//! 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::expr::as_place::PlaceBuilder; 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 rustc_data_structures::{ fx::{FxHashSet, FxIndexMap, FxIndexSet}, stack::ensure_sufficient_stack, }; use rustc_index::bit_set::BitSet; use rustc_middle::middle::region; use rustc_middle::mir::*; use rustc_middle::thir::{self, *}; use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty}; use rustc_span::symbol::Symbol; use rustc_span::{BytePos, Pos, Span}; use rustc_target::abi::VariantIdx; use smallvec::{smallvec, SmallVec}; // helper functions, broken out by category: mod simplify; mod test; mod util; use std::borrow::Borrow; use std::mem; impl<'a, 'tcx> Builder<'a, 'tcx> { pub(crate) fn then_else_break( &mut self, mut block: BasicBlock, expr: &Expr<'tcx>, temp_scope_override: Option, break_scope: region::Scope, variable_source_info: SourceInfo, ) -> BlockAnd<()> { let this = self; let expr_span = expr.span; match expr.kind { ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => { let lhs_then_block = unpack!(this.then_else_break( block, &this.thir[lhs], temp_scope_override, break_scope, variable_source_info, )); let rhs_then_block = unpack!(this.then_else_break( lhs_then_block, &this.thir[rhs], temp_scope_override, break_scope, variable_source_info, )); rhs_then_block.unit() } ExprKind::Scope { region_scope, lint_level, value } => { let region_scope = (region_scope, this.source_info(expr_span)); this.in_scope(region_scope, lint_level, |this| { this.then_else_break( block, &this.thir[value], temp_scope_override, break_scope, variable_source_info, ) }) } ExprKind::Let { expr, ref pat } => this.lower_let_expr( block, &this.thir[expr], pat, break_scope, Some(variable_source_info.scope), variable_source_info.span, true, ), _ => { let temp_scope = temp_scope_override.unwrap_or_else(|| this.local_scope()); let mutability = Mutability::Mut; let place = unpack!(block = this.as_temp(block, Some(temp_scope), expr, mutability)); let operand = Operand::Move(Place::from(place)); let then_block = this.cfg.start_new_block(); let else_block = this.cfg.start_new_block(); let term = TerminatorKind::if_(operand, then_block, else_block); let source_info = this.source_info(expr_span); this.cfg.terminate(block, source_info, term); this.break_for_else(else_block, break_scope, source_info); then_block.unit() } } } /// 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 ([Builder::lower_scrutinee]). /// 2. Create the decision tree ([Builder::lower_match_tree]). /// 3. Determine the fake borrows that are needed from the places that were /// matched against and create the required temporaries for them /// ([Builder::calculate_fake_borrows]). /// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]). /// /// ## 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 pre-binding block to the next pre-binding block. /// * From each otherwise block to the next pre-binding block. #[instrument(level = "debug", skip(self, arms))] pub(crate) fn match_expr( &mut self, destination: Place<'tcx>, span: Span, mut block: BasicBlock, scrutinee: &Expr<'tcx>, arms: &[ArmId], ) -> BlockAnd<()> { let scrutinee_span = scrutinee.span; let scrutinee_place = unpack!(block = self.lower_scrutinee(block, scrutinee, scrutinee_span,)); let mut arm_candidates = self.create_match_candidates(&scrutinee_place, &arms); let match_has_guard = arm_candidates.iter().any(|(_, candidate)| candidate.has_guard); let mut candidates = arm_candidates.iter_mut().map(|(_, candidate)| candidate).collect::>(); let match_start_span = span.shrink_to_lo().to(scrutinee.span); let fake_borrow_temps = self.lower_match_tree( block, scrutinee_span, match_start_span, match_has_guard, &mut candidates, ); self.lower_match_arms( destination, scrutinee_place, scrutinee_span, arm_candidates, self.source_info(span), fake_borrow_temps, ) } /// Evaluate the scrutinee and add the fake read of it. fn lower_scrutinee( &mut self, mut block: BasicBlock, scrutinee: &Expr<'tcx>, scrutinee_span: Span, ) -> BlockAnd> { let scrutinee_place_builder = unpack!