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;
89 use rustc_index::vec::IndexVec;
90 use rustc_middle::middle::region;
91 use rustc_middle::mir::*;
92 use rustc_middle::thir::{Expr, LintLevel};
94 use rustc_span::{Span, DUMMY_SP};
97 pub struct Scopes<'tcx> {
100 /// The current set of breakable scopes. See module comment for more details.
101 breakable_scopes: Vec<BreakableScope<'tcx>>,
103 /// The scope of the innermost if-then currently being lowered.
104 if_then_scope: Option<IfThenScope>,
106 /// Drops that need to be done on unwind paths. See the comment on
107 /// [DropTree] for more details.
108 unwind_drops: DropTree,
110 /// Drops that need to be done on paths to the `GeneratorDrop` terminator.
111 generator_drops: DropTree,
116 /// The source scope this scope was created in.
117 source_scope: SourceScope,
119 /// the region span of this scope within source code.
120 region_scope: region::Scope,
122 /// set of places to drop when exiting this scope. This starts
123 /// out empty but grows as variables are declared during the
124 /// building process. This is a stack, so we always drop from the
125 /// end of the vector (top of the stack) first.
126 drops: Vec<DropData>,
128 moved_locals: Vec<Local>,
130 /// The drop index that will drop everything in and below this scope on an
132 cached_unwind_block: Option<DropIdx>,
134 /// The drop index that will drop everything in and below this scope on a
135 /// generator drop path.
136 cached_generator_drop_block: Option<DropIdx>,
139 #[derive(Clone, Copy, Debug)]
141 /// The `Span` where drop obligation was incurred (typically where place was
143 source_info: SourceInfo,
148 /// Whether this is a value Drop or a StorageDead.
152 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
153 pub(crate) enum DropKind {
159 struct BreakableScope<'tcx> {
160 /// Region scope of the loop
161 region_scope: region::Scope,
162 /// The destination of the loop/block expression itself (i.e., where to put
163 /// the result of a `break` or `return` expression)
164 break_destination: Place<'tcx>,
165 /// Drops that happen on the `break`/`return` path.
166 break_drops: DropTree,
167 /// Drops that happen on the `continue` path.
168 continue_drops: Option<DropTree>,
173 /// The if-then scope or arm scope
174 region_scope: region::Scope,
175 /// Drops that happen on the `else` path.
176 else_drops: DropTree,
179 /// The target of an expression that breaks out of a scope
180 #[derive(Clone, Copy, Debug)]
181 pub(crate) enum BreakableTarget {
182 Continue(region::Scope),
183 Break(region::Scope),
187 rustc_index::newtype_index! {
188 struct DropIdx { .. }
191 const ROOT_NODE: DropIdx = DropIdx::from_u32(0);
193 /// A tree of drops that we have deferred lowering. It's used for:
195 /// * Drops on unwind paths
196 /// * Drops on generator drop paths (when a suspended generator is dropped)
197 /// * Drops on return and loop exit paths
198 /// * Drops on the else path in an `if let` chain
200 /// Once no more nodes could be added to the tree, we lower it to MIR in one go
204 /// Drops in the tree.
205 drops: IndexVec<DropIdx, (DropData, DropIdx)>,
206 /// Map for finding the inverse of the `next_drop` relation:
208 /// `previous_drops[(drops[i].1, drops[i].0.local, drops[i].0.kind)] == i`
209 previous_drops: FxHashMap<(DropIdx, Local, DropKind), DropIdx>,
210 /// Edges into the `DropTree` that need to be added once it's lowered.
211 entry_points: Vec<(DropIdx, BasicBlock)>,
215 /// Whether there's anything to do for the cleanup path, that is,
216 /// when unwinding through this scope. This includes destructors,
217 /// but not StorageDead statements, which don't get emitted at all
218 /// for unwinding, for several reasons:
219 /// * clang doesn't emit llvm.lifetime.end for C++ unwinding
220 /// * LLVM's memory dependency analysis can't handle it atm
221 /// * polluting the cleanup MIR with StorageDead creates
222 /// landing pads even though there's no actual destructors
223 /// * freeing up stack space has no effect during unwinding
224 /// Note that for generators we do emit StorageDeads, for the
225 /// use of optimizations in the MIR generator transform.
226 fn needs_cleanup(&self) -> bool {
227 self.drops.iter().any(|drop| match drop.kind {
228 DropKind::Value => true,
229 DropKind::Storage => false,
233 fn invalidate_cache(&mut self) {
234 self.cached_unwind_block = None;
235 self.cached_generator_drop_block = None;
239 /// A trait that determined how [DropTree] creates its blocks and
240 /// links to any entry nodes.
241 trait DropTreeBuilder<'tcx> {
242 /// Create a new block for the tree. This should call either
243 /// `cfg.start_new_block()` or `cfg.start_new_cleanup_block()`.
244 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock;
246 /// Links a block outside the drop tree, `from`, to the block `to` inside
248 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock);
253 // The root node of the tree doesn't represent a drop, but instead
254 // represents the block in the tree that should be jumped to once all
255 // of the required drops have been performed.
256 let fake_source_info = SourceInfo::outermost(DUMMY_SP);
258 DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
259 let drop_idx = DropIdx::MAX;
260 let drops = IndexVec::from_elem_n((fake_data, drop_idx), 1);
261 Self { drops, entry_points: Vec::new(), previous_drops: FxHashMap::default() }
264 fn add_drop(&mut self, drop: DropData, next: DropIdx) -> DropIdx {
265 let drops = &mut self.drops;
268 .entry((next, drop.local, drop.kind))
269 .or_insert_with(|| drops.push((drop, next)))
272 fn add_entry(&mut self, from: BasicBlock, to: DropIdx) {
273 debug_assert!(to < self.drops.next_index());
274 self.entry_points.push((to, from));
277 /// Builds the MIR for a given drop tree.
