enter generator drop tree from {:?}",
1366 impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
1367 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1368 cfg.start_new_cleanup_block()
1370 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1371 let term = &mut cfg.block_data_mut(from).terminator_mut();
1372 match &mut term.kind {
1373 TerminatorKind::Drop { unwind, .. }
1374 | TerminatorKind::DropAndReplace { unwind, .. }
1375 | TerminatorKind::FalseUnwind { unwind, .. }
1376 | TerminatorKind::Call { cleanup: unwind, .. }
1377 | TerminatorKind::Assert { cleanup: unwind, .. }
1378 | TerminatorKind::InlineAsm { cleanup: unwind, .. } => {
1381 TerminatorKind::Goto { .. }
1382 | TerminatorKind::SwitchInt { .. }
1383 | TerminatorKind::Resume
1384 | TerminatorKind::Abort
1385 | TerminatorKind::Return
1386 | TerminatorKind::Unreachable
1387 | TerminatorKind::Yield { .. }
1388 | TerminatorKind::GeneratorDrop
1389 | TerminatorKind::FalseEdge { .. } => {
1390 span_bug!(term.source_info.span, "cannot unwind from {:?}", term.kind)