(block = self.as_place_builder(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 cause_matched_place = FakeReadCause::ForMatchedPlace(None); let source_info = self.source_info(scrutinee_span); if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) { self.cfg.push_fake_read(block, source_info, cause_matched_place, scrutinee_place); } block.and(scrutinee_place_builder) } /// Create the initial `Candidate`s for a `match` expression. fn create_match_candidates<'pat>( &mut self, scrutinee: &PlaceBuilder<'tcx>, arms: &'pat [ArmId], ) -> Vec<(&'pat Arm<'tcx>, Candidate<'pat, 'tcx>)> where 'a: 'pat, { // Assemble a list of candidates: there is one candidate per pattern, // which means there may be more than one candidate *per arm*. arms.iter() .copied() .map(|arm| { let arm = &self.thir[arm]; let arm_has_guard = arm.guard.is_some(); let arm_candidate = Candidate::new(scrutinee.clone(), &arm.pattern, arm_has_guard, self); (arm, arm_candidate) }) .collect() } /// Create the decision tree for the match expression, starting from `block`. /// /// Modifies `candidates` to store the bindings and type ascriptions for /// that candidate. /// /// Returns the places that need fake borrows because we bind or test them. fn lower_match_tree<'pat>( &mut self, block: BasicBlock, scrutinee_span: Span, match_start_span: Span, match_has_guard: bool, candidates: &mut [&mut Candidate<'pat, 'tcx>], ) -> Vec<(Place<'tcx>, Local)> { // 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 = match_has_guard.then(FxIndexSet::default); let mut otherwise = None; // This will generate code to test scrutinee_place and // branch to the appropriate arm block self.match_candidates( match_start_span, scrutinee_span, block, &mut otherwise, candidates, &mut fake_borrows, ); if let Some(otherwise_block) = otherwise { // See the doc comment on `match_candidates` for why we may have an // otherwise block. Match checking will ensure this is actually // unreachable. let source_info = self.source_info(scrutinee_span); self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable); } // Link each leaf candidate to the `pre_binding_block` of the next one. let mut previous_candidate: Option<&mut Candidate<'_, '_>> = None; for candidate in candidates { candidate.visit_leaves(|leaf_candidate| { if let Some(ref mut prev) = previous_candidate { prev.next_candidate_pre_binding_block = leaf_candidate.pre_binding_block; } previous_candidate = Some(leaf_candidate); }); } if let Some(ref borrows) = fake_borrows { self.calculate_fake_borrows(borrows, scrutinee_span) } else { Vec::new() } } /// Lower the bindings, guards and arm bodies of a `match` expression. /// /// The decision tree should have already been created /// (by [Builder::lower_match_tree]). /// /// `outer_source_info` is the SourceInfo for the whole match. fn lower_match_arms( &mut self, destination: Place<'tcx>, scrutinee_place_builder: PlaceBuilder<'tcx>, scrutinee_span: Span, arm_candidates: Vec<(&'_ Arm<'tcx>, Candidate<'_, 'tcx>)>, outer_source_info: SourceInfo, fake_borrow_temps: Vec<(Place<'tcx>, Local)>, ) -> BlockAnd<()> { let arm_end_blocks: Vec<_> = arm_candidates .into_iter() .map(|(arm, candidate)| { debug!("lowering arm {:?}\ncandidate = {:?}", arm, candidate); let arm_source_info = self.source_info(arm.span); let arm_scope = (arm.scope, arm_source_info); let match_scope = self.local_scope(); self.in_scope(arm_scope, arm.lint_level, |this| { // `try_to_place` may fail if it is unable to resolve the given // `PlaceBuilder` inside a closure. In this case, we don't want to include // a scrutinee place. `scrutinee_place_builder` will fail to be resolved // if the only match arm is a wildcard (`_`). // Example: // ``` // let foo = (0, 1); // let c = || { // match foo { _ => () }; // }; // ``` let scrutinee_place = scrutinee_place_builder.try_to_place(this); let opt_scrutinee_place = scrutinee_place.as_ref().map(|place| (Some(place), scrutinee_span)); let scope = this.declare_bindings( None, arm.span, &arm.pattern, arm.guard.as_ref(), opt_scrutinee_place, ); let arm_block = this.bind_pattern( outer_source_info, candidate, &fake_borrow_temps, scrutinee_span, Some((arm, match_scope)), false, ); if let Some(source_scope) = scope { this.source_scope = source_scope; } this.expr_into_dest(destination, arm_block, &&this.thir[arm.body]) }) }) .collect(); // all the arm blocks will rejoin here let end_block = self.cfg.start_new_block(); let end_brace = self.source_info( outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)), ); for arm_block in arm_end_blocks { let block = &self.cfg.basic_blocks[arm_block.0]; let last_location = block.statements.last().map(|s| s.source_info); self.cfg.goto(unpack!