| //! Code related to match expressions. These are sufficiently complex to |
| //! warrant their own module and submodules. :) This main module includes the |
| //! high-level algorithm, the submodules contain the details. |
| //! |
| //! This also includes code for pattern bindings in `let` statements and |
| //! function parameters. |
| |
| use crate::build::scope::DropKind; |
| use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard}; |
| use crate::build::{BlockAnd, BlockAndExtension, Builder}; |
| use crate::build::{GuardFrame, GuardFrameLocal, LocalsForNode}; |
| use crate::hair::{self, *}; |
| use rustc::hir::HirId; |
| use rustc::mir::*; |
| use rustc::middle::region; |
| use rustc::ty::{self, CanonicalUserTypeAnnotation, Ty}; |
| use rustc::ty::layout::VariantIdx; |
| use rustc_data_structures::bit_set::BitSet; |
| use rustc_data_structures::fx::{FxHashMap, FxHashSet}; |
| use syntax::ast::Name; |
| use syntax_pos::Span; |
| |
| // helper functions, broken out by category: |
| mod simplify; |
| mod test; |
| mod util; |
| |
| use std::convert::TryFrom; |
| |
| impl<'a, 'tcx> Builder<'a, 'tcx> { |
| /// Generates MIR for a `match` expression. |
| /// |
| /// The MIR that we generate for a match looks like this. |
| /// |
| /// ```text |
| /// [ 0. Pre-match ] |
| /// | |
| /// [ 1. Evaluate Scrutinee (expression being matched on) ] |
| /// [ (fake read of scrutinee) ] |
| /// | |
| /// [ 2. Decision tree -- check discriminants ] <--------+ |
| /// | | |
| /// | (once a specific arm is chosen) | |
| /// | | |
| /// [pre_binding_block] [otherwise_block] |
| /// | | |
| /// [ 3. Create "guard bindings" for arm ] | |
| /// [ (create fake borrows) ] | |
| /// | | |
| /// [ 4. Execute guard code ] | |
| /// [ (read fake borrows) ] --(guard is false)-----------+ |
| /// | |
| /// | (guard results in true) |
| /// | |
| /// [ 5. Create real bindings and execute arm ] |
| /// | |
| /// [ Exit match ] |
| /// ``` |
| /// |
| /// All of the different arms have been stacked on top of each other to |
| /// simplify the diagram. For an arm with no guard the blocks marked 3 and |
| /// 4 and the fake borrows are omitted. |
| /// |
| /// We generate MIR in the following steps: |
| /// |
| /// 1. Evaluate the scrutinee and add the fake read of it. |
| /// 2. Create the prebinding and otherwise blocks. |
| /// 3. Create the decision tree and record the places that we bind or test. |
| /// 4. Determine the fake borrows that are needed from the above places. |
| /// Create the required temporaries for them. |
| /// 5. Create everything else: Create everything else: the guards and the |
| /// arms. |
| /// |
| /// ## Fake Reads and borrows |
| /// |
| /// Match exhaustiveness checking is not able to handle the case where the |
| /// place being matched on is mutated in the guards. There is an AST check |
| /// that tries to stop this but it is buggy and overly restrictive. Instead |
| /// we add "fake borrows" to the guards that prevent any mutation of the |
| /// place being matched. There are a some subtleties: |
| /// |
| /// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared |
| /// refence, the borrow isn't even tracked. As such we have to add fake |
| /// borrows of any prefixes of a place |
| /// 2. We don't want `match x { _ => (), }` to conflict with mutable |
| /// borrows of `x`, so we only add fake borrows for places which are |
| /// bound or tested by the match. |
| /// 3. We don't want the fake borrows to conflict with `ref mut` bindings, |
| /// so we use a special BorrowKind for them. |
| /// 4. The fake borrows may be of places in inactive variants, so it would |
| /// be UB to generate code for them. They therefore have to be removed |
| /// by a MIR pass run after borrow checking. |
| /// |
| /// ## False edges |
| /// |
| /// We don't want to have the exact structure of the decision tree be |
| /// visible through borrow checking. False edges ensure that the CFG as |
| /// seen by borrow checking doesn't encode this. False edges are added: |
| /// |
| /// * From each prebinding block to the next prebinding block. |
| /// * From each otherwise block to the next prebinding block. |
| pub fn match_expr( |
| &mut self, |
| destination: &Place<'tcx>, |
| span: Span, |
| mut block: BasicBlock, |
| scrutinee: ExprRef<'tcx>, |
| arms: Vec<Arm<'tcx>>, |
| ) -> BlockAnd<()> { |
| let tcx = self.hir.tcx(); |
| |
| // Step 1. Evaluate the scrutinee and add the fake read of it. |
| |
| let scrutinee_span = scrutinee.span(); |
| let scrutinee_place = unpack!(block = self.as_place(block, scrutinee)); |
| |
| // Matching on a `scrutinee_place` with an uninhabited type doesn't |
| // generate any memory reads by itself, and so if the place "expression" |
| // contains unsafe operations like raw pointer dereferences or union |
| // field projections, we wouldn't know to require an `unsafe` block |
| // around a `match` equivalent to `std::intrinsics::unreachable()`. |
| // See issue #47412 for this hole being discovered in the wild. |
| // |
| // HACK(eddyb) Work around the above issue by adding a dummy inspection |
| // of `scrutinee_place`, specifically by applying `ReadForMatch`. |
| // |
| // NOTE: ReadForMatch also checks that the scrutinee is initialized. |
| // This is currently needed to not allow matching on an uninitialized, |
| // uninhabited value. If we get never patterns, those will check that |
| // the place is initialized, and so this read would only be used to |
| // check safety. |
| |
| let source_info = self.source_info(scrutinee_span); |
| self.cfg.push(block, Statement { |
| source_info, |
| kind: StatementKind::FakeRead( |
| FakeReadCause::ForMatchedPlace, |
| box(scrutinee_place.clone()), |
| ), |
| }); |
| |
| // Step 2. Create the otherwise and prebinding blocks. |
| |
| // create binding start block for link them by false edges |
| let candidate_count = arms.iter().map(|c| c.patterns.len()).sum::<usize>(); |
| let pre_binding_blocks: Vec<_> = (0..candidate_count) |
| .map(|_| self.cfg.start_new_block()) |
| .collect(); |
| |
| let mut match_has_guard = false; |
| |
| let mut candidate_pre_binding_blocks = pre_binding_blocks.iter(); |
| let mut next_candidate_pre_binding_blocks = pre_binding_blocks.iter().skip(1); |
| |
| // Assemble a list of candidates: there is one candidate per pattern, |
| // which means there may be more than one candidate *per arm*. |
| let mut arm_candidates: Vec<_> = arms |
| .iter() |
| .map(|arm| { |
| let arm_has_guard = arm.guard.is_some(); |
| match_has_guard |= arm_has_guard; |
| let arm_candidates: Vec<_> = arm.patterns |
| .iter() |
| .zip(candidate_pre_binding_blocks.by_ref()) |
| .map( |
| |(pattern, pre_binding_block)| { |
| Candidate { |
| span: pattern.span, |
| match_pairs: vec![ |
| MatchPair::new(scrutinee_place.clone(), pattern), |
| ], |
| bindings: vec![], |
| ascriptions: vec![], |
| otherwise_block: if arm_has_guard { |
| Some(self.cfg.start_new_block()) |
| } else { |
| None |
| }, |
| pre_binding_block: *pre_binding_block, |
| next_candidate_pre_binding_block: |
| next_candidate_pre_binding_blocks.next().copied(), |
| } |
| }, |
| ) |
| .collect(); |
| (arm, arm_candidates) |
| }) |
| .collect(); |
| |
| // Step 3. Create the decision tree and record the places that we bind or test. |