279 /// `blocks` should have the same length as `self.drops`, and may have its
280 /// first value set to some already existing block.
281 fn build_mir<'tcx, T: DropTreeBuilder<'tcx>>(
284 blocks: &mut IndexVec<DropIdx, Option<BasicBlock>>,
286 debug!("DropTree::build_mir(drops = {:#?})", self);
287 assert_eq!(blocks.len(), self.drops.len());
289 self.assign_blocks::<T>(cfg, blocks);
290 self.link_blocks(cfg, blocks)
293 /// Assign blocks for all of the drops in the drop tree that need them.
294 fn assign_blocks<'tcx, T: DropTreeBuilder<'tcx>>(
297 blocks: &mut IndexVec<DropIdx, Option<BasicBlock>>,
299 // StorageDead statements can share blocks with each other and also with
300 // a Drop terminator. We iterate through the drops to find which drops
301 // need their own block.
302 #[derive(Clone, Copy)]
304 // This drop is unreachable
306 // This drop is only reachable through the `StorageDead` with the
309 // This drop has more than one way of being reached, or it is
310 // branched to from outside the tree, or its predecessor is a
315 let mut needs_block = IndexVec::from_elem(Block::None, &self.drops);
316 if blocks[ROOT_NODE].is_some() {
317 // In some cases (such as drops for `continue`) the root node
318 // already has a block. In this case, make sure that we don't
320 needs_block[ROOT_NODE] = Block::Own;
323 // Sort so that we only need to check the last value.
324 let entry_points = &mut self.entry_points;
327 for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
328 if entry_points.last().map_or(false, |entry_point| entry_point.0 == drop_idx) {
329 let block = *blocks[drop_idx].get_or_insert_with(|| T::make_block(cfg));
330 needs_block[drop_idx] = Block::Own;
331 while entry_points.last().map_or(false, |entry_point| entry_point.0 == drop_idx) {
332 let entry_block = entry_points.pop().unwrap().1;
333 T::add_entry(cfg, entry_block, block);
336 match needs_block[drop_idx] {
337 Block::None => continue,
339 blocks[drop_idx].get_or_insert_with(|| T::make_block(cfg));
341 Block::Shares(pred) => {
342 blocks[drop_idx] = blocks[pred];
345 if let DropKind::Value = drop_data.0.kind {
346 needs_block[drop_data.1] = Block::Own;
347 } else if drop_idx != ROOT_NODE {
348 match &mut needs_block[drop_data.1] {
349 pred @ Block::None => *pred = Block::Shares(drop_idx),
350 pred @ Block::Shares(_) => *pred = Block::Own,
356 debug!("assign_blocks: blocks = {:#?}", blocks);
357 assert!(entry_points.is_empty());
360 fn link_blocks<'tcx>(
363 blocks: &IndexVec<DropIdx, Option<BasicBlock>>,
365 for (drop_idx, drop_data) in self.drops.iter_enumerated().rev() {
366 let Some(block) = blocks[drop_idx] else { continue };
367 match drop_data.0.kind {
369 let terminator = TerminatorKind::Drop {
370 target: blocks[drop_data.1].unwrap(),
371 // The caller will handle this if needed.
373 place: drop_data.0.local.into(),
375 cfg.terminate(block, drop_data.0.source_info, terminator);
377 // Root nodes don't correspond to a drop.
378 DropKind::Storage if drop_idx == ROOT_NODE => {}
379 DropKind::Storage => {
380 let stmt = Statement {
381 source_info: drop_data.0.source_info,
382 kind: StatementKind::StorageDead(drop_data.0.local),
384 cfg.push(block, stmt);
385 let target = blocks[drop_data.1].unwrap();
387 // Diagnostics don't use this `Span` but debuginfo
388 // might. Since we don't want breakpoints to be placed
389 // here, especially when this is on an unwind path, we
391 let source_info = SourceInfo { span: DUMMY_SP, ..drop_data.0.source_info };
392 let terminator = TerminatorKind::Goto { target };
393 cfg.terminate(block, source_info, terminator);
401 impl<'tcx> Scopes<'tcx> {
402 pub(crate) fn new() -> Self {
405 breakable_scopes: Vec::new(),
407 unwind_drops: DropTree::new(),
408 generator_drops: DropTree::new(),
412 fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo), vis_scope: SourceScope) {
413 debug!("push_scope({:?})", region_scope);
414 self.scopes.push(Scope {
415 source_scope: vis_scope,
416 region_scope: region_scope.0,
418 moved_locals: vec![],
419 cached_unwind_block: None,
420 cached_generator_drop_block: None,
424 fn pop_scope(&mut self, region_scope: (region::Scope, SourceInfo)) -> Scope {
425 let scope = self.scopes.pop().unwrap();
426 assert_eq!(scope.region_scope, region_scope.0);
430 fn scope_index(&self, region_scope: region::Scope, span: Span) -> usize {
433 .rposition(|scope| scope.region_scope == region_scope)
434 .unwrap_or_else(|| span_bug!(span, "region_scope {:?} does not enclose", region_scope))
437 /// Returns the topmost active scope, which is known to be alive until
438 /// the next scope expression.