(arm_block), last_location.unwrap_or(end_brace), end_block); } self.source_scope = outer_source_info.scope; end_block.unit() } /// Binds the variables and ascribes types for a given `match` arm or /// `let` binding. /// /// Also check if the guard matches, if it's provided. /// `arm_scope` should be `Some` if and only if this is called for a /// `match` arm. fn bind_pattern( &mut self, outer_source_info: SourceInfo, candidate: Candidate<'_, 'tcx>, fake_borrow_temps: &[(Place<'tcx>, Local)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>, storages_alive: bool, ) -> BasicBlock { if candidate.subcandidates.is_empty() { // Avoid generating another `BasicBlock` when we only have one // candidate. self.bind_and_guard_matched_candidate( candidate, &[], fake_borrow_temps, scrutinee_span, arm_match_scope, true, storages_alive, ) } else { // It's helpful to avoid scheduling drops multiple times to save // drop elaboration from having to clean up the extra drops. // // If we are in a `let` then we only schedule drops for the first // candidate. // // If we're in a `match` arm then we could have a case like so: // // Ok(x) | Err(x) if return => { /* ... */ } // // In this case we don't want a drop of `x` scheduled when we // return: it isn't bound by move until right before enter the arm. // To handle this we instead unschedule it's drop after each time // we lower the guard. let target_block = self.cfg.start_new_block(); let mut schedule_drops = true; let arm = arm_match_scope.unzip().0; // We keep a stack of all of the bindings and type ascriptions // from the parent candidates that we visit, that also need to // be bound for each candidate. traverse_candidate( candidate, &mut Vec::new(), &mut |leaf_candidate, parent_bindings| { if let Some(arm) = arm { self.clear_top_scope(arm.scope); } let binding_end = self.bind_and_guard_matched_candidate( leaf_candidate, parent_bindings, &fake_borrow_temps, scrutinee_span, arm_match_scope, schedule_drops, storages_alive, ); if arm.is_none() { schedule_drops = false; } self.cfg.goto(binding_end, outer_source_info, target_block); }, |inner_candidate, parent_bindings| { parent_bindings.push((inner_candidate.bindings, inner_candidate.ascriptions)); inner_candidate.subcandidates.into_iter() }, |parent_bindings| { parent_bindings.pop(); }, ); target_block } } pub(super) fn expr_into_pattern( &mut self, mut block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer: &Expr<'tcx>, ) -> BlockAnd<()> { match irrefutable_pat.kind { // Optimize the case of `let x = ...` to write directly into `x` PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. } => { let place = self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true); unpack!(block = self.expr_into_dest(place, block, initializer)); // Inject a fake read, see comments on `FakeReadCause::ForLet`. let source_info = self.source_info(irrefutable_pat.span); self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), 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. PatKind::AscribeUserType { subpattern: box Pat { kind: PatKind::Binding { mode: BindingMode::ByValue, var, subpattern: None, .. }, .. }, ascription: thir::Ascription { ref annotation, variance: _ }, } => { let place = self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard, true); unpack!(block = self.expr_into_dest(place, block, initializer)); // Inject a fake read, see comments on `FakeReadCause::ForLet`. let pattern_source_info = self.source_info(irrefutable_pat.span); let cause_let = FakeReadCause::ForLet(None); self.cfg.push_fake_read(block, pattern_source_info, cause_let, place); let ty_source_info = self.source_info(annotation.span); let base = self.canonical_user_type_annotations.push(annotation.clone()); self.cfg.push( block, Statement { source_info: ty_source_info, kind: StatementKind::AscribeUserType( Box::new((place, UserTypeProjection { base, projs: Vec::new() })), // 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 = // ``` // // 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 // ``. ty::Variance::Invariant, ), }, ); self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard); block.unit() } _ => { let place_builder = unpack!(block = self.as_place_builder(block, initializer)); self.place_into_pattern(block, &irrefutable_pat, place_builder, true) } } } pub(crate) fn place_into_pattern( &mut self, block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer: PlaceBuilder<'tcx>, set_match_place: bool, ) -> BlockAnd<()> { let mut candidate = Candidate::new(initializer.clone(), &irrefutable_pat, false, self); let fake_borrow_temps = self.lower_match_tree( block, irrefutable_pat.span, irrefutable_pat.span, false, &mut [&mut candidate], ); // 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 { let mut candidate_ref = &candidate; while let Some(next) = { for binding in &candidate_ref.bindings { let local = self.var_local_id(binding.var_id, OutsideGuard); // `try_to_place` may