1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
12 Managing the scope stack. The scopes are tied to lexical scopes, so as
13 we descend the HAIR, we push a scope on the stack, translate ite
14 contents, and then pop it off. Every scope is named by a
19 When pushing a new scope, we record the current point in the graph (a
20 basic block); this marks the entry to the scope. We then generate more
21 stuff in the control-flow graph. Whenever the scope is exited, either
22 via a `break` or `return` or just by fallthrough, that marks an exit
23 from the scope. Each lexical scope thus corresponds to a single-entry,
24 multiple-exit (SEME) region in the control-flow graph.
26 For now, we keep a mapping from each `region::Scope` to its
27 corresponding SEME region for later reference (see caveat in next
28 paragraph). This is because region scopes are tied to
29 them. Eventually, when we shift to non-lexical lifetimes, there should
30 be no need to remember this mapping.
32 There is one additional wrinkle, actually, that I wanted to hide from
33 you but duty compels me to mention. In the course of translating
34 matches, it sometimes happen that certain code (namely guards) gets
35 executed multiple times. This means that the scope lexical scope may
36 in fact correspond to multiple, disjoint SEME regions. So in fact our
37 mapping is from one scope to a vector of SEME regions.
41 The primary purpose for scopes is to insert drops: while translating
42 the contents, we also accumulate lvalues that need to be dropped upon
43 exit from each scope. This is done by calling `schedule_drop`. Once a
44 drop is scheduled, whenever we branch out we will insert drops of all
45 those lvalues onto the outgoing edge. Note that we don't know the full
46 set of scheduled drops up front, and so whenever we exit from the
47 scope we only drop the values scheduled thus far. For example, consider
48 the scope S corresponding to this loop:
59 When processing the `let x`, we will add one drop to the scope for
60 `x`. The break will then insert a drop for `x`. When we process `let
61 y`, we will add another drop (in fact, to a subscope, but let's ignore
62 that for now); any later drops would also drop `y`.
66 There are numerous "normal" ways to early exit a scope: `break`,
67 `continue`, `return` (panics are handled separately). Whenever an
68 early exit occurs, the method `exit_scope` is called. It is given the
69 current point in execution where the early exit occurs, as well as the
70 scope you want to branch to (note that all early exits from to some
71 other enclosing scope). `exit_scope` will record this exit point and
74 Panics are handled in a similar fashion, except that a panic always
75 returns out to the `DIVERGE_BLOCK`. To trigger a panic, simply call
76 `panic(p)` with the current point `p`. Or else you can call
77 `diverge_cleanup`, which will produce a block that you can branch to
78 which does the appropriate cleanup and then diverges. `panic(p)`
79 simply calls `diverge_cleanup()` and adds an edge from `p` to the
84 In addition to the normal scope stack, we track a loop scope stack
85 that contains only loops. It tracks where a `break` and `continue`
90 use build::{BlockAnd, BlockAndExtension, Builder, CFG};
92 use rustc::middle::region;
93 use rustc::ty::{Ty, TyCtxt};
94 use rustc::hir::def_id::LOCAL_CRATE;
96 use rustc::mir::transform::MirSource;
97 use syntax_pos::{Span};
98 use rustc_data_structures::indexed_vec::Idx;
99 use rustc_data_structures::fx::FxHashMap;
102 pub struct Scope<'tcx> {
103 /// The visibility scope this scope was created in.
104 visibility_scope: VisibilityScope,
106 /// the region span of this scope within source code.
107 region_scope: region::Scope,
109 /// the span of that region_scope
110 region_scope_span: Span,
112 /// Whether there's anything to do for the cleanup path, that is,
113 /// when unwinding through this scope. This includes destructors,
114 /// but not StorageDead statements, which don't get emitted at all
115 /// for unwinding, for several reasons:
116 /// * clang doesn't emit llvm.lifetime.end for C++ unwinding
117 /// * LLVM's memory dependency analysis can't handle it atm
118 /// * polluting the cleanup MIR with StorageDead creates
119 /// landing pads even though there's no actual destructors
120 /// * freeing up stack space has no effect during unwinding
123 /// set of lvalues to drop when exiting this scope. This starts
124 /// out empty but grows as variables are declared during the
125 /// building process. This is a stack, so we always drop from the
126 /// end of the vector (top of the stack) first.