439 fn topmost(&self) -> region::Scope {
440 self.scopes.last().expect("topmost_scope: no scopes present").region_scope
444 impl<'a, 'tcx> Builder<'a, 'tcx> {
445 // Adding and removing scopes
446 // ==========================
448 /// Start a breakable scope, which tracks where `continue`, `break` and
449 /// `return` should branch to.
450 pub(crate) fn in_breakable_scope<F>(
452 loop_block: Option<BasicBlock>,
453 break_destination: Place<'tcx>,
458 F: FnOnce(&mut Builder<'a, 'tcx>) -> Option<BlockAnd<()>>,
460 let region_scope = self.scopes.topmost();
461 let scope = BreakableScope {
464 break_drops: DropTree::new(),
465 continue_drops: loop_block.map(|_| DropTree::new()),
467 self.scopes.breakable_scopes.push(scope);
468 let normal_exit_block = f(self);
469 let breakable_scope = self.scopes.breakable_scopes.pop().unwrap();
470 assert!(breakable_scope.region_scope == region_scope);
472 self.build_exit_tree(breakable_scope.break_drops, region_scope, span, None);
473 if let Some(drops) = breakable_scope.continue_drops {
474 self.build_exit_tree(drops, region_scope, span, loop_block);
476 match (normal_exit_block, break_block) {
477 (Some(block), None) | (None, Some(block)) => block,
478 (None, None) => self.cfg.start_new_block().unit(),
479 (Some(normal_block), Some(exit_block)) => {
480 let target = self.cfg.start_new_block();
481 let source_info = self.source_info(span);
483 unpack!(normal_block),
485 TerminatorKind::Goto { target },
490 TerminatorKind::Goto { target },
497 /// Start an if-then scope which tracks drop for `if` expressions and `if`
500 /// For an if-let chain:
502 /// if let Some(x) = a && let Some(y) = b && let Some(z) = c { ... }
504 /// There are three possible ways the condition can be false and we may have
505 /// to drop `x`, `x` and `y`, or neither depending on which binding fails.
506 /// To handle this correctly we use a `DropTree` in a similar way to a
507 /// `loop` expression and 'break' out on all of the 'else' paths.
510 /// - We don't need to keep a stack of scopes in the `Builder` because the
511 /// 'else' paths will only leave the innermost scope.
512 /// - This is also used for match guards.
513 pub(crate) fn in_if_then_scope<F>(
515 region_scope: region::Scope,
518 ) -> (BasicBlock, BasicBlock)
520 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<()>,
522 let scope = IfThenScope { region_scope, else_drops: DropTree::new() };
523 let previous_scope = mem::replace(&mut self.scopes.if_then_scope, Some(scope));
525 let then_block = unpack!(f(self));
527 let if_then_scope = mem::replace(&mut self.scopes.if_then_scope, previous_scope).unwrap();
528 assert!(if_then_scope.region_scope == region_scope);
530 let else_block = self
531 .build_exit_tree(if_then_scope.else_drops, region_scope, span, None)
532 .map_or_else(|| self.cfg.start_new_block(), |else_block_and| unpack!(else_block_and));
534 (then_block, else_block)
537 pub(crate) fn in_opt_scope<F, R>(
539 opt_scope: Option<(region::Scope, SourceInfo)>,
543 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>,
545 debug!("in_opt_scope(opt_scope={:?})", opt_scope);
546 if let Some(region_scope) = opt_scope {
547 self.push_scope(region_scope);
550 let rv = unpack!(block = f(self));
551 if let Some(region_scope) = opt_scope {
552 unpack!(block = self.pop_scope(region_scope, block));
554 debug!("in_scope: exiting opt_scope={:?} block={:?}", opt_scope, block);
558 /// Convenience wrapper that pushes a scope and then executes `f`
559 /// to build its contents, popping the scope afterwards.
560 #[instrument(skip(self, f), level = "debug")]
561 pub(crate) fn in_scope<F, R>(
563 region_scope: (region::Scope, SourceInfo),
564 lint_level: LintLevel,
568 F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>,
570 let source_scope = self.source_scope;
571 if let LintLevel::Explicit(current_hir_id) = lint_level {
573 self.source_scopes[source_scope].local_data.as_ref().assert_crate_local().lint_root;
574 self.maybe_new_source_scope(region_scope.1.span, None, current_hir_id, parent_id);
576 self.push_scope(region_scope);
578 let rv = unpack!(block = f(self));
579 unpack!(block = self.pop_scope(region_scope, block));
580 self.source_scope = source_scope;
585 /// Push a scope onto the stack. You can then build code in this
586 /// scope and call `pop_scope` afterwards. Note that these two
587 /// calls must be paired; using `in_scope` as a convenience
588 /// wrapper maybe preferable.
589 pub(crate) fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo)) {
590 self.scopes.push_scope(region_scope, self.source_scope);
593 /// Pops a scope, which should have region scope `region_scope`,
594 /// adding any drops onto the end of `block` that are needed.
595 /// This must match 1-to-1 with `push_scope`.
596 pub(crate) fn pop_scope(
598 region_scope: (region::Scope, SourceInfo),
599 mut block: BasicBlock,
601 debug!("pop_scope({:?}, {:?})", region_scope, block);
603 block = self.leave_top_scope(block);
605 self.scopes.pop_scope(region_scope);
610 /// Sets up the drops for breaking from `block` to `target`.
611 pub(crate) fn break_scope(
613 mut block: BasicBlock,
614 value: Option<&Expr<'tcx>>,
615 target: BreakableTarget,
616 source_info: SourceInfo,
618 let span = source_info.span;
620 let get_scope_index = |scope: region::Scope| {
621 // find the loop-scope by its `region::Scope`.
625 .rposition(|breakable_scope| breakable_scope.region_scope == scope)
626 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
628 let (break_index, destination) = match target {
629 BreakableTarget::Return => {
630 let scope = &self.scopes.breakable_scopes[0];
631 if scope.break_destination != Place::return_place() {
632 span_bug!(span, "`return` in item with no return scope");
634 (0, Some(scope.break_destination))
636 BreakableTarget::Break(scope) => {
637 let break_index = get_scope_index(scope);
638 let scope = &self.scopes.breakable_scopes[break_index];
639 (break_index, Some(scope.break_destination))
641 BreakableTarget::Continue(scope) => {
642 let break_index = get_scope_index(scope);
647 if let Some(destination) = destination {
648 if let Some(value) = value {
649 debug!("stmt_expr Break val block_context.push(SubExpr)");
650 self.block_context.push(BlockFrame::SubExpr);
651 unpack!(block = self.expr_into_dest(destination, block, value));
652 self.block_context.pop();
654 self.cfg.push_assign_unit(block, source_info, destination, self.tcx)
657 assert!(value.is_none(), "`return` and `break` should have a destination");
658 if self.tcx.sess.instrument_coverage() {
659 // Unlike `break` and `return`, which push an `Assign` statement to MIR, from which
660 // a Coverage code region can be generated, `continue` needs no `Assign`; but
661 // without one, the `InstrumentCoverage` MIR pass cannot generate a code region for
662 // `continue`. Coverage will be missing unless we add a dummy `Assign` to MIR.
663 self.add_dummy_assignment(span, block, source_info);
667 let region_scope = self.scopes.breakable_scopes[break_index].region_scope;
668 let scope_index = self.scopes.scope_index(region_scope, span);
669 let drops = if destination.is_some() {
670 &mut self.scopes.breakable_scopes[break_index].break_drops
672 self.scopes.breakable_scopes[break_index].continue_drops.as_mut().unwrap()
674 let mut drop_idx = ROOT_NODE;
675 for scope in &self.scopes.scopes[scope_index + 1..] {
676 for drop in &scope.drops {
677 drop_idx = drops.add_drop(*drop, drop_idx);
680 drops.add_entry(block, drop_idx);
682 // `build_drop_trees` doesn't have access to our source_info, so we
683 // create a dummy terminator now. `TerminatorKind::Resume` is used
684 // because MIR type checking will panic if it hasn't been overwritten.
685 self.cfg.terminate(block, source_info, TerminatorKind::Resume);
687 self.cfg.start_new_block().unit()
690 pub(crate) fn break_for_else(
693 target: region::Scope,
694 source_info: SourceInfo,
696 let scope_index = self.scopes.scope_index(target, source_info.span);
697 let if_then_scope = self
701 .unwrap_or_else(|| span_bug!(source_info.span, "no if-then scope found"));
703 assert_eq!(if_then_scope.region_scope, target, "breaking to incorrect scope");
705 let mut drop_idx = ROOT_NODE;
706 let drops = &mut if_then_scope.else_drops;
707 for scope in &self.scopes.scopes[scope_index + 1..] {
708 for drop in &scope.drops {
709 drop_idx = drops.add_drop(*drop, drop_idx);
712 drops.add_entry(block, drop_idx);
714 // `build_drop_trees` doesn't have access to our source_info, so we
715 // create a dummy terminator now. `TerminatorKind::Resume` is used
716 // because MIR type checking will panic if it hasn't been overwritten.
717 self.cfg.terminate(block, source_info, TerminatorKind::Resume);
720 // Add a dummy `Assign` statement to the CFG, with the span for the source code's `continue`
722 fn add_dummy_assignment(&mut self, span: Span, block: BasicBlock, source_info: SourceInfo) {
723 let local_decl = LocalDecl::new(self.tcx.mk_unit(), span).internal();
724 let temp_place = Place::from(self.local_decls.push(local_decl));
725 self.cfg.push_assign_unit(block, source_info, temp_place, self.tcx);
728 fn leave_top_scope(&mut self, block: BasicBlock) -> BasicBlock {
729 // If we are emitting a `drop` statement, we need to have the cached
730 // diverge cleanup pads ready in case that drop panics.
731 let needs_cleanup = self.scopes.scopes.last().map_or(false, |scope| scope.needs_cleanup());
732 let is_generator = self.generator_kind.is_some();
733 let unwind_to = if needs_cleanup { self.diverge_cleanup() } else { DropIdx::MAX };
735 let scope = self.scopes.scopes.last().expect("leave_top_scope called with no scopes");
736 unpack!(build_scope_drops(
738 &mut self.scopes.unwind_drops,
742 is_generator && needs_cleanup,
747 /// Possibly creates a new source scope if `current_root` and `parent_root`
748 /// are different, or if -Zmaximal-hir-to-mir-coverage is enabled.
749 pub(crate) fn maybe_new_source_scope(
752 safety: Option<Safety>,
756 let (current_root, parent_root) =
757 if self.tcx.sess.opts.unstable_opts.maximal_hir_to_mir_coverage {
758 // Some consumers of rustc need to map MIR locations back to HIR nodes. Currently the
759 // the only part of rustc that tracks MIR -> HIR is the `SourceScopeLocalData::lint_root`
760 // field that tracks lint levels for MIR locations. Normally the number of source scopes
761 // is limited to the set of nodes with lint annotations. The -Zmaximal-hir-to-mir-coverage
762 // flag changes this behavior to maximize the number of source scopes, increasing the
763 // granularity of the MIR->HIR mapping.
764 (current_id, parent_id)
766 // Use `maybe_lint_level_root_bounded` with `self.hir_id` as a bound
767 // to avoid adding Hir dependencies on our parents.
768 // We estimate the true lint roots here to avoid creating a lot of source scopes.
770 self.tcx.maybe_lint_level_root_bounded(current_id, self.hir_id),
771 self.tcx.maybe_lint_level_root_bounded(parent_id, self.hir_id),
775 if current_root != parent_root {
776 let lint_level = LintLevel::Explicit(current_root);
777 self.source_scope = self.new_source_scope(span, lint_level, safety);
781 /// Creates a new source scope, nested in the current one.
782 pub(crate) fn new_source_scope(
785 lint_level: LintLevel,
786 safety: Option<Safety>,
788 let parent = self.source_scope;
790 "new_source_scope({:?}, {:?}, {:?}) - parent({:?})={:?}",
795 self.source_scopes.get(parent)
797 let scope_local_data = SourceScopeLocalData {
798 lint_root: if let LintLevel::Explicit(lint_root) = lint_level {
801 self.source_scopes[parent].local_data.as_ref().assert_crate_local().lint_root
803 safety: safety.unwrap_or_else(|| {
804 self.source_scopes[parent].local_data.as_ref().assert_crate_local().safety
807 self.source_scopes.push(SourceScopeData {
809 parent_scope: Some(parent),
811 inlined_parent_scope: None,
812 local_data: ClearCrossCrate::Set(scope_local_data),
816 /// Given a span and the current source scope, make a SourceInfo.
817 pub(crate) fn source_info(&self, span: Span) -> SourceInfo {
818 SourceInfo { span, scope: self.source_scope }
824 /// Returns the scope that we should use as the lifetime of an
825 /// operand. Basically, an operand must live until it is consumed.
826 /// This is similar to, but not quite the same as, the temporary
827 /// scope (which can be larger or smaller).
830 /// ```ignore (illustrative)
831 /// let x = foo(bar(X, Y));
833 /// We wish to pop the storage for X and Y after `bar()` is
834 /// called, not after the whole `let` is completed.
836 /// As another example, if the second argument diverges:
837 /// ```ignore (illustrative)
838 /// foo(Box::new(2), panic!())
840 /// We would allocate the box but then free it on the unwinding
841 /// path; we would also emit a free on the 'success' path from
842 /// panic, but that will turn out to be removed as dead-code.
843 pub(crate) fn local_scope(&self) -> region::Scope {
844 self.scopes.topmost()
850 pub(crate) fn schedule_drop_storage_and_value(
853 region_scope: region::Scope,
856 self.schedule_drop(span, region_scope, local, DropKind::Storage);
857 self.schedule_drop(span, region_scope, local, DropKind::Value);
860 /// Indicates that `place` should be dropped on exit from `region_scope`.
862 /// When called with `DropKind::Storage`, `place` shouldn't be the return
863 /// place, or a function parameter.
864 pub(crate) fn schedule_drop(
867 region_scope: region::Scope,
871 let needs_drop = match drop_kind {
873 if !self.local_decls[local].ty.needs_drop(self.tcx, self.param_env) {
878 DropKind::Storage => {
879 if local.index() <= self.arg_count {
882 "`schedule_drop` called with local {:?} and arg_count {}",
891 // When building drops, we try to cache chains of drops to reduce the
892 // number of `DropTree::add_drop` calls. This, however, means that
893 // whenever we add a drop into a scope which already had some entries
894 // in the drop tree built (and thus, cached) for it, we must invalidate
895 // all caches which might branch into the scope which had a drop just
896 // added to it. This is necessary, because otherwise some other code
897 // might use the cache to branch into already built chain of drops,
898 // essentially ignoring the newly added drop.
900 // For example consider there’s two scopes with a drop in each. These
901 // are built and thus the caches are filled:
903 // +--------------------------------------------------------+
904 // | +---------------------------------+ |
905 // | | +--------+ +-------------+ | +---------------+ |
906 // | | | return | <-+ | drop(outer) | <-+ | drop(middle) | |
907 // | | +--------+ +-------------+ | +---------------+ |
908 // | +------------|outer_scope cache|--+ |
909 // +------------------------------|middle_scope cache|------+
911 // Now, a new, inner-most scope is added along with a new drop into
912 // both inner-most and outer-most scopes:
914 // +------------------------------------------------------------+
915 // | +----------------------------------+ |
916 // | | +--------+ +-------------+ | +---------------+ | +-------------+
917 // | | | return | <+ | drop(new) | <-+ | drop(middle) | <--+| drop(inner) |
918 // | | +--------+ | | drop(outer) | | +---------------+ | +-------------+
919 // | | +-+ +-------------+ | |
920 // | +---|invalid outer_scope cache|----+ |
921 // +----=----------------|invalid middle_scope cache|-----------+
923 // If, when adding `drop(new)` we do not invalidate the cached blocks for both
924 // outer_scope and middle_scope, then, when building drops for the inner (right-most)
925 // scope, the old, cached blocks, without `drop(new)` will get used, producing the
928 // Note that this code iterates scopes from the inner-most to the outer-most,
929 // invalidating caches of each scope visited. This way bare minimum of the
930 // caches gets invalidated. i.e., if a new drop is added into the middle scope, the
931 // cache of outer scope stays intact.
933 // Since we only cache drops for the unwind path and the generator drop
934 // path, we only need to invalidate the cache for drops that happen on
935 // the unwind or generator drop paths. This means that for
936 // non-generators we don't need to invalidate caches for `DropKind::Storage`.
937 let invalidate_caches = needs_drop || self.generator_kind.is_some();
938 for scope in self.scopes.scopes.iter_mut().rev() {
939 if invalidate_caches {
940 scope.invalidate_cache();
943 if scope.region_scope == region_scope {
944 let region_scope_span = region_scope.span(self.tcx, &self.region_scope_tree);
945 // Attribute scope exit drops to scope's closing brace.
946 let scope_end = self.tcx.sess.source_map().end_point(region_scope_span);
948 scope.drops.push(DropData {
949 source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
958 span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, local);
961 /// Indicates that the "local operand" stored in `local` is
962 /// *moved* at some point during execution (see `local_scope` for
963 /// more information about what a "local operand" is -- in short,
964 /// it's an intermediate operand created as part of preparing some
965 /// MIR instruction). We use this information to suppress
966 /// redundant drops on the non-unwind paths. This results in less
967 /// MIR, but also avoids spurious borrow check errors
970 /// Example: when compiling the call to `foo` here:
972 /// ```ignore (illustrative)
976 /// we would evaluate `bar()` to an operand `_X`. We would also
977 /// schedule `_X` to be dropped when the expression scope for
978 /// `foo(bar())` is exited. This is relevant, for example, if the
979 /// later arguments should unwind (it would ensure that `_X` gets
980 /// dropped). However, if no unwind occurs, then `_X` will be
981 /// unconditionally consumed by the `call`:
983 /// ```ignore (illustrative)
986 /// _R = CALL(foo, _X, ...)
990 /// However, `_X` is still registered to be dropped, and so if we
991 /// do nothing else, we would generate a `DROP(_X)` that occurs
992 /// after the call. This will later be optimized out by the
993 /// drop-elaboration code, but in the meantime it can lead to
994 /// spurious borrow-check errors -- the problem, ironically, is
995 /// not the `DROP(_X)` itself, but the (spurious) unwind pathways
996 /// that it creates. See #64391 for an example.
997 pub(crate) fn record_operands_moved(&mut self, operands: &[Operand<'tcx>]) {
998 let local_scope = self.local_scope();
999 let scope = self.scopes.scopes.last_mut().unwrap();
1001 assert_eq!(scope.region_scope, local_scope, "local scope is not the topmost scope!",);
1003 // look for moves of a local variable, like `MOVE(_X)`
1004 let locals_moved = operands.iter().flat_map(|operand| match operand {
1005 Operand::Copy(_) | Operand::Constant(_) => None,
1006 Operand::Move(place) => place.as_local(),
1009 for local in locals_moved {
1010 // check if we have a Drop for this operand and -- if so
1011 // -- add it to the list of moved operands. Note that this
1012 // local might not have been an operand created for this
1013 // call, it could come from other places too.
1014 if scope.drops.iter().any(|drop| drop.local == local && drop.kind == DropKind::Value) {
1015 scope.moved_locals.push(local);
1023 /// Returns the [DropIdx] for the innermost drop if the function unwound at
1024 /// this point. The `DropIdx` will be created if it doesn't already exist.
1025 fn diverge_cleanup(&mut self) -> DropIdx {
1026 // It is okay to use dummy span because the getting scope index on the topmost scope
1027 // must always succeed.
1028 self.diverge_cleanup_target(self.scopes.topmost(), DUMMY_SP)
1031 /// This is similar to [diverge_cleanup](Self::diverge_cleanup) except its target is set to
1032 /// some ancestor scope instead of the current scope.
1033 /// It is possible to unwind to some ancestor scope if some drop panics as
1034 /// the program breaks out of a if-then scope.
1035 fn diverge_cleanup_target(&mut self, target_scope: region::Scope, span: Span) -> DropIdx {
1036 let target = self.scopes.scope_index(target_scope, span);
1037 let (uncached_scope, mut cached_drop) = self.scopes.scopes[..=target]
1041 .find_map(|(scope_idx, scope)| {
1042 scope.cached_unwind_block.map(|cached_block| (scope_idx + 1, cached_block))
1044 .unwrap_or((0, ROOT_NODE));
1046 if uncached_scope > target {
1050 let is_generator = self.generator_kind.is_some();
1051 for scope in &mut self.scopes.scopes[uncached_scope..=target] {
1052 for drop in &scope.drops {
1053 if is_generator || drop.kind == DropKind::Value {
1054 cached_drop = self.scopes.unwind_drops.add_drop(*drop, cached_drop);
1057 scope.cached_unwind_block = Some(cached_drop);
1063 /// Prepares to create a path that performs all required cleanup for a
1064 /// terminator that can unwind at the given basic block.
1066 /// This path terminates in Resume. The path isn't created until after all
1067 /// of the non-unwind paths in this item have been lowered.
1068 pub(crate) fn diverge_from(&mut self, start: BasicBlock) {
1071 self.cfg.block_data(start).terminator().kind,
1072 TerminatorKind::Assert { .. }
1073 | TerminatorKind::Call { .. }
1074 | TerminatorKind::Drop { .. }
1075 | TerminatorKind::DropAndReplace { .. }
1076 | TerminatorKind::FalseUnwind { .. }
1077 | TerminatorKind::InlineAsm { .. }
1079 "diverge_from called on block with terminator that cannot unwind."
1082 let next_drop = self.diverge_cleanup();
1083 self.scopes.unwind_drops.add_entry(start, next_drop);
1086 /// Sets up a path that performs all required cleanup for dropping a
1087 /// generator, starting from the given block that ends in
1088 /// [TerminatorKind::Yield].
1090 /// This path terminates in GeneratorDrop.
1091 pub(crate) fn generator_drop_cleanup(&mut self, yield_block: BasicBlock) {
1094 self.cfg.block_data(yield_block).terminator().kind,
1095 TerminatorKind::Yield { .. }
1097 "generator_drop_cleanup called on block with non-yield terminator."
1099 let (uncached_scope, mut cached_drop) = self
1105 .find_map(|(scope_idx, scope)| {
1106 scope.cached_generator_drop_block.map(|cached_block| (scope_idx + 1, cached_block))
1108 .unwrap_or((0, ROOT_NODE));
1110 for scope in &mut self.scopes.scopes[uncached_scope..] {
1111 for drop in &scope.drops {
1112 cached_drop = self.scopes.generator_drops.add_drop(*drop, cached_drop);
1114 scope.cached_generator_drop_block = Some(cached_drop);
1117 self.scopes.generator_drops.add_entry(yield_block, cached_drop);
1120 /// Utility function for *non*-scope code to build their own drops
1121 pub(crate) fn build_drop_and_replace(
1126 value: Operand<'tcx>,
1128 let source_info = self.source_info(span);
1129 let next_target = self.cfg.start_new_block();
1134 TerminatorKind::DropAndReplace { place, value, target: next_target, unwind: None },
1136 self.diverge_from(block);
1141 /// Creates an `Assert` terminator and return the success block.
1142 /// If the boolean condition operand is not the expected value,
1143 /// a runtime panic will be caused with the given message.
1144 pub(crate) fn assert(
1147 cond: Operand<'tcx>,
1149 msg: AssertMessage<'tcx>,
1152 let source_info = self.source_info(span);
1153 let success_block = self.cfg.start_new_block();
1158 TerminatorKind::Assert { cond, expected, msg, target: success_block, cleanup: None },
1160 self.diverge_from(block);
1165 /// Unschedules any drops in the top scope.
1167 /// This is only needed for `match` arm scopes, because they have one
1168 /// entrance per pattern, but only one exit.
1169 pub(crate) fn clear_top_scope(&mut self, region_scope: region::Scope) {
1170 let top_scope = self.scopes.scopes.last_mut().unwrap();
1172 assert_eq!(top_scope.region_scope, region_scope);
1174 top_scope.drops.clear();
1175 top_scope.invalidate_cache();
1179 /// Builds drops for `pop_scope` and `leave_top_scope`.
1180 fn build_scope_drops<'tcx>(
1181 cfg: &mut CFG<'tcx>,
1182 unwind_drops: &mut DropTree,
1184 mut block: BasicBlock,
1185 mut unwind_to: DropIdx,
1186 storage_dead_on_unwind: bool,
1189 debug!("build_scope_drops({:?} -> {:?})", block, scope);
1191 // Build up the drops in evaluation order. The end result will
1194 // [SDs, drops[n]] --..> [SDs, drop[1]] -> [SDs, drop[0]] -> [[SDs]]
1198 // [drop[n]] -...-> [drop[1]] ------> [drop[0]] ------> [last_unwind_to]
1200 // The horizontal arrows represent the execution path when the drops return
1201 // successfully. The downwards arrows represent the execution path when the
1202 // drops panic (panicking while unwinding will abort, so there's no need for
1203 // another set of arrows).
1205 // For generators, we unwind from a drop on a local to its StorageDead
1206 // statement. For other functions we don't worry about StorageDead. The
1207 // drops for the unwind path should have already been generated by
1208 // `diverge_cleanup_gen`.
1210 for drop_data in scope.drops.iter().rev() {
1211 let source_info = drop_data.source_info;
1212 let local = drop_data.local;
1214 match drop_data.kind {
1215 DropKind::Value => {
1216 // `unwind_to` should drop the value that we're about to
1217 // schedule. If dropping this value panics, then we continue
1218 // with the *next* value on the unwind path.
1219 debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
1220 debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
1221 unwind_to = unwind_drops.drops[unwind_to].1;
1223 // If the operand has been moved, and we are not on an unwind
1224 // path, then don't generate the drop. (We only take this into
1225 // account for non-unwind paths so as not to disturb the
1226 // caching mechanism.)
1227 if scope.moved_locals.iter().any(|&o| o == local) {
1231 unwind_drops.add_entry(block, unwind_to);
1233 let next = cfg.start_new_block();
1237 TerminatorKind::Drop { place: local.into(), target: next, unwind: None },
1241 DropKind::Storage => {
1242 if storage_dead_on_unwind {
1243 debug_assert_eq!(unwind_drops.drops[unwind_to].0.local, drop_data.local);
1244 debug_assert_eq!(unwind_drops.drops[unwind_to].0.kind, drop_data.kind);
1245 unwind_to = unwind_drops.drops[unwind_to].1;
1247 // Only temps and vars need their storage dead.
1248 assert!(local.index() > arg_count);
1249 cfg.push(block, Statement { source_info, kind: StatementKind::StorageDead(local) });
1256 impl<'a, 'tcx: 'a> Builder<'a, 'tcx> {
1257 /// Build a drop tree for a breakable scope.
1259 /// If `continue_block` is `Some`, then the tree is for `continue` inside a
1260 /// loop. Otherwise this is for `break` or `return`.
1263 mut drops: DropTree,
1264 else_scope: region::Scope,
1266 continue_block: Option<BasicBlock>,
1267 ) -> Option<BlockAnd<()>> {
1268 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1269 blocks[ROOT_NODE] = continue_block;
1271 drops.build_mir::<ExitScopes>(&mut self.cfg, &mut blocks);
1272 let is_generator = self.generator_kind.is_some();
1274 // Link the exit drop tree to unwind drop tree.
1275 if drops.drops.iter().any(|(drop, _)| drop.kind == DropKind::Value) {
1276 let unwind_target = self.diverge_cleanup_target(else_scope, span);
1277 let mut unwind_indices = IndexVec::from_elem_n(unwind_target, 1);
1278 for (drop_idx, drop_data) in drops.drops.iter_enumerated().skip(1) {
1279 match drop_data.0.kind {
1280 DropKind::Storage => {
1282 let unwind_drop = self
1285 .add_drop(drop_data.0, unwind_indices[drop_data.1]);
1286 unwind_indices.push(unwind_drop);
1288 unwind_indices.push(unwind_indices[drop_data.1]);
1291 DropKind::Value => {
1292 let unwind_drop = self
1295 .add_drop(drop_data.0, unwind_indices[drop_data.1]);
1298 .add_entry(blocks[drop_idx].unwrap(), unwind_indices[drop_data.1]);
1299 unwind_indices.push(unwind_drop);
1304 blocks[ROOT_NODE].map(BasicBlock::unit)
1307 /// Build the unwind and generator drop trees.
1308 pub(crate) fn build_drop_trees(&mut self) {
1309 if self.generator_kind.is_some() {
1310 self.build_generator_drop_trees();
1312 Self::build_unwind_tree(
1314 &mut self.scopes.unwind_drops,
1321 fn build_generator_drop_trees(&mut self) {
1322 // Build the drop tree for dropping the generator while it's suspended.
1323 let drops = &mut self.scopes.generator_drops;
1324 let cfg = &mut self.cfg;
1325 let fn_span = self.fn_span;
1326 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1327 drops.build_mir::<GeneratorDrop>(cfg, &mut blocks);
1328 if let Some(root_block) = blocks[ROOT_NODE] {
1331 SourceInfo::outermost(fn_span),
1332 TerminatorKind::GeneratorDrop,
1336 // Build the drop tree for unwinding in the normal control flow paths.
1337 let resume_block = &mut None;
1338 let unwind_drops = &mut self.scopes.unwind_drops;
1339 Self::build_unwind_tree(cfg, unwind_drops, fn_span, resume_block);
1341 // Build the drop tree for unwinding when dropping a suspended
1344 // This is a different tree to the standard unwind paths here to
1345 // prevent drop elaboration from creating drop flags that would have
1346 // to be captured by the generator. I'm not sure how important this
1347 // optimization is, but it is here.
1348 for (drop_idx, drop_data) in drops.drops.iter_enumerated() {
1349 if let DropKind::Value = drop_data.0.kind {
1350 debug_assert!(drop_data.1 < drops.drops.next_index());
1351 drops.entry_points.push((drop_data.1, blocks[drop_idx].unwrap()));
1354 Self::build_unwind_tree(cfg, drops, fn_span, resume_block);
1357 fn build_unwind_tree(
1358 cfg: &mut CFG<'tcx>,
1359 drops: &mut DropTree,
1361 resume_block: &mut Option<BasicBlock>,
1363 let mut blocks = IndexVec::from_elem(None, &drops.drops);
1364 blocks[ROOT_NODE] = *resume_block;
1365 drops.build_mir::<Unwind>(cfg, &mut blocks);
1366 if let (None, Some(resume)) = (*resume_block, blocks[ROOT_NODE]) {
1367 cfg.terminate(resume, SourceInfo::outermost(fn_span), TerminatorKind::Resume);
1369 *resume_block = blocks[ROOT_NODE];
1374 // DropTreeBuilder implementations.
1378 impl<'tcx> DropTreeBuilder<'tcx> for ExitScopes {
1379 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1380 cfg.start_new_block()
1382 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1383 cfg.block_data_mut(from).terminator_mut().kind = TerminatorKind::Goto { target: to };
1387 struct GeneratorDrop;
1389 impl<'tcx> DropTreeBuilder<'tcx> for GeneratorDrop {
1390 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1391 cfg.start_new_block()
1393 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1394 let term = cfg.block_data_mut(from).terminator_mut();
1395 if let TerminatorKind::Yield { ref mut drop, .. } = term.kind {
1399 term.source_info.span,
1400 "cannot enter generator drop tree from {:?}",
1409 impl<'tcx> DropTreeBuilder<'tcx> for Unwind {
1410 fn make_block(cfg: &mut CFG<'tcx>) -> BasicBlock {
1411 cfg.start_new_cleanup_block()
1413 fn add_entry(cfg: &mut CFG<'tcx>, from: BasicBlock, to: BasicBlock) {
1414 let term = &mut cfg.block_data_mut(from).terminator_mut();
1415 match &mut term.kind {
1416 TerminatorKind::Drop { unwind, .. }
1417 | TerminatorKind::DropAndReplace { unwind, .. }
1418 | TerminatorKind::FalseUnwind { unwind, .. }
1419 | TerminatorKind::Call { cleanup: unwind, .. }
1420 | TerminatorKind::Assert { cleanup: unwind, .. }
1421 | TerminatorKind::InlineAsm { cleanup: unwind, .. } => {
1424 TerminatorKind::Goto { .. }
1425 | TerminatorKind::SwitchInt { .. }
1426 | TerminatorKind::Resume
1427 | TerminatorKind::Abort
1428 | TerminatorKind::Return
1429 | TerminatorKind::Unreachable
1430 | TerminatorKind::Yield { .. }
1431 | TerminatorKind::GeneratorDrop
1432 | TerminatorKind::FalseEdge { .. } => {
1433 span_bug!(term.source_info.span, "cannot unwind from {:?}", term.kind)