fail if it is unable to resolve the given // `PlaceBuilder` inside a closure. In this case, we don't want to include // a scrutinee place. `scrutinee_place_builder` will fail for destructured // assignments. This is because a closure only captures the precise places // that it will read and as a result a closure may not capture the entire // tuple/struct and rather have individual places that will be read in the // final MIR. // Example: // ``` // let foo = (0, 1); // let c = || { // let (v1, v2) = foo; // }; // ``` if let Some(place) = initializer.try_to_place(self) { let Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var( VarBindingForm { opt_match_place: Some((ref mut match_place, _)), .. }, )))) = self.local_decls[local].local_info else { bug!("Let binding to non-user variable.") }; *match_place = Some(place); } } // All of the subcandidates should bind the same locals, so we // only visit the first one. candidate_ref.subcandidates.get(0) } { candidate_ref = next; } } self.bind_pattern( self.source_info(irrefutable_pat.span), candidate, &fake_borrow_temps, irrefutable_pat.span, None, false, ) .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. #[instrument(skip(self), level = "debug")] pub(crate) fn declare_bindings( &mut self, mut visibility_scope: Option, scope_span: Span, pattern: &Pat<'tcx>, guard: Option<&Guard<'tcx>>, opt_match_place: Option<(Option<&Place<'tcx>>, Span)>, ) -> Option { self.visit_primary_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, ArmHasGuard(guard.is_some()), opt_match_place.map(|(x, y)| (x.cloned(), y)), pattern.span, ); }, ); if let Some(Guard::IfLet(guard_pat, _)) = guard { // FIXME: pass a proper `opt_match_place` self.declare_bindings(visibility_scope, scope_span, guard_pat, None, None); } visibility_scope } pub(crate) fn storage_live_binding( &mut self, block: BasicBlock, var: LocalVarId, span: Span, for_guard: ForGuard, schedule_drop: bool, ) -> 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) }); // Although there is almost always scope for given variable in corner cases // like #92893 we might get variable with no scope. if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) && schedule_drop { self.schedule_drop(span, region_scope, local_id, DropKind::Storage); } Place::from(local_id) } pub(crate) fn schedule_drop_for_binding( &mut self, var: LocalVarId, span: Span, for_guard: ForGuard, ) { let local_id = self.var_local_id(var, for_guard); if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) { self.schedule_drop(span, region_scope, local_id, DropKind::Value); } } /// Visit all of the primary bindings in a patterns, that is, visit the /// leftmost occurrence of each variable bound in a pattern. A variable /// will occur more than once in an or-pattern. pub(super) fn visit_primary_bindings( &mut self, pattern: &Pat<'tcx>, pattern_user_ty: UserTypeProjections, f: &mut impl FnMut( &mut Self, Mutability, Symbol, BindingMode, LocalVarId, Span, Ty<'tcx>, UserTypeProjections, ), ) { debug!( "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}", pattern, pattern_user_ty ); match pattern.kind { PatKind::Binding { mutability, name, mode, var, ty, ref subpattern, is_primary, .. } => { if is_primary { f(self, mutability, name, mode, var, pattern.span, ty, pattern_user_ty.clone()); } if let Some(subpattern) = subpattern.as_ref() { self.visit_primary_bindings(subpattern, pattern_user_ty, f); } } PatKind::Array { ref prefix, ref slice, ref suffix } | PatKind::Slice { ref prefix, ref slice, ref suffix } => { let from = u64::try_from(prefix.len()).unwrap(); let to = u64::try_from(suffix.len()).unwrap(); for subpattern in prefix.iter() { self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f); } for subpattern in slice { self.visit_primary_bindings( subpattern, pattern_user_ty.clone().subslice(from, to), f, ); } for subpattern in suffix.iter() { self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f); } } PatKind::Constant { .. } | PatKind::Range { .. } | PatKind::Wild => {} PatKind::Deref { ref subpattern } => { self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f); } PatKind::AscribeUserType { ref subpattern, ascription: thir::Ascription { ref annotation, 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 projection = UserTypeProjection { base: self.canonical_user_type_annotations.push(annotation.clone()), projs: Vec::new(), }; let subpattern_user_ty = pattern_user_ty.push_projection(&projection, annotation.span); self.visit_primary_bindings(subpattern, subpattern_user_ty, f) } PatKind::Leaf { ref subpatterns } => { for subpattern in subpatterns { let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field); debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty); self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f); } } PatKind::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_primary_bindings(&subpattern.pattern, subpattern_user_ty, f); } } PatKind::Or { ref pats } => { // In cases where we recover from errors the primary bindings // may not all be in the leftmost subpattern. For example in // `let (x | y) = ...