| |
| // The set of places that we are creating fake borrows of. If there are |
| // no match guards then we don't need any fake borrows, so don't track |
| // them. |
| let mut fake_borrows = if match_has_guard && tcx.generate_borrow_of_any_match_input() { |
| Some(FxHashSet::default()) |
| } else { |
| None |
| }; |
| |
| // These candidates are kept sorted such that the highest priority |
| // candidate comes first in the list. (i.e., same order as in source) |
| // As we gnerate the decision tree, |
| let candidates = &mut arm_candidates |
| .iter_mut() |
| .flat_map(|(_, candidates)| candidates) |
| .collect::<Vec<_>>(); |
| |
| let outer_source_info = self.source_info(span); |
| |
| // this will generate code to test scrutinee_place and |
| // branch to the appropriate arm block |
| self.match_candidates( |
| scrutinee_span, |
| &mut Some(block), |
| None, |
| candidates, |
| &mut fake_borrows, |
| ); |
| |
| // Step 4. Determine the fake borrows that are needed from the above |
| // places. Create the required temporaries for them. |
| |
| let fake_borrow_temps = if let Some(ref borrows) = fake_borrows { |
| self.calculate_fake_borrows(borrows, scrutinee_span) |
| } else { |
| Vec::new() |
| }; |
| |
| // Step 5. Create everything else: the guards and the arms. |
| let match_scope = self.scopes.topmost(); |
| |
| let arm_end_blocks: Vec<_> = arm_candidates.into_iter().map(|(arm, mut candidates)| { |
| let arm_source_info = self.source_info(arm.span); |
| let arm_scope = (arm.scope, arm_source_info); |
| self.in_scope(arm_scope, arm.lint_level, |this| { |
| let body = this.hir.mirror(arm.body.clone()); |
| let scope = this.declare_bindings( |
| None, |
| arm.span, |
| &arm.patterns[0], |
| ArmHasGuard(arm.guard.is_some()), |
| Some((Some(&scrutinee_place), scrutinee_span)), |
| ); |
| |
| let arm_block; |
| if candidates.len() == 1 { |
| arm_block = this.bind_and_guard_matched_candidate( |
| candidates.pop().unwrap(), |
| arm.guard.clone(), |
| &fake_borrow_temps, |
| scrutinee_span, |
| match_scope, |
| ); |
| } else { |
| arm_block = this.cfg.start_new_block(); |
| for candidate in candidates { |
| this.clear_top_scope(arm.scope); |
| let binding_end = this.bind_and_guard_matched_candidate( |
| candidate, |
| arm.guard.clone(), |
| &fake_borrow_temps, |
| scrutinee_span, |
| match_scope, |
| ); |
| this.cfg.terminate( |
| binding_end, |
| source_info, |
| TerminatorKind::Goto { target: arm_block }, |
| ); |
| } |
| } |
| |
| if let Some(source_scope) = scope { |
| this.source_scope = source_scope; |
| } |
| |
| this.into(destination, arm_block, body) |
| }) |
| }).collect(); |
| |
| // all the arm blocks will rejoin here |
| let end_block = self.cfg.start_new_block(); |
| |
| for arm_block in arm_end_blocks { |
| self.cfg.terminate( |
| unpack!(arm_block), |
| outer_source_info, |
| TerminatorKind::Goto { target: end_block }, |
| ); |
| } |
| |
| self.source_scope = outer_source_info.scope; |
| |
| end_block.unit() |
| } |
| |
| pub(super) fn expr_into_pattern( |
| &mut self, |
| mut block: BasicBlock, |
| irrefutable_pat: Pattern<'tcx>, |
| initializer: ExprRef<'tcx>, |
| ) -> BlockAnd<()> { |
| match *irrefutable_pat.kind { |
| // Optimize the case of `let x = ...` to write directly into `x` |
| PatternKind::Binding { |
| mode: BindingMode::ByValue, |
| var, |
| subpattern: None, |
| .. |
| } => { |
| let place = |
| self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard); |
| unpack!(block = self.into(&place, block, initializer)); |
| |
| |
| // Inject a fake read, see comments on `FakeReadCause::ForLet`. |
| let source_info = self.source_info(irrefutable_pat.span); |
| self.cfg.push( |
| block, |
| Statement { |
| source_info, |
| kind: StatementKind::FakeRead(FakeReadCause::ForLet, box(place)), |
| }, |
| ); |
| |
| self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard); |
| block.unit() |
| } |
| |
| // Optimize the case of `let x: T = ...` to write directly |
| // into `x` and then require that `T == typeof(x)`. |
| // |
| // Weirdly, this is needed to prevent the |
| // `intrinsic-move-val.rs` test case from crashing. That |
| // test works with uninitialized values in a rather |
| // dubious way, so it may be that the test is kind of |
| // broken. |
| PatternKind::AscribeUserType { |
| subpattern: Pattern { |
| kind: box PatternKind::Binding { |
| mode: BindingMode::ByValue, |
| var, |
| subpattern: None, |
| .. |
| }, |
| .. |
| }, |
| ascription: hair::pattern::Ascription { |
| user_ty: pat_ascription_ty, |
| variance: _, |
| user_ty_span, |
| }, |
| } => { |
| let place = |
| self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard); |
| unpack!(block = self.into(&place, block, initializer)); |
| |
| // Inject a fake read, see comments on `FakeReadCause::ForLet`. |
| let pattern_source_info = self.source_info(irrefutable_pat.span); |
| self.cfg.push( |
| block, |
| Statement { |
| source_info: pattern_source_info, |
| kind: StatementKind::FakeRead(FakeReadCause::ForLet, box(place.clone())), |
| }, |
| ); |
| |
| let ty_source_info = self.source_info(user_ty_span); |
| let user_ty = pat_ascription_ty.user_ty( |
| &mut self.canonical_user_type_annotations, |
| place.ty(&self.local_decls, self.hir.tcx()).ty, |
| ty_source_info.span, |
| ); |
| self.cfg.push( |
| block, |
| Statement { |
| source_info: ty_source_info, |
| kind: StatementKind::AscribeUserType( |
| box( |
| place, |
| user_ty, |
| ), |
| // We always use invariant as the variance here. This is because the |
| // variance field from the ascription refers to the variance to use |
| // when applying the type to the value being matched, but this |
| // ascription applies rather to the type of the binding. e.g., in this |
| // example: |
| // |
| // ``` |
| // let x: T = <expr> |
| // ``` |
| // |
| // We are creating an ascription that defines the type of `x` to be |
| // exactly `T` (i.e., with invariance). The variance field, in |
| // contrast, is intended to be used to relate `T` to the type of |
| // `<expr>`. |
| ty::Variance::Invariant, |
| ), |
| }, |
| ); |
| |
| self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard); |
| block.unit() |
| } |
| |
| _ => { |
| let place = unpack!(block = self.as_place(block, initializer)); |
| self.place_into_pattern(block, irrefutable_pat, &place, true) |
| } |
| } |
| } |
| |
| pub fn place_into_pattern( |
| &mut self, |
| block: BasicBlock, |
| irrefutable_pat: Pattern<'tcx>, |
| initializer: &Place<'tcx>, |
| set_match_place: bool, |
| ) -> BlockAnd<()> { |
| // create a dummy candidate |
| let mut candidate = Candidate { |
| span: irrefutable_pat.span, |
| match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)], |
| bindings: vec![], |
| ascriptions: vec![], |
| |
| // since we don't call `match_candidates`, next fields are unused |
| otherwise_block: None, |
| pre_binding_block: block, |
| next_candidate_pre_binding_block: None, |
| }; |
| |
| // Simplify the candidate. Since the pattern is irrefutable, this should |
| // always convert all match-pairs into bindings. |
| self.simplify_candidate(&mut candidate); |
| |
| if !candidate.match_pairs.is_empty() { |
| // ICE if no other errors have been emitted. This used to be a hard error that wouldn't |
| // be reached because `hair::pattern::check_match::check_match` wouldn't have let the |
| // compiler continue. In our tests this is only ever hit by |
| // `ui/consts/const-match-check.rs` with `--cfg eval1`, and that file already generates |
| // a different error before hand. |
| self.hir.tcx().sess.delay_span_bug( |
| candidate.match_pairs[0].pattern.span, |
| &format!( |
| "match pairs {:?} remaining after simplifying irrefutable pattern", |
| candidate.match_pairs, |
| ), |
| ); |
| } |
| |
| // for matches and function arguments, the place that is being matched |
| // can be set when creating the variables. But the place for |
| // let PATTERN = ... might not even exist until we do the assignment. |
| // so we set it here instead |
| if set_match_place { |
| for binding in &candidate.bindings { |
| let local = self.var_local_id(binding.var_id, OutsideGuard); |
| |
| if let Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm { |
| opt_match_place: Some((ref mut match_place, _)), |
| .. |
| }))) = self.local_decls[local].is_user_variable |
| { |
| *match_place = Some(initializer.clone()); |
| } else { |
| bug!("Let binding to non-user variable.") |
| } |
| } |
| } |
| |
| self.ascribe_types(block, &candidate.ascriptions); |
| |
| // now apply the bindings, which will also declare the variables |
| self.bind_matched_candidate_for_arm_body(block, &candidate.bindings); |
| |
| block.unit() |
| } |
| |
| /// Declares the bindings of the given patterns and returns the visibility |
| /// scope for the bindings in these patterns, if such a scope had to be |
| /// created. NOTE: Declaring the bindings should always be done in their |
| /// drop scope. |
| pub fn declare_bindings( |
| &mut self, |
| mut visibility_scope: Option<SourceScope>, |
| scope_span: Span, |
| pattern: &Pattern<'tcx>, |
| has_guard: ArmHasGuard, |
| opt_match_place: Option<(Option<&Place<'tcx>>, Span)>, |
| ) -> Option<SourceScope> { |
| debug!("declare_bindings: pattern={:?}", pattern); |
| self.visit_bindings( |
| &pattern, |
| UserTypeProjections::none(), |
| &mut |this, mutability, name, mode, var, span, ty, user_ty| { |
| if visibility_scope.is_none() { |
| visibility_scope = |
| Some(this.new_source_scope(scope_span, LintLevel::Inherited, None)); |
| } |
| let source_info = SourceInfo { span, scope: this.source_scope }; |
| let visibility_scope = visibility_scope.unwrap(); |
| this.declare_binding( |
| source_info, |
| visibility_scope, |
| mutability, |
| name, |
| mode, |
| var, |
| ty, |
| user_ty, |
| has_guard, |
| opt_match_place.map(|(x, y)| (x.cloned(), y)), |
| pattern.span, |
| ); |
| }, |
| ); |
| visibility_scope |
| } |
| |
| pub fn storage_live_binding( |
| &mut self, |
| block: BasicBlock, |
| var: HirId, |
| span: Span, |
| for_guard: ForGuard, |
| ) -> Place<'tcx> { |
| let local_id = self.var_local_id(var, for_guard); |
| let source_info = self.source_info(span); |
| self.cfg.push( |
| block, |
| Statement { |
| source_info, |
| kind: StatementKind::StorageLive(local_id), |
| }, |
| ); |
| let var_ty = self.local_decls[local_id].ty; |
| let region_scope = self.hir.region_scope_tree.var_scope(var.local_id); |
| self.schedule_drop(span, region_scope, local_id, var_ty, DropKind::Storage); |
| Place::from(local_id) |
| } |
| |
| pub fn schedule_drop_for_binding(&mut self, var: HirId, span: Span, for_guard: ForGuard) { |
| let local_id = self.var_local_id(var, for_guard); |
| let var_ty = self.local_decls[local_id].ty; |
| let region_scope = self.hir.region_scope_tree.var_scope(var.local_id); |
| self.schedule_drop( |
| span, |
| region_scope, |
| local_id, |
| var_ty, |
| DropKind::Value, |
| ); |
| } |
| |
| pub(super) fn visit_bindings( |
| &mut self, |
| pattern: &Pattern<'tcx>, |
| pattern_user_ty: UserTypeProjections, |
| f: &mut impl FnMut( |
| &mut Self, |
| Mutability, |
| Name, |
| BindingMode, |
| HirId, |
| Span, |
| Ty<'tcx>, |
| UserTypeProjections, |
| ), |
| ) { |
| debug!("visit_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty); |
| match *pattern.kind { |
| PatternKind::Binding { |
| mutability, |
| name, |
| mode, |
| var, |
| ty, |
| ref subpattern, |
| .. |
| } => { |
| f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone()); |
| if let Some(subpattern) = subpattern.as_ref() { |
| self.visit_bindings(subpattern, pattern_user_ty, f); |
| } |
| } |
| |
| PatternKind::Array { |
| ref prefix, |
| ref slice, |
| ref suffix, |
| } |
| | PatternKind::Slice { |
| ref prefix, |
| ref slice, |
| ref suffix, |
| } => { |
| let from = u32::try_from(prefix.len()).unwrap(); |
| let to = u32::try_from(suffix.len()).unwrap(); |
| for subpattern in prefix { |
| self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f); |
| } |
| for subpattern in slice { |
| self.visit_bindings(subpattern, pattern_user_ty.clone().subslice(from, to), f); |
| } |
| for subpattern in suffix { |
| self.visit_bindings(subpattern, pattern_user_ty.clone().index(), f); |
| } |
| } |
| |
| PatternKind::Constant { .. } | PatternKind::Range { .. } | PatternKind::Wild => {} |
| |
| PatternKind::Deref { ref subpattern } => { |
| self.visit_bindings(subpattern, pattern_user_ty.deref(), f); |
| } |
| |
| PatternKind::AscribeUserType { |
| ref subpattern, |
| ascription: hair::pattern::Ascription { |
| ref user_ty, |
| user_ty_span, |
| variance: _, |
| }, |
| } => { |
| // This corresponds to something like |
| // |
| // ``` |
| // let A::<'a>(_): A<'static> = ...; |
| // ``` |
| // |
| // Note that the variance doesn't apply here, as we are tracking the effect |
| // of `user_ty` on any bindings contained with subpattern. |
| let annotation = CanonicalUserTypeAnnotation { |
| span: user_ty_span, |
| user_ty: user_ty.user_ty, |
| inferred_ty: subpattern.ty, |
| }; |
| let projection = UserTypeProjection { |
| base: self.canonical_user_type_annotations.push(annotation), |
| projs: Vec::new(), |
| }; |
| let subpattern_user_ty = pattern_user_ty.push_projection(&projection, user_ty_span); |
| self.visit_bindings(subpattern, subpattern_user_ty, f) |
| } |
| |
| PatternKind::Leaf { ref subpatterns } => { |
| for subpattern in subpatterns { |
| let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field); |
| debug!("visit_bindings: subpattern_user_ty={:?}", subpattern_user_ty); |
| self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f); |
| } |
| } |
| |
| PatternKind::Variant { adt_def, substs: _, variant_index, ref subpatterns } => { |
| for subpattern in subpatterns { |
| let subpattern_user_ty = pattern_user_ty.clone().variant( |
| adt_def, variant_index, subpattern.field); |
| self.visit_bindings(&subpattern.pattern, subpattern_user_ty, f); |
| } |
| } |
| PatternKind::Or { ref pats } => { |
| for pat in pats { |
| self.visit_bindings(&pat, pattern_user_ty.clone(), f); |
| } |
| } |
| } |
| } |
| } |
| |
| #[derive(Debug)] |
| pub struct Candidate<'pat, 'tcx> { |
| // span of the original pattern that gave rise to this candidate |
| span: Span, |
| |
| // all of these must be satisfied... |
| match_pairs: Vec<MatchPair<'pat, 'tcx>>, |
| |
| // ...these bindings established... |
| bindings: Vec<Binding<'tcx>>, |
| |
| // ...and these types asserted... |
| ascriptions: Vec<Ascription<'tcx>>, |
| |
| // ...and the guard must be evaluated, if false branch to Block... |
| otherwise_block: Option<BasicBlock>, |
| |
| // ...and the blocks for add false edges between candidates |
| pre_binding_block: BasicBlock, |
| next_candidate_pre_binding_block: Option<BasicBlock>, |
| } |
| |
| #[derive(Clone, Debug)] |
| struct Binding<'tcx> { |
| span: Span, |
| source: Place<'tcx>, |
| name: Name, |
| var_id: HirId, |
| var_ty: Ty<'tcx>, |
| mutability: Mutability, |
| binding_mode: BindingMode, |
| } |
| |
| /// Indicates that the type of `source` must be a subtype of the |
| /// user-given type `user_ty`; this is basically a no-op but can |
| /// influence region inference. |
| #[derive(Clone, Debug)] |
| struct Ascription<'tcx> { |
| span: Span, |
| source: Place<'tcx>, |
| user_ty: PatternTypeProjection<'tcx>, |
| variance: ty::Variance, |
| } |
| |
| #[derive(Clone, Debug)] |
| pub struct MatchPair<'pat, 'tcx> { |
| // this place... |
| place: Place<'tcx>, |
| |
| // ... must match this pattern. |
| pattern: &'pat Pattern<'tcx>, |
| } |
| |
| #[derive(Clone, Debug, PartialEq)] |
| enum TestKind<'tcx> { |
| /// Test the branches of enum. |
| Switch { |
| /// The enum being tested |
| adt_def: &'tcx ty::AdtDef, |
| /// The set of variants that we should create a branch for. We also |
| /// create an additional "otherwise" case. |
| variants: BitSet<VariantIdx>, |
| }, |
| |
| /// Test what value an `integer`, `bool` or `char` has. |
| SwitchInt { |
| /// The type of the value that we're testing. |
| switch_ty: Ty<'tcx>, |
| /// The (ordered) set of values that we test for. |
| /// |
| /// For integers and `char`s we create a branch to each of the values in |
| /// `options`, as well as an "otherwise" branch for all other values, even |
| /// in the (rare) case that options is exhaustive. |
| /// |
| /// For `bool` we always generate two edges, one for `true` and one for |
| /// `false`. |
| options: Vec<u128>, |
| /// Reverse map used to ensure that the values in `options` are unique. |
| indices: FxHashMap<&'tcx ty::Const<'tcx>, usize>, |
| }, |
| |
| /// Test for equality with value, possibly after an unsizing coercion to |
| /// `ty`, |
| Eq { |
| value: &'tcx ty::Const<'tcx>, |
| // Integer types are handled by `SwitchInt`, and constants with ADT |
| // types are converted back into patterns, so this can only be `&str`, |
| // `&[T]`, `f32` or `f64`. |
| ty: Ty<'tcx>, |
| }, |
| |
| /// Test whether the value falls within an inclusive or exclusive range |
| Range(PatternRange<'tcx>), |
| |
| /// Test length of the slice is equal to len |
| Len { |
| len: u64, |
| op: BinOp, |
| }, |
| } |
| |
| #[derive(Debug)] |
| pub struct Test<'tcx> { |
| span: Span, |
| kind: TestKind<'tcx>, |
| } |
| |
| /// ArmHasGuard is isomorphic to a boolean flag. It indicates whether |
| /// a match arm has a guard expression attached to it. |
| #[derive(Copy, Clone, Debug)] |
| pub(crate) struct ArmHasGuard(pub bool); |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // Main matching algorithm |
| |
| impl<'a, 'tcx> Builder<'a, 'tcx> { |
| /// The main match algorithm. It begins with a set of candidates |
| /// `candidates` and has the job of generating code to determine |
| /// which of these candidates, if any, is the correct one. The |
| /// candidates are sorted such that the first item in the list |
| /// has the highest priority. When a candidate is found to match |
| /// the value, we will generate a branch to the appropriate |
| /// prebinding block. |
| /// |
| /// If we find that *NONE* of the candidates apply, we branch to the |
| /// `otherwise_block`. In principle, this means that the input list was not |
| /// exhaustive, though at present we sometimes are not smart enough to |
| /// recognize all exhaustive inputs. |
| /// |
| /// It might be surprising that the input can be inexhaustive. |
| /// Indeed, initially, it is not, because all matches are |
| /// exhaustive in Rust. But during processing we sometimes divide |
| /// up the list of candidates and recurse with a non-exhaustive |
| /// list. This is important to keep the size of the generated code |
| /// under control. See `test_candidates` for more details. |
| /// |
| /// If `fake_borrows` is Some, then places which need fake borrows |
| /// will be added to it. |
| fn match_candidates<'pat>( |
| &mut self, |
| span: Span, |
| start_block: &mut Option<BasicBlock>, |
| otherwise_block: Option<BasicBlock>, |
| candidates: &mut [&mut Candidate<'pat, 'tcx>], |
| fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>, |
| ) { |
| debug!( |
| "matched_candidate(span={:?}, candidates={:?}, start_block={:?}, otherwise_block={:?})", |
| span, |
| candidates, |
| start_block, |
| otherwise_block, |
| ); |
| |
| // Start by simplifying candidates. Once this process is complete, all |
| // the match pairs which remain require some form of test, whether it |
| // be a switch or pattern comparison. |
| for candidate in &mut *candidates { |
| self.simplify_candidate(candidate); |
| } |
| |
| // The candidates are sorted by priority. Check to see whether the |
| // higher priority candidates (and hence at the front of the slice) |
| // have satisfied all their match pairs. |
| let fully_matched = candidates |
| .iter() |
| .take_while(|c| c.match_pairs.is_empty()) |
| .count(); |
| debug!( |
| "match_candidates: {:?} candidates fully matched", |
| fully_matched |
| ); |
| let (matched_candidates, unmatched_candidates) = candidates.split_at_mut(fully_matched); |
| |
| let block: BasicBlock; |
| |
| if !matched_candidates.is_empty() { |
| let otherwise_block = self.select_matched_candidates( |
| matched_candidates, |
| start_block, |
| fake_borrows, |
| ); |
| |
| if let Some(last_otherwise_block) = otherwise_block { |
| block = last_otherwise_block |
| } else { |
| // Any remaining candidates are unreachable. |
| if unmatched_candidates.is_empty() { |
| return; |
| } |
| block = self.cfg.start_new_block(); |
| }; |
| } else { |
| block = *start_block.get_or_insert_with(|| self.cfg.start_new_block()); |
| } |
| |
| // If there are no candidates that still need testing, we're |
| // done. Since all matches are exhaustive, execution should |
| // never reach this point. |
| if unmatched_candidates.is_empty() { |
| let source_info = self.source_info(span); |
| if let Some(otherwise) = otherwise_block { |
| self.cfg.terminate( |
| block, |
| source_info, |
| TerminatorKind::Goto { target: otherwise }, |
| ); |
| } else { |
| self.cfg.terminate( |
| block, |
| source_info, |
| TerminatorKind::Unreachable, |
| ) |
| } |
| return; |
| } |
| |
| // Test for the remaining candidates. |
| self.test_candidates( |
| span, |
| unmatched_candidates, |
| block, |
| otherwise_block, |
| fake_borrows, |
| ); |
| } |
| |
| /// Link up matched candidates. For example, if we have something like |
| /// this: |
| /// |
| /// ... |
| /// Some(x) if cond => ... |
| /// Some(x) => ... |
| /// Some(x) if cond => ... |
| /// ... |
| /// |
| /// We generate real edges from: |
| /// * `block` to the prebinding_block of the first pattern, |
| /// * the otherwise block of the first pattern to the second pattern, |
| /// * the otherwise block of the third pattern to the a block with an |
| /// Unreachable terminator. |
| /// |
| /// As well as that we add fake edges from the otherwise blocks to the |
| /// prebinding block of the next candidate in the original set of |
| /// candidates. |
| fn select_matched_candidates( |
| &mut self, |
| matched_candidates: &mut [&mut Candidate<'_, 'tcx>], |
| start_block: &mut Option<BasicBlock>, |
| fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>, |
| ) -> Option<BasicBlock> { |
| debug_assert!( |
| !matched_candidates.is_empty(), |
| "select_matched_candidates called with no candidates", |
| ); |
| |
| // Insert a borrows of prefixes of places that are bound and are |
| // behind a dereference projection. |
| // |
| // These borrows are taken to avoid situations like the following: |
| // |
| // match x[10] { |
| // _ if { x = &[0]; false } => (), |
| // y => (), // Out of bounds array access! |
| // } |
| // |
| // match *x { |
| // // y is bound by reference in the guard and then by copy in the |
| // // arm, so y is 2 in the arm! |
| // y if { y == 1 && (x = &2) == () } => y, |
| // _ => 3, |
| // } |
| if let Some(fake_borrows) = fake_borrows { |
| for Binding { source, .. } |
| in matched_candidates.iter().flat_map(|candidate| &candidate.bindings) |
| { |
| if let Some(i) = |
| source.projection.iter().rposition(|elem| *elem == ProjectionElem::Deref) |
| { |
| let proj_base = &source.projection[..i]; |
| |
| fake_borrows.insert(Place { |
| base: source.base.clone(), |
| projection: proj_base.to_vec().into_boxed_slice(), |
| }); |
| } |
| } |
| } |
| |
| let fully_matched_with_guard = matched_candidates |
| .iter() |
| .position(|c| c.otherwise_block.is_none()) |
| .unwrap_or(matched_candidates.len() - 1); |
| |
| let (reachable_candidates, unreachable_candidates) |
| = matched_candidates.split_at_mut(fully_matched_with_guard + 1); |
| |
| let first_candidate = &reachable_candidates[0]; |
| let first_prebinding_block = first_candidate.pre_binding_block; |
| |
| if let Some(start_block) = *start_block { |
| let source_info = self.source_info(first_candidate.span); |
| self.cfg.terminate( |
| start_block, |
| source_info, |
| TerminatorKind::Goto { target: first_prebinding_block }, |
| ); |
| } else { |
| *start_block = Some(first_prebinding_block); |
| } |
| |
| for window in reachable_candidates.windows(2) { |
| if let [first_candidate, second_candidate] = window { |
| let source_info = self.source_info(first_candidate.span); |
| if let Some(otherwise_block) = first_candidate.otherwise_block { |
| self.false_edges( |
| otherwise_block, |
| second_candidate.pre_binding_block, |
| first_candidate.next_candidate_pre_binding_block, |
| source_info, |
| ); |
| } else { |
| bug!("candidate other than the last has no guard"); |
| } |
| } else { |
| bug!("<[_]>::windows returned incorrectly sized window"); |
| } |
| } |
| |
| debug!("match_candidates: add false edges for unreachable {:?}", unreachable_candidates); |
| for candidate in unreachable_candidates { |
| if let Some(otherwise) = candidate.otherwise_block { |
| let source_info = self.source_info(candidate.span); |
| let unreachable = self.cfg.start_new_block(); |
| self.false_edges( |
| otherwise, |
| unreachable, |
| candidate.next_candidate_pre_binding_block, |
| source_info, |
| ); |
| self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable); |
| } |
| } |
| |
| let last_candidate = reachable_candidates.last().unwrap(); |
| |
| if let Some(otherwise) = last_candidate.otherwise_block { |
| let source_info = self.source_info(last_candidate.span); |
| let block = self.cfg.start_new_block(); |
| self.false_edges( |
| otherwise, |
| block, |
| last_candidate.next_candidate_pre_binding_block, |
| source_info, |
| ); |
| Some(block) |
| } else { |
| None |
| } |
| } |
| |
| /// This is the most subtle part of the matching algorithm. At |
| /// this point, the input candidates have been fully simplified, |
| /// and so we know that all remaining match-pairs require some |
| /// sort of test. To decide what test to do, we take the highest |
| /// priority candidate (last one in the list) and extract the |
| /// first match-pair from the list. From this we decide what kind |
| /// of test is needed using `test`, defined in the `test` module. |
| /// |
| /// *Note:* taking the first match pair is somewhat arbitrary, and |
| /// we might do better here by choosing more carefully what to |
| /// test. |
| /// |
| /// For example, consider the following possible match-pairs: |
| /// |
| /// 1. `x @ Some(P)` -- we will do a `Switch` to decide what variant `x` has |
| /// 2. `x @ 22` -- we will do a `SwitchInt` |
| /// 3. `x @ 3..5` -- we will do a range test |
| /// 4. etc. |
| /// |
| /// Once we know what sort of test we are going to perform, this |
| /// Tests may also help us with other candidates. So we walk over |
| /// the candidates (from high to low priority) and check. This |
| /// gives us, for each outcome of the test, a transformed list of |
| /// candidates. For example, if we are testing the current |
| /// variant of `x.0`, and we have a candidate `{x.0 @ Some(v), x.1 |
| /// @ 22}`, then we would have a resulting candidate of `{(x.0 as |
| /// Some).0 @ v, x.1 @ 22}`. Note that the first match-pair is now |
| /// simpler (and, in fact, irrefutable). |
| /// |
| /// But there may also be candidates that the test just doesn't |
| /// apply to. The classical example involves wildcards: |
| /// |
| /// ``` |
| /// # let (x, y, z) = (true, true, true); |
| /// match (x, y, z) { |
| /// (true, _, true) => true, // (0) |
| /// (_, true, _) => true, // (1) |
| /// (false, false, _) => false, // (2) |
| /// (true, _, false) => false, // (3) |
| /// } |
| /// ``` |
| /// |
| /// In that case, after we test on `x`, there are 2 overlapping candidate |
| /// sets: |
| /// |
| /// - If the outcome is that `x` is true, candidates 0, 1, and 3 |
| /// - If the outcome is that `x` is false, candidates 1 and 2 |
| /// |
| /// Here, the traditional "decision tree" method would generate 2 |
| /// separate code-paths for the 2 separate cases. |
| /// |
| /// In some cases, this duplication can create an exponential amount of |
| /// code. This is most easily seen by noticing that this method terminates |
| /// with precisely the reachable arms being reachable - but that problem |
| /// is trivially NP-complete: |
| /// |
| /// ```rust |
| /// match (var0, var1, var2, var3, ..) { |
| /// (true, _, _, false, true, ...) => false, |
| /// (_, true, true, false, _, ...) => false, |
| /// (false, _, false, false, _, ...) => false, |
| /// ... |
| /// _ => true |
| /// } |
| /// ``` |
| /// |
| /// Here the last arm is reachable only if there is an assignment to |
| /// the variables that does not match any of the literals. Therefore, |
| /// compilation would take an exponential amount of time in some cases. |
| /// |
| /// That kind of exponential worst-case might not occur in practice, but |
| /// our simplistic treatment of constants and guards would make it occur |
| /// in very common situations - for example #29740: |
| /// |
| /// ```rust |
| /// match x { |
| /// "foo" if foo_guard => ..., |
| /// "bar" if bar_guard => ..., |
| /// "baz" if baz_guard => ..., |
| /// ... |
| /// } |
| /// ``` |
| /// |
| /// Here we first test the match-pair `x @ "foo"`, which is an `Eq` test. |
| /// |
| /// It might seem that we would end up with 2 disjoint candidate |
| /// sets, consisting of the first candidate or the other 3, but our |
| /// algorithm doesn't reason about "foo" being distinct from the other |
| /// constants; it considers the latter arms to potentially match after |
| /// both outcomes, which obviously leads to an exponential amount |
| /// of tests. |
| /// |
| /// To avoid these kinds of problems, our algorithm tries to ensure |
| /// the amount of generated tests is linear. When we do a k-way test, |
| /// we return an additional "unmatched" set alongside the obvious `k` |
| /// sets. When we encounter a candidate that would be present in more |
| /// than one of the sets, we put it and all candidates below it into the |
| /// "unmatched" set. This ensures these `k+1` sets are disjoint. |
| /// |
| /// After we perform our test, we branch into the appropriate candidate |
| /// set and recurse with `match_candidates`. These sub-matches are |
| /// obviously inexhaustive - as we discarded our otherwise set - so |
| /// we set their continuation to do `match_candidates` on the |
| /// "unmatched" set (which is again inexhaustive). |
| /// |
| /// If you apply this to the above test, you basically wind up |
| /// with an if-else-if chain, testing each candidate in turn, |
| /// which is precisely what we want. |
| /// |
| /// In addition to avoiding exponential-time blowups, this algorithm |
| /// also has nice property that each guard and arm is only generated |
| /// once. |
| fn test_candidates<'pat, 'b, 'c>( |
| &mut self, |
| span: Span, |
| mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], |
| block: BasicBlock, |
| mut otherwise_block: Option<BasicBlock>, |
| fake_borrows: &mut Option<FxHashSet<Place<'tcx>>>, |
| ) { |
| // extract the match-pair from the highest priority candidate |
| let match_pair = &candidates.first().unwrap().match_pairs[0]; |
| let mut test = self.test(match_pair); |
| let match_place = match_pair.place.clone(); |
| |
| // most of the time, the test to perform is simply a function |
| // of the main candidate; but for a test like SwitchInt, we |
| // may want to add cases based on the candidates that are |
| // available |
| match test.kind { |
| TestKind::SwitchInt { |
| switch_ty, |
| ref mut options, |
| ref mut indices, |
| } => { |
| for candidate in candidates.iter() { |
| if !self.add_cases_to_switch( |
| &match_place, |
| candidate, |
| switch_ty, |
| options, |
| indices, |
| ) { |
| break; |
| } |
| } |
| } |
| TestKind::Switch { |
| adt_def: _, |
| ref mut variants, |
| } => { |
| for candidate in candidates.iter() { |
| if !self.add_variants_to_switch(&match_place, candidate, variants) { |
| break; |
| } |
| } |
| } |
| _ => {} |
| } |
| |
| // Insert a Shallow borrow of any places that is switched on. |
| fake_borrows.as_mut().map(|fb| { |
| fb.insert(match_place.clone()) |
| }); |
| |
| // perform the test, branching to one of N blocks. For each of |
| // those N possible outcomes, create a (initially empty) |
| // vector of candidates. Those are the candidates that still |
| // apply if the test has that particular outcome. |
| debug!( |
| "match_candidates: test={:?} match_pair={:?}", |
| test, match_pair |
| ); |
| let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![]; |
| target_candidates.resize_with(test.targets(), Default::default); |
| |
| let total_candidate_count = candidates.len(); |
| |
| // Sort the candidates into the appropriate vector in |
| // `target_candidates`. Note that at some point we may |
| // encounter a candidate where the test is not relevant; at |
| // that point, we stop sorting. |
| while let Some(candidate) = candidates.first_mut() { |
| if let Some(idx) = self.sort_candidate(&match_place, &test, candidate) { |
| let (candidate, rest) = candidates.split_first_mut().unwrap(); |
| target_candidates[idx].push(candidate); |
| candidates = rest; |
| } else { |
| break; |
| } |
| } |
| // at least the first candidate ought to be tested |
| assert!(total_candidate_count > candidates.len()); |
| debug!("tested_candidates: {}", total_candidate_count - candidates.len()); |
| debug!("untested_candidates: {}", candidates.len()); |
| |
| // HACK(matthewjasper) This is a closure so that we can let the test |
| // create its blocks before the rest of the match. This currently |
| // improves the speed of llvm when optimizing long string literal |
| // matches |
| let make_target_blocks = move |this: &mut Self| -> Vec<BasicBlock> { |
| // For each outcome of test, process the candidates that still |
| // apply. Collect a list of blocks where control flow will |
| // branch if one of the `target_candidate` sets is not |
| // exhaustive. |
| if !candidates.is_empty() { |
| let remainder_start = &mut None; |
| this.match_candidates( |
| span, |
| remainder_start, |
| otherwise_block, |
| candidates, |
| fake_borrows, |
| ); |
| otherwise_block = Some(remainder_start.unwrap()); |
| }; |
| |
| target_candidates.into_iter().map(|mut candidates| { |
| if candidates.len() != 0 { |
| let candidate_start = &mut None; |
| this.match_candidates( |
| span, |
| candidate_start, |
| otherwise_block, |
| &mut *candidates, |
| fake_borrows, |
| ); |
| candidate_start.unwrap() |
| } else { |
| *otherwise_block.get_or_insert_with(|| { |
| let unreachable = this.cfg.start_new_block(); |
| let source_info = this.source_info(span); |
| this.cfg.terminate( |
| unreachable, |
| source_info, |
| TerminatorKind::Unreachable, |
| ); |
| unreachable |
| }) |
| } |
| }).collect() |
| }; |
| |
| self.perform_test( |
| block, |
| &match_place, |
| &test, |
| make_target_blocks, |
| ); |
| } |
| |
| // Determine the fake borrows that are needed to ensure that the place |
| // will evaluate to the same thing until an arm has been chosen. |
| fn calculate_fake_borrows<'b>( |
| &mut self, |
| fake_borrows: &'b FxHashSet<Place<'tcx>>, |
| temp_span: Span, |
| ) -> Vec<(PlaceRef<'b, 'tcx>, Local)> { |
| let tcx = self.hir.tcx(); |
| |
| debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows); |
| |
| let mut all_fake_borrows = Vec::with_capacity(fake_borrows.len()); |
| |
| // Insert a Shallow borrow of the prefixes of any fake borrows. |
| for place in fake_borrows |
| { |
| for (i, elem) in place.projection.iter().enumerate().rev() { |
| let proj_base = &place.projection[..i]; |
| |
| if let ProjectionElem::Deref = elem { |
| // Insert a shallow borrow after a deref. For other |
| // projections the borrow of prefix_cursor will |
| // conflict with any mutation of base. |
| all_fake_borrows.push(PlaceRef { |
| base: &place.base, |
| projection: proj_base, |
| }); |
| } |
| } |
| |
| all_fake_borrows.push(place.as_ref()); |
| } |
| |
| // Deduplicate and ensure a deterministic order. |
| all_fake_borrows.sort(); |
| all_fake_borrows.dedup(); |
| |
| debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows); |
| |
| all_fake_borrows.into_iter().map(|matched_place| { |
| let fake_borrow_deref_ty = Place::ty_from( |
| matched_place.base, |
| matched_place.projection, |
| &self.local_decls, |
| tcx, |
| ) |
| .ty; |
| let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty); |
| let fake_borrow_temp = self.local_decls.push( |
| LocalDecl::new_temp(fake_borrow_ty, temp_span) |
| ); |
| |
| (matched_place, fake_borrow_temp) |
| }).collect() |
| } |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // Pattern binding - used for `let` and function parameters as well. |
| |
| impl<'a, 'tcx> Builder<'a, 'tcx> { |
| /// Initializes each of the bindings from the candidate by |
| /// moving/copying/ref'ing the source as appropriate. Tests the guard, if |
| /// any, and then branches to the arm. Returns the block for the case where |
| /// the guard fails. |
| /// |
| /// Note: we do not check earlier that if there is a guard, |
| /// there cannot be move bindings. We avoid a use-after-move by only |
| /// moving the binding once the guard has evaluated to true (see below). |
| fn bind_and_guard_matched_candidate<'pat>( |
| &mut self, |
| candidate: Candidate<'pat, 'tcx>, |
| guard: Option<Guard<'tcx>>, |
| fake_borrows: &Vec<(PlaceRef<'_, 'tcx>, Local)>, |
| scrutinee_span: Span, |
| region_scope: region::Scope, |
| ) -> BasicBlock { |
| debug!("bind_and_guard_matched_candidate(candidate={:?})", candidate); |
| |
| debug_assert!(candidate.match_pairs.is_empty()); |
| |
| let candidate_source_info = self.source_info(candidate.span); |
| |
| let mut block = candidate.pre_binding_block; |
| |
| // If we are adding our own statements, then we need a fresh block. |
| let create_fresh_block = candidate.next_candidate_pre_binding_block.is_some() |
| || !candidate.bindings.is_empty() |
| || !candidate.ascriptions.is_empty() |
| || guard.is_some(); |
| |
| if create_fresh_block { |
| let fresh_block = self.cfg.start_new_block(); |
| self.false_edges( |
| block, |
| fresh_block, |
| candidate.next_candidate_pre_binding_block, |
| candidate_source_info, |
| ); |
| block = fresh_block; |
| self.ascribe_types(block, &candidate.ascriptions); |
| } else { |
| return block; |
| } |
| |
| // rust-lang/rust#27282: The `autoref` business deserves some |
| // explanation here. |
| // |
| // The intent of the `autoref` flag is that when it is true, |
| // then any pattern bindings of type T will map to a `&T` |
| // within the context of the guard expression, but will |
| // continue to map to a `T` in the context of the arm body. To |
| // avoid surfacing this distinction in the user source code |
| // (which would be a severe change to the language and require |
| // far more revision to the compiler), when `autoref` is true, |
| // then any occurrence of the identifier in the guard |
| // expression will automatically get a deref op applied to it. |
| // |
| // So an input like: |
| // |
| // ``` |
| // let place = Foo::new(); |
| // match place { foo if inspect(foo) |
| // => feed(foo), ... } |
| // ``` |
| // |
| // will be treated as if it were really something like: |
| // |
| // ``` |
| // let place = Foo::new(); |
| // match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) } |
| // => { let tmp2 = place; feed(tmp2) }, ... } |
| // |
| // And an input like: |
| // |
| // ``` |
| // let place = Foo::new(); |
| // match place { ref mut foo if inspect(foo) |
| // => feed(foo), ... } |
| // ``` |
| // |
| // will be treated as if it were really something like: |
| // |
| // ``` |
| // let place = Foo::new(); |
| // match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) } |
| // => { let tmp2 = &mut place; feed(tmp2) }, ... } |
| // ``` |
| // |
| // In short, any pattern binding will always look like *some* |
| // kind of `&T` within the guard at least in terms of how the |
| // MIR-borrowck views it, and this will ensure that guard |
| // expressions cannot mutate their the match inputs via such |
| // bindings. (It also ensures that guard expressions can at |
| // most *copy* values from such bindings; non-Copy things |
| // cannot be moved via pattern bindings in guard expressions.) |
| // |
| // ---- |
| // |
| // Implementation notes (under assumption `autoref` is true). |
| // |
| // To encode the distinction above, we must inject the |
| // temporaries `tmp1` and `tmp2`. |
| // |
| // There are two cases of interest: binding by-value, and binding by-ref. |
| // |
| // 1. Binding by-value: Things are simple. |
| // |
| // * Establishing `tmp1` creates a reference into the |
| // matched place. This code is emitted by |
| // bind_matched_candidate_for_guard. |
| // |
| // * `tmp2` is only initialized "lazily", after we have |
| // checked the guard. Thus, the code that can trigger |
| // moves out of the candidate can only fire after the |
| // guard evaluated to true. This initialization code is |
| // emitted by bind_matched_candidate_for_arm. |
| // |
| // 2. Binding by-reference: Things are tricky. |
| // |
| // * Here, the guard expression wants a `&&` or `&&mut` |
| // into the original input. This means we need to borrow |
| // the reference that we create for the arm. |
| // * So we eagerly create the reference for the arm and then take a |
| // reference to that. |
| if let Some(guard) = guard { |
| let tcx = self.hir.tcx(); |
| |
| self.bind_matched_candidate_for_guard( |
| block, |
| &candidate.bindings, |
| ); |
| let guard_frame = GuardFrame { |
| locals: candidate |
| .bindings |
| .iter() |
| .map(|b| GuardFrameLocal::new(b.var_id, b.binding_mode)) |
| .collect(), |
| }; |
| debug!("entering guard building context: {:?}", guard_frame); |
| self.guard_context.push(guard_frame); |
| |
| let re_erased = tcx.lifetimes.re_erased; |
| let scrutinee_source_info = self.source_info(scrutinee_span); |
| for (place, temp) in fake_borrows { |
| let borrow = Rvalue::Ref( |
| re_erased, |
| BorrowKind::Shallow, |
| Place { |
| base: place.base.clone(), |
| projection: place.projection.to_vec().into_boxed_slice(), |
| }, |
| ); |
| self.cfg.push_assign( |
| block, |
| scrutinee_source_info, |
| &Place::from(*temp), |
| borrow, |
| ); |
| } |
| |
| // the block to branch to if the guard fails; if there is no |
| // guard, this block is simply unreachable |
| let guard = match guard { |
| Guard::If(e) => self.hir.mirror(e), |
| }; |
| let source_info = self.source_info(guard.span); |
| let guard_end = self.source_info(tcx.sess.source_map().end_point(guard.span)); |
| let (post_guard_block, otherwise_post_guard_block) |
| = self.test_bool(block, guard, source_info); |
| let guard_frame = self.guard_context.pop().unwrap(); |
| debug!( |
| "Exiting guard building context with locals: {:?}", |
| guard_frame |
| ); |
| |
| for &(_, temp) in fake_borrows { |
| self.cfg.push(post_guard_block, Statement { |
| source_info: guard_end, |
| kind: StatementKind::FakeRead( |
| FakeReadCause::ForMatchGuard, |
| box(Place::from(temp)), |
| ), |
| }); |
| } |
| |
| self.exit_scope( |
| source_info.span, |
| region_scope, |
| otherwise_post_guard_block, |
| candidate.otherwise_block.unwrap(), |
| ); |
| |
| // We want to ensure that the matched candidates are bound |
| // after we have confirmed this candidate *and* any |
| // associated guard; Binding them on `block` is too soon, |
| // because that would be before we've checked the result |
| // from the guard. |
| // |
| // But binding them on the arm is *too late*, because |
| // then all of the candidates for a single arm would be |
| // bound in the same place, that would cause a case like: |
| // |
| // ```rust |
| // match (30, 2) { |
| // (mut x, 1) | (2, mut x) if { true } => { ... } |
| // ... // ^^^^^^^ (this is `arm_block`) |
| // } |
| // ``` |
| // |
| // would yield a `arm_block` something like: |
| // |
| // ``` |
| // StorageLive(_4); // _4 is `x` |
| // _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case |
| // _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case |
| // ``` |
| // |
| // and that is clearly not correct. |
| let by_value_bindings = candidate.bindings.iter().filter(|binding| { |
| if let BindingMode::ByValue = binding.binding_mode { true } else { false } |
| }); |
| // Read all of the by reference bindings to ensure that the |
| // place they refer to can't be modified by the guard. |
| for binding in by_value_bindings.clone() { |
| let local_id = self.var_local_id(binding.var_id, RefWithinGuard); |
| let place = Place::from(local_id); |
| self.cfg.push( |
| post_guard_block, |
| Statement { |
| source_info: guard_end, |
| kind: StatementKind::FakeRead(FakeReadCause::ForGuardBinding, box(place)), |
| }, |
| ); |
| } |
| self.bind_matched_candidate_for_arm_body( |
| post_guard_block, |
| by_value_bindings, |
| ); |
| |
| post_guard_block |
| } else { |
| assert!(candidate.otherwise_block.is_none()); |
| // (Here, it is not too early to bind the matched |
| // candidate on `block`, because there is no guard result |
| // that we have to inspect before we bind them.) |
| self.bind_matched_candidate_for_arm_body(block, &candidate.bindings); |
| block |
| } |
| } |
| |
| /// Append `AscribeUserType` statements onto the end of `block` |
| /// for each ascription |
| fn ascribe_types(&mut self, block: BasicBlock, ascriptions: &[Ascription<'tcx>]) { |
| for ascription in ascriptions { |
| let source_info = self.source_info(ascription.span); |
| |
| debug!( |
| "adding user ascription at span {:?} of place {:?} and {:?}", |
| source_info.span, |
| ascription.source, |
| ascription.user_ty, |
| ); |
| |
| let user_ty = ascription.user_ty.clone().user_ty( |
| &mut self.canonical_user_type_annotations, |
| ascription.source.ty(&self.local_decls, self.hir.tcx()).ty, |
| source_info.span |
| ); |
| self.cfg.push( |
| block, |
| Statement { |
| source_info, |
| kind: StatementKind::AscribeUserType( |
| box( |
| ascription.source.clone(), |
| user_ty, |
| ), |
| ascription.variance, |
| ), |
| }, |
| ); |
| } |
| } |
| |
| fn bind_matched_candidate_for_guard( |
| &mut self, |
| block: BasicBlock, |
| bindings: &[Binding<'tcx>], |
| ) { |
| debug!("bind_matched_candidate_for_guard(block={:?}, bindings={:?})", block, bindings); |
| |
| // Assign each of the bindings. Since we are binding for a |
| // guard expression, this will never trigger moves out of the |
| // candidate. |
| let re_erased = self.hir.tcx().lifetimes.re_erased; |
| for binding in bindings { |
| let source_info = self.source_info(binding.span); |
| |
| // For each pattern ident P of type T, `ref_for_guard` is |
| // a reference R: &T pointing to the location matched by |
| // the pattern, and every occurrence of P within a guard |
| // denotes *R. |
| let ref_for_guard = |
| self.storage_live_binding(block, binding.var_id, binding.span, RefWithinGuard); |
| match binding.binding_mode { |
| BindingMode::ByValue => { |
| let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source.clone()); |
| self.cfg |
| .push_assign(block, source_info, &ref_for_guard, rvalue); |
| } |
| BindingMode::ByRef(borrow_kind) => { |
| let value_for_arm = self.storage_live_binding( |
| block, |
| binding.var_id, |
| binding.span, |
| OutsideGuard, |
| ); |
| |
| let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source.clone()); |
| self.cfg |
| .push_assign(block, source_info, &value_for_arm, rvalue); |
| let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm); |
| self.cfg |
| .push_assign(block, source_info, &ref_for_guard, rvalue); |
| } |
| } |
| } |
| } |
| |
| fn bind_matched_candidate_for_arm_body<'b>( |
| &mut self, |
| block: BasicBlock, |
| bindings: impl IntoIterator<Item = &'b Binding<'tcx>>, |
| ) where 'tcx: 'b { |
| debug!("bind_matched_candidate_for_arm_body(block={:?})", block); |
| |
| let re_erased = self.hir.tcx().lifetimes.re_erased; |
| // Assign each of the bindings. This may trigger moves out of the candidate. |
| for binding in bindings { |
| let source_info = self.source_info(binding.span); |
| let local = |
| self.storage_live_binding(block, binding.var_id, binding.span, OutsideGuard); |
| self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard); |
| let rvalue = match binding.binding_mode { |
| BindingMode::ByValue => { |
| Rvalue::Use(self.consume_by_copy_or_move(binding.source.clone())) |
| } |
| BindingMode::ByRef(borrow_kind) => { |
| Rvalue::Ref(re_erased, borrow_kind, binding.source.clone()) |
| } |
| }; |
| self.cfg.push_assign(block, source_info, &local, rvalue); |
| } |
| } |
| |
| /// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound |
| /// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The |
| /// first local is a binding for occurrences of `var` in the guard, which |
| /// will have type `&T`. The second local is a binding for occurrences of |
| /// `var` in the arm body, which will have type `T`. |
| fn declare_binding( |
| &mut self, |
| source_info: SourceInfo, |
| visibility_scope: SourceScope, |
| mutability: Mutability, |
| name: Name, |
| mode: BindingMode, |
| var_id: HirId, |
| var_ty: Ty<'tcx>, |
| user_ty: UserTypeProjections, |
| has_guard: ArmHasGuard, |
| opt_match_place: Option<(Option<Place<'tcx>>, Span)>, |
| pat_span: Span, |
| ) { |
| debug!( |
| "declare_binding(var_id={:?}, name={:?}, mode={:?}, var_ty={:?}, \ |
| visibility_scope={:?}, source_info={:?})", |
| var_id, name, mode, var_ty, visibility_scope, source_info |
| ); |
| |
| let tcx = self.hir.tcx(); |
| let binding_mode = match mode { |
| BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()), |
| BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()), |
| }; |
| debug!("declare_binding: user_ty={:?}", user_ty); |
| let local = LocalDecl::<'tcx> { |
| mutability, |
| ty: var_ty, |
| user_ty, |
| name: Some(name), |
| source_info, |
| visibility_scope, |
| internal: false, |
| is_block_tail: None, |
| is_user_variable: Some(ClearCrossCrate::Set(BindingForm::Var(VarBindingForm { |
| binding_mode, |
| // hypothetically, `visit_bindings` could try to unzip |
| // an outermost hir::Ty as we descend, matching up |
| // idents in pat; but complex w/ unclear UI payoff. |
| // Instead, just abandon providing diagnostic info. |
| opt_ty_info: None, |
| opt_match_place, |
| pat_span, |
| }))), |
| }; |
| let for_arm_body = self.local_decls.push(local); |
| let locals = if has_guard.0 { |
| let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> { |
| // This variable isn't mutated but has a name, so has to be |
| // immutable to avoid the unused mut lint. |
| mutability: Mutability::Not, |
| ty: tcx.mk_imm_ref(tcx.lifetimes.re_erased, var_ty), |
| user_ty: UserTypeProjections::none(), |
| name: Some(name), |
| source_info, |
| visibility_scope, |
| internal: false, |
| is_block_tail: None, |
| is_user_variable: Some(ClearCrossCrate::Set(BindingForm::RefForGuard)), |
| }); |
| LocalsForNode::ForGuard { |
| ref_for_guard, |
| for_arm_body, |
| } |
| } else { |
| LocalsForNode::One(for_arm_body) |
| }; |
| debug!("declare_binding: vars={:?}", locals); |
| self.var_indices.insert(var_id, locals); |
| } |
| } |