127 drops: Vec<DropData<'tcx>>,
129 /// The cache for drop chain on “normal” exit into a particular BasicBlock.
130 cached_exits: FxHashMap<(BasicBlock, region::Scope), BasicBlock>,
132 /// The cache for drop chain on "generator drop" exit.
133 cached_generator_drop: Option<BasicBlock>,
137 struct DropData<'tcx> {
138 /// span where drop obligation was incurred (typically where lvalue was declared)
142 location: Lvalue<'tcx>,
144 /// Whether this is a full value Drop, or just a StorageDead.
148 #[derive(Debug, Default, Clone, Copy)]
150 /// The cached block for the cleanups-on-diverge path. This block
151 /// contains code to run the current drop and all the preceding
152 /// drops (i.e. those having lower index in Drop’s Scope drop
154 unwind: Option<BasicBlock>,
156 /// The cached block for unwinds during cleanups-on-generator-drop path
158 /// This is split from the standard unwind path here to prevent drop
159 /// elaboration from creating drop flags that would have to be captured
160 /// by the generator. I'm not sure how important this optimization is,
162 generator_drop: Option<BasicBlock>,
168 cached_block: CachedBlock,
173 #[derive(Clone, Debug)]
174 pub struct BreakableScope<'tcx> {
175 /// Region scope of the loop
176 pub region_scope: region::Scope,
177 /// Where the body of the loop begins. `None` if block
178 pub continue_block: Option<BasicBlock>,
179 /// Block to branch into when the loop or block terminates (either by being `break`-en out
180 /// from, or by having its condition to become false)
181 pub break_block: BasicBlock,
182 /// The destination of the loop/block expression itself (i.e. where to put the result of a
183 /// `break` expression)
184 pub break_destination: Lvalue<'tcx>,
188 fn invalidate(&mut self) {
189 self.generator_drop = None;
193 fn get(&self, generator_drop: bool) -> Option<BasicBlock> {
201 fn ref_mut(&mut self, generator_drop: bool) -> &mut Option<BasicBlock> {
203 &mut self.generator_drop
211 fn may_panic(&self) -> bool {
213 DropKind::Value { .. } => true,
214 DropKind::Storage => false
219 impl<'tcx> Scope<'tcx> {
220 /// Invalidate all the cached blocks in the scope.
222 /// Should always be run for all inner scopes when a drop is pushed into some scope enclosing a
223 /// larger extent of code.
225 /// `storage_only` controls whether to invalidate only drop paths run `StorageDead`.
226 /// `this_scope_only` controls whether to invalidate only drop paths that refer to the current
227 /// top-of-scope (as opposed to dependent scopes).
228 fn invalidate_cache(&mut self, storage_only: bool, this_scope_only: bool) {
229 // FIXME: maybe do shared caching of `cached_exits` etc. to handle functions
230 // with lots of `try!`?
232 // cached exits drop storage and refer to the top-of-scope
233 self.cached_exits.clear();
236 // the current generator drop ignores storage but refers to top-of-scope
237 self.cached_generator_drop = None;
240 if !storage_only && !this_scope_only {
241 for dropdata in &mut self.drops {
242 if let DropKind::Value { ref mut cached_block } = dropdata.kind {
243 cached_block.invalidate();
249 /// Returns the cached entrypoint for diverging exit from this scope.
251 /// Precondition: the caches must be fully filled (i.e. diverge_cleanup is called) in order for
252 /// this method to work correctly.
253 fn cached_block(&self, generator_drop: bool) -> Option<BasicBlock> {
254 let mut drops = self.drops.iter().rev().filter_map(|data| {
256 DropKind::Value { cached_block } => {
257 Some(cached_block.get(generator_drop))
259 DropKind::Storage => None
262 if let Some(cached_block) = drops.next() {
263 Some(cached_block.expect("drop cache is not filled"))
269 /// Given a span and this scope's visibility scope, make a SourceInfo.
270 fn source_info(&self, span: Span) -> SourceInfo {
273 scope: self.visibility_scope
278 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
279 // Adding and removing scopes
280 // ==========================
281 /// Start a breakable scope, which tracks where `continue` and `break`
282 /// should branch to. See module comment for more details.
284 /// Returns the might_break attribute of the BreakableScope used.
285 pub fn in_breakable_scope<F, R>(&mut self,
286 loop_block: Option<BasicBlock>,
287 break_block: BasicBlock,
288 break_destination: Lvalue<'tcx>,
290 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> R
292 let region_scope = self.topmost_scope();
293 let scope = BreakableScope {
295 continue_block: loop_block,
299 self.breakable_scopes.push(scope);
301 let breakable_scope = self.breakable_scopes.pop().unwrap();
302 assert!(breakable_scope.region_scope == region_scope);
306 pub fn in_opt_scope<F, R>(&mut self,
307 opt_scope: Option<(region::Scope, SourceInfo)>,
308 mut block: BasicBlock,
311 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> BlockAnd<R>
313 debug!("in_opt_scope(opt_scope={:?}, block={:?})", opt_scope, block);
314 if let Some(region_scope) = opt_scope { self.push_scope(region_scope); }
315 let rv = unpack!(block = f(self));
316 if let Some(region_scope) = opt_scope {
317 unpack!(block = self.pop_scope(region_scope, block));
319 debug!("in_scope: exiting opt_scope={:?} block={:?}", opt_scope, block);
323 /// Convenience wrapper that pushes a scope and then executes `f`
324 /// to build its contents, popping the scope afterwards.
325 pub fn in_scope<F, R>(&mut self,
326 region_scope: (region::Scope, SourceInfo),
327 lint_level: LintLevel,
328 mut block: BasicBlock,
331 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> BlockAnd<R>
333 debug!("in_scope(region_scope={:?}, block={:?})", region_scope, block);
334 let visibility_scope = self.visibility_scope;
335 let tcx = self.hir.tcx();
336 if let LintLevel::Explicit(node_id) = lint_level {
337 let same_lint_scopes = tcx.dep_graph.with_ignore(|| {
338 let sets = tcx.lint_levels(LOCAL_CRATE);
340 tcx.hir.definitions().node_to_hir_id(
341 self.visibility_scope_info[visibility_scope].lint_root
344 tcx.hir.definitions().node_to_hir_id(node_id);
345 sets.lint_level_set(parent_hir_id) ==
346 sets.lint_level_set(current_hir_id)
349 if !same_lint_scopes {
350 self.visibility_scope =
351 self.new_visibility_scope(region_scope.1.span, lint_level,
355 self.push_scope(region_scope);
356 let rv = unpack!(block = f(self));
357 unpack!(block = self.pop_scope(region_scope, block));
358 self.visibility_scope = visibility_scope;
359 debug!("in_scope: exiting region_scope={:?} block={:?}", region_scope, block);
363 /// Push a scope onto the stack. You can then build code in this
364 /// scope and call `pop_scope` afterwards. Note that these two
365 /// calls must be paired; using `in_scope` as a convenience
366 /// wrapper maybe preferable.
367 pub fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo)) {
368 debug!("push_scope({:?})", region_scope);
369 let vis_scope = self.visibility_scope;
370 self.scopes.push(Scope {
371 visibility_scope: vis_scope,
372 region_scope: region_scope.0,
373 region_scope_span: region_scope.1.span,
374 needs_cleanup: false,
376 cached_generator_drop: None,
377 cached_exits: FxHashMap()
381 /// Pops a scope, which should have region scope `region_scope`,
382 /// adding any drops onto the end of `block` that are needed.
383 /// This must match 1-to-1 with `push_scope`.
384 pub fn pop_scope(&mut self,
385 region_scope: (region::Scope, SourceInfo),
386 mut block: BasicBlock)
388 debug!("pop_scope({:?}, {:?})", region_scope, block);
389 // If we are emitting a `drop` statement, we need to have the cached
390 // diverge cleanup pads ready in case that drop panics.
392 self.scopes.last().unwrap().drops.iter().any(|s| s.kind.may_panic());
394 self.diverge_cleanup();
396 let scope = self.scopes.pop().unwrap();
397 assert_eq!(scope.region_scope, region_scope.0);
399 self.cfg.push_end_region(self.hir.tcx(), block, region_scope.1, scope.region_scope);
400 unpack!(block = build_scope_drops(&mut self.cfg,
411 /// Branch out of `block` to `target`, exiting all scopes up to
412 /// and including `region_scope`. This will insert whatever drops are
413 /// needed, as well as tracking this exit for the SEME region. See
414 /// module comment for details.
415 pub fn exit_scope(&mut self,
417 region_scope: (region::Scope, SourceInfo),
418 mut block: BasicBlock,
419 target: BasicBlock) {
420 debug!("exit_scope(region_scope={:?}, block={:?}, target={:?})",
421 region_scope, block, target);
422 let scope_count = 1 + self.scopes.iter().rev()
423 .position(|scope| scope.region_scope == region_scope.0)
425 span_bug!(span, "region_scope {:?} does not enclose", region_scope)
427 let len = self.scopes.len();
428 assert!(scope_count < len, "should not use `exit_scope` to pop ALL scopes");
430 // If we are emitting a `drop` statement, we need to have the cached
431 // diverge cleanup pads ready in case that drop panics.
432 let may_panic = self.scopes[(len - scope_count)..].iter()
433 .any(|s| s.drops.iter().any(|s| s.kind.may_panic()));
435 self.diverge_cleanup();
439 let mut rest = &mut self.scopes[(len - scope_count)..];
440 while let Some((scope, rest_)) = {rest}.split_last_mut() {
442 block = if let Some(&e) = scope.cached_exits.get(&(target, region_scope.0)) {
443 self.cfg.terminate(block, scope.source_info(span),
444 TerminatorKind::Goto { target: e });
447 let b = self.cfg.start_new_block();
448 self.cfg.terminate(block, scope.source_info(span),
449 TerminatorKind::Goto { target: b });
450 scope.cached_exits.insert((target, region_scope.0), b);
454 // End all regions for scopes out of which we are breaking.
455 self.cfg.push_end_region(self.hir.tcx(), block, region_scope.1, scope.region_scope);
457 unpack!(block = build_scope_drops(&mut self.cfg,
465 let scope = &self.scopes[len - scope_count];
466 self.cfg.terminate(block, scope.source_info(span),
467 TerminatorKind::Goto { target: target });
470 /// Creates a path that performs all required cleanup for dropping a generator.
472 /// This path terminates in GeneratorDrop. Returns the start of the path.
473 /// None indicates there’s no cleanup to do at this point.
474 pub fn generator_drop_cleanup(&mut self) -> Option<BasicBlock> {
475 if !self.scopes.iter().any(|scope| scope.needs_cleanup) {
480 self.diverge_cleanup_gen(true);
482 let src_info = self.scopes[0].source_info(self.fn_span);
483 let mut block = self.cfg.start_new_block();
485 let mut rest = &mut self.scopes[..];
487 while let Some((scope, rest_)) = {rest}.split_last_mut() {
489 if !scope.needs_cleanup {
492 block = if let Some(b) = scope.cached_generator_drop {
493 self.cfg.terminate(block, src_info,
494 TerminatorKind::Goto { target: b });
497 let b = self.cfg.start_new_block();
498 scope.cached_generator_drop = Some(b);
499 self.cfg.terminate(block, src_info,
500 TerminatorKind::Goto { target: b });
503 unpack!(block = build_scope_drops(&mut self.cfg,
510 // End all regions for scopes out of which we are breaking.
511 self.cfg.push_end_region(self.hir.tcx(), block, src_info, scope.region_scope);
514 self.cfg.terminate(block, src_info, TerminatorKind::GeneratorDrop);
519 /// Creates a new visibility scope, nested in the current one.
520 pub fn new_visibility_scope(&mut self,
522 lint_level: LintLevel,
523 safety: Option<Safety>) -> VisibilityScope {
524 let parent = self.visibility_scope;
525 debug!("new_visibility_scope({:?}, {:?}, {:?}) - parent({:?})={:?}",
526 span, lint_level, safety,
527 parent, self.visibility_scope_info.get(parent));
528 let scope = self.visibility_scopes.push(VisibilityScopeData {
530 parent_scope: Some(parent),
532 let scope_info = VisibilityScopeInfo {
533 lint_root: if let LintLevel::Explicit(lint_root) = lint_level {
536 self.visibility_scope_info[parent].lint_root
538 safety: safety.unwrap_or_else(|| {
539 self.visibility_scope_info[parent].safety
542 self.visibility_scope_info.push(scope_info);
548 /// Finds the breakable scope for a given label. This is used for
549 /// resolving `break` and `continue`.
550 pub fn find_breakable_scope(&mut self,
552 label: region::Scope)
553 -> &mut BreakableScope<'tcx> {
554 // find the loop-scope with the correct id
555 self.breakable_scopes.iter_mut()
557 .filter(|breakable_scope| breakable_scope.region_scope == label)
559 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
562 /// Given a span and the current visibility scope, make a SourceInfo.
563 pub fn source_info(&self, span: Span) -> SourceInfo {
566 scope: self.visibility_scope
570 /// Returns the `region::Scope` of the scope which should be exited by a
572 pub fn region_scope_of_return_scope(&self) -> region::Scope {
573 // The outermost scope (`scopes[0]`) will be the `CallSiteScope`.
574 // We want `scopes[1]`, which is the `ParameterScope`.
575 assert!(self.scopes.len() >= 2);
576 assert!(match self.scopes[1].region_scope.data() {
577 region::ScopeData::Arguments(_) => true,
580 self.scopes[1].region_scope
583 /// Returns the topmost active scope, which is known to be alive until
584 /// the next scope expression.
585 pub fn topmost_scope(&self) -> region::Scope {
586 self.scopes.last().expect("topmost_scope: no scopes present").region_scope
589 /// Returns the scope that we should use as the lifetime of an
590 /// operand. Basically, an operand must live until it is consumed.
591 /// This is similar to, but not quite the same as, the temporary
592 /// scope (which can be larger or smaller).
596 /// let x = foo(bar(X, Y));
598 /// We wish to pop the storage for X and Y after `bar()` is
599 /// called, not after the whole `let` is completed.
601 /// As another example, if the second argument diverges:
603 /// foo(Box::new(2), panic!())
605 /// We would allocate the box but then free it on the unwinding
606 /// path; we would also emit a free on the 'success' path from
607 /// panic, but that will turn out to be removed as dead-code.
609 /// When building statics/constants, returns `None` since
610 /// intermediate values do not have to be dropped in that case.
611 pub fn local_scope(&self) -> Option<region::Scope> {
613 MirSource::Const(_) |
614 MirSource::Static(..) =>
615 // No need to free storage in this context.
618 Some(self.topmost_scope()),
619 MirSource::Promoted(..) |
620 MirSource::GeneratorDrop(..) =>
627 /// Indicates that `lvalue` should be dropped on exit from
629 pub fn schedule_drop(&mut self,
631 region_scope: region::Scope,
632 lvalue: &Lvalue<'tcx>,
633 lvalue_ty: Ty<'tcx>) {
634 let needs_drop = self.hir.needs_drop(lvalue_ty);
635 let drop_kind = if needs_drop {
636 DropKind::Value { cached_block: CachedBlock::default() }
638 // Only temps and vars need their storage dead.
640 Lvalue::Local(index) if index.index() > self.arg_count => DropKind::Storage,
645 for scope in self.scopes.iter_mut().rev() {
646 let this_scope = scope.region_scope == region_scope;
647 // When building drops, we try to cache chains of drops in such a way so these drops
648 // could be reused by the drops which would branch into the cached (already built)
649 // blocks. This, however, means that whenever we add a drop into a scope which already
650 // had some blocks built (and thus, cached) for it, we must invalidate all caches which
651 // might branch into the scope which had a drop just added to it. This is necessary,
652 // because otherwise some other code might use the cache to branch into already built
653 // chain of drops, essentially ignoring the newly added drop.
655 // For example consider there’s two scopes with a drop in each. These are built and
656 // thus the caches are filled:
658 // +--------------------------------------------------------+
659 // | +---------------------------------+ |
660 // | | +--------+ +-------------+ | +---------------+ |
661 // | | | return | <-+ | drop(outer) | <-+ | drop(middle) | |
662 // | | +--------+ +-------------+ | +---------------+ |
663 // | +------------|outer_scope cache|--+ |
664 // +------------------------------|middle_scope cache|------+
666 // Now, a new, inner-most scope is added along with a new drop into both inner-most and
667 // outer-most scopes:
669 // +------------------------------------------------------------+
670 // | +----------------------------------+ |
671 // | | +--------+ +-------------+ | +---------------+ | +-------------+
672 // | | | return | <+ | drop(new) | <-+ | drop(middle) | <--+| drop(inner) |
673 // | | +--------+ | | drop(outer) | | +---------------+ | +-------------+
674 // | | +-+ +-------------+ | |
675 // | +---|invalid outer_scope cache|----+ |
676 // +----=----------------|invalid middle_scope cache|-----------+
678 // If, when adding `drop(new)` we do not invalidate the cached blocks for both
679 // outer_scope and middle_scope, then, when building drops for the inner (right-most)
680 // scope, the old, cached blocks, without `drop(new)` will get used, producing the
683 // The cache and its invalidation for unwind branch is somewhat special. The cache is
684 // per-drop, rather than per scope, which has a several different implications. Adding
685 // a new drop into a scope will not invalidate cached blocks of the prior drops in the
686 // scope. That is true, because none of the already existing drops will have an edge
687 // into a block with the newly added drop.
689 // Note that this code iterates scopes from the inner-most to the outer-most,
690 // invalidating caches of each scope visited. This way bare minimum of the
691 // caches gets invalidated. i.e. if a new drop is added into the middle scope, the
692 // cache of outer scpoe stays intact.
693 scope.invalidate_cache(!needs_drop, this_scope);
695 if let DropKind::Value { .. } = drop_kind {
696 scope.needs_cleanup = true;
698 let region_scope_span = region_scope.span(self.hir.tcx(),
699 &self.hir.region_scope_tree);
700 // Attribute scope exit drops to scope's closing brace
701 let scope_end = region_scope_span.with_lo(region_scope_span.hi());
702 scope.drops.push(DropData {
704 location: lvalue.clone(),
710 span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, lvalue);
715 /// Creates a path that performs all required cleanup for unwinding.
717 /// This path terminates in Resume. Returns the start of the path.
718 /// See module comment for more details. None indicates there’s no
719 /// cleanup to do at this point.
720 pub fn diverge_cleanup(&mut self) -> Option<BasicBlock> {
721 self.diverge_cleanup_gen(false)
724 fn diverge_cleanup_gen(&mut self, generator_drop: bool) -> Option<BasicBlock> {
725 if !self.scopes.iter().any(|scope| scope.needs_cleanup) {
728 assert!(!self.scopes.is_empty()); // or `any` above would be false
730 let Builder { ref mut cfg, ref mut scopes,
731 ref mut cached_resume_block, .. } = *self;
733 // Build up the drops in **reverse** order. The end result will
736 // scopes[n] -> scopes[n-1] -> ... -> scopes[0]
738 // However, we build this in **reverse order**. That is, we
739 // process scopes[0], then scopes[1], etc, pointing each one at
740 // the result generates from the one before. Along the way, we
741 // store caches. If everything is cached, we'll just walk right
742 // to left reading the cached results but never created anything.
744 // To start, create the resume terminator.
745 let mut target = if let Some(target) = *cached_resume_block {
748 let resumeblk = cfg.start_new_cleanup_block();
749 cfg.terminate(resumeblk,
750 scopes[0].source_info(self.fn_span),
751 TerminatorKind::Resume);
752 *cached_resume_block = Some(resumeblk);
756 for scope in scopes.iter_mut() {
757 target = build_diverge_scope(self.hir.tcx(), cfg, scope.region_scope_span,
758 scope, target, generator_drop);
763 /// Utility function for *non*-scope code to build their own drops
764 pub fn build_drop(&mut self,
767 location: Lvalue<'tcx>,
768 ty: Ty<'tcx>) -> BlockAnd<()> {
769 if !self.hir.needs_drop(ty) {
772 let source_info = self.source_info(span);
773 let next_target = self.cfg.start_new_block();
774 let diverge_target = self.diverge_cleanup();
775 self.cfg.terminate(block, source_info,
776 TerminatorKind::Drop {
779 unwind: diverge_target,
784 /// Utility function for *non*-scope code to build their own drops
785 pub fn build_drop_and_replace(&mut self,
788 location: Lvalue<'tcx>,
789 value: Operand<'tcx>) -> BlockAnd<()> {
790 let source_info = self.source_info(span);
791 let next_target = self.cfg.start_new_block();
792 let diverge_target = self.diverge_cleanup();
793 self.cfg.terminate(block, source_info,
794 TerminatorKind::DropAndReplace {
798 unwind: diverge_target,
803 /// Create an Assert terminator and return the success block.
804 /// If the boolean condition operand is not the expected value,
805 /// a runtime panic will be caused with the given message.
806 pub fn assert(&mut self, block: BasicBlock,
809 msg: AssertMessage<'tcx>,
812 let source_info = self.source_info(span);
814 let success_block = self.cfg.start_new_block();
815 let cleanup = self.diverge_cleanup();
817 self.cfg.terminate(block, source_info,
818 TerminatorKind::Assert {
822 target: success_block,
830 /// Builds drops for pop_scope and exit_scope.
831 fn build_scope_drops<'tcx>(cfg: &mut CFG<'tcx>,
833 earlier_scopes: &[Scope<'tcx>],
834 mut block: BasicBlock,
836 generator_drop: bool)
838 debug!("build_scope_drops({:?} -> {:?})", block, scope);
839 let mut iter = scope.drops.iter().rev();
840 while let Some(drop_data) = iter.next() {
841 let source_info = scope.source_info(drop_data.span);
842 match drop_data.kind {
843 DropKind::Value { .. } => {
844 // Try to find the next block with its cached block
845 // for us to diverge into in case the drop panics.
846 let on_diverge = iter.clone().filter_map(|dd| {
848 DropKind::Value { cached_block } => Some(cached_block),
849 DropKind::Storage => None
851 }).map(|cached_block| {
854 .unwrap_or_else(|| span_bug!(drop_data.span, "cached block not present?"))
856 // If there’s no `cached_block`s within current scope,
857 // we must look for one in the enclosing scope.
858 let on_diverge = on_diverge.or_else(|| {
859 earlier_scopes.iter().rev().flat_map(|s| s.cached_block(generator_drop)).next()
861 let next = cfg.start_new_block();
862 cfg.terminate(block, source_info, TerminatorKind::Drop {
863 location: drop_data.location.clone(),
869 DropKind::Storage => {}
872 // We do not need to emit StorageDead for generator drops
877 // Drop the storage for both value and storage drops.
878 // Only temps and vars need their storage dead.
879 match drop_data.location {
880 Lvalue::Local(index) if index.index() > arg_count => {
881 cfg.push(block, Statement {
883 kind: StatementKind::StorageDead(index)
892 fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
895 scope: &mut Scope<'tcx>,
896 mut target: BasicBlock,
897 generator_drop: bool)
900 // Build up the drops in **reverse** order. The end result will
903 // [EndRegion Block] -> [drops[n]] -...-> [drops[0]] -> [Free] -> [target]
905 // +---------------------------------------------------------+
908 // The code in this function reads from right to left. At each
909 // point, we check for cached blocks representing the
910 // remainder. If everything is cached, we'll just walk right to
911 // left reading the cached results but never create anything.
913 let visibility_scope = scope.visibility_scope;
914 let source_info = |span| SourceInfo {
916 scope: visibility_scope
919 // Next, build up the drops. Here we iterate the vector in
920 // *forward* order, so that we generate drops[0] first (right to
921 // left in diagram above).
922 for (j, drop_data) in scope.drops.iter_mut().enumerate() {
923 debug!("build_diverge_scope drop_data[{}]: {:?}", j, drop_data);
924 // Only full value drops are emitted in the diverging path,
927 // Note: This may not actually be what we desire (are we
928 // "freeing" stack storage as we unwind, or merely observing a
929 // frozen stack)? In particular, the intent may have been to
930 // match the behavior of clang, but on inspection eddyb says
931 // this is not what clang does.
932 let cached_block = match drop_data.kind {
933 DropKind::Value { ref mut cached_block } => cached_block.ref_mut(generator_drop),
934 DropKind::Storage => continue
936 target = if let Some(cached_block) = *cached_block {
939 let block = cfg.start_new_cleanup_block();
940 cfg.terminate(block, source_info(drop_data.span),
941 TerminatorKind::Drop {
942 location: drop_data.location.clone(),
946 *cached_block = Some(block);
951 // Finally, push the EndRegion block, used by mir-borrowck. (Block
952 // becomes trivial goto after pass that removes all EndRegions.)
954 let block = cfg.start_new_cleanup_block();
955 cfg.push_end_region(tcx, block, source_info(span), scope.region_scope);
956 cfg.terminate(block, source_info(span), TerminatorKind::Goto { target: target });
960 debug!("build_diverge_scope({:?}, {:?}) = {:?}", scope, span, target);