`, the primary binding of `y` occurs in // the right subpattern for subpattern in pats.iter() { self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f); } } } } } #[derive(Debug)] struct Candidate<'pat, 'tcx> { /// [`Span`] of the original pattern that gave rise to this candidate. span: Span, /// Whether this `Candidate` has a guard. has_guard: bool, /// All of these must be satisfied... match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>, /// ...these bindings established... bindings: Vec>, /// ...and these types asserted... ascriptions: Vec>, /// ...and if this is non-empty, one of these subcandidates also has to match... subcandidates: Vec>, /// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`. otherwise_block: Option, /// The block before the `bindings` have been established. pre_binding_block: Option, /// The pre-binding block of the next candidate. next_candidate_pre_binding_block: Option, } impl<'tcx, 'pat> Candidate<'pat, 'tcx> { fn new( place: PlaceBuilder<'tcx>, pattern: &'pat Pat<'tcx>, has_guard: bool, cx: &Builder<'_, 'tcx>, ) -> Self { Candidate { span: pattern.span, has_guard, match_pairs: smallvec![MatchPair::new(place, pattern, cx)], bindings: Vec::new(), ascriptions: Vec::new(), subcandidates: Vec::new(), otherwise_block: None, pre_binding_block: None, next_candidate_pre_binding_block: None, } } /// Visit the leaf candidates (those with no subcandidates) contained in /// this candidate. fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) { traverse_candidate( self, &mut (), &mut move |c, _| visit_leaf(c), move |c, _| c.subcandidates.iter_mut(), |_| {}, ); } } /// A depth-first traversal of the `Candidate` and all of its recursive /// subcandidates. fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>( candidate: C, context: &mut T, visit_leaf: &mut impl FnMut(C, &mut T), get_children: impl Copy + Fn(C, &mut T) -> I, complete_children: impl Copy + Fn(&mut T), ) where C: Borrow>, I: Iterator, { if candidate.borrow().subcandidates.is_empty() { visit_leaf(candidate, context) } else { for child in get_children(candidate, context) { traverse_candidate(child, context, visit_leaf, get_children, complete_children); } complete_children(context) } } #[derive(Clone, Debug)] struct Binding<'tcx> { span: Span, source: Place<'tcx>, var_id: LocalVarId, 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> { source: Place<'tcx>, annotation: CanonicalUserTypeAnnotation<'tcx>, variance: ty::Variance, } #[derive(Clone, Debug)] pub(crate) struct MatchPair<'pat, 'tcx> { // this place... place: PlaceBuilder<'tcx>, // ... must match this pattern. pattern: &'pat Pat<'tcx>, } /// See [`Test`] for more. #[derive(Clone, Debug, PartialEq)] enum TestKind<'tcx> { /// Test what enum variant a value is. Switch { /// The enum type being tested. adt_def: ty::AdtDef<'tcx>, /// The set of variants that we should create a branch for. We also /// create an additional "otherwise" case. variants: BitSet, }, /// 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: FxIndexMap, u128>, }, /// Test for equality with value, possibly after an unsizing coercion to /// `ty`, Eq { value: ConstantKind<'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(Box>), /// Test that the length of the slice is equal to `len`. Len { len: u64, op: BinOp }, } /// A test to perform to determine which [`Candidate`] matches a value. /// /// [`Test`] is just the test to perform; it does not include the value /// to be tested. #[derive(Debug)] pub(crate) struct Test<'tcx> { span: Span, kind: TestKind<'tcx>, } /// `ArmHasGuard` is a wrapper around 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(crate) 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 set and generate a branch to the appropriate /// pre-binding block. /// /// If we find that *NONE* of the candidates apply, we branch to the /// `otherwise_block`, setting it to `Some` if required. 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 non-exhaustive. /// 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 [`Builder::test_candidates`] for more details. /// /// If `fake_borrows` is `Some`, then places which need fake borrows /// will be added to it. /// /// For an example of a case where we set `otherwise_block`, even for an /// exhaustive match, consider: /// /// ``` /// # fn foo(x: (bool, bool)) { /// match x { /// (true, true) => (), /// (_, false) => (), /// (false, true) => (), /// } /// # } /// ``` /// /// For this match, we check if `x.0` matches `true` (for the first /// arm). If it doesn't match, we check `x.1`. If `x.1` is `true` we check /// if `x.0` matches `false` (for the third arm). In the (impossible at /// runtime) case when `x.0` is now `true`, we branch to /// `otherwise_block`. #[instrument(skip(self, fake_borrows), level = "debug")] fn match_candidates<'pat>( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, otherwise_block: &mut Option, candidates: &mut [&mut Candidate<'pat, 'tcx>], fake_borrows: &mut Option>>, ) { // 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. let mut split_or_candidate = false; for candidate in &mut *candidates { split_or_candidate |= self.simplify_candidate(candidate); } ensure_sufficient_stack(|| { if split_or_candidate { // At least one of the candidates has been split into subcandidates. // We need to change the candidate list to include those. let mut new_candidates = Vec::new(); for candidate in candidates { candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate)); } self.match_simplified_candidates( span, scrutinee_span, start_block, otherwise_block, &mut *new_candidates, fake_borrows, ); } else { self.match_simplified_candidates( span, scrutinee_span, start_block, otherwise_block, candidates, fake_borrows, ); } }); } fn match_simplified_candidates( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, otherwise_block: &mut Option, candidates: &mut [&mut Candidate<'_, 'tcx>], fake_borrows: &mut Option>>, ) { // 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 = 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 { last_otherwise_block } else { // Any remaining candidates are unreachable. if unmatched_candidates.is_empty() { return; } self.cfg.start_new_block() } } else { start_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.goto(block, source_info, otherwise); } else { *otherwise_block = Some(block); } return; } // Test for the remaining candidates. self.test_candidates_with_or( span, scrutinee_span, unmatched_candidates, block, otherwise_block, fake_borrows, ); } /// Link up matched candidates. /// /// For example, if we have something like this: /// /// ```ignore (illustrative) /// ... /// Some(x) if cond1 => ... /// Some(x) => ... /// Some(x) if cond2 => ... /// ... /// ``` /// /// We generate real edges from: /// /// * `start_block` to the [pre-binding block] of the first pattern, /// * the [otherwise block] of the first pattern to the second pattern, /// * the [otherwise block] of the third pattern to a block with an /// [`Unreachable` terminator](TerminatorKind::Unreachable). /// /// In addition, we add fake edges from the otherwise blocks to the /// pre-binding block of the next candidate in the original set of /// candidates. /// /// [pre-binding block]: Candidate::pre_binding_block /// [otherwise block]: Candidate::otherwise_block fn select_matched_candidates( &mut self, matched_candidates: &mut [&mut Candidate<'_, 'tcx>], start_block: BasicBlock, fake_borrows: &mut Option>>, ) -> Option { debug_assert!( !matched_candidates.is_empty(), "select_matched_candidates called with no candidates", ); debug_assert!( matched_candidates.iter().all(|c| c.subcandidates.is_empty()), "subcandidates should be empty in select_matched_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 { local: source.local, projection: self.tcx.mk_place_elems(proj_base), }); } } } let fully_matched_with_guard = matched_candidates .iter() .position(|c| !c.has_guard) .unwrap_or(matched_candidates.len() - 1); let (reachable_candidates, unreachable_candidates) = matched_candidates.split_at_mut(fully_matched_with_guard + 1); let mut next_prebinding = start_block; for candidate in reachable_candidates.iter_mut() { assert!(candidate.otherwise_block.is_none()); assert!(candidate.pre_binding_block.is_none()); candidate.pre_binding_block = Some(next_prebinding); if candidate.has_guard { // Create the otherwise block for this candidate, which is the // pre-binding block for the next candidate. next_prebinding = self.cfg.start_new_block(); candidate.otherwise_block = Some(next_prebinding); } } debug!( "match_candidates: add pre_binding_blocks for unreachable {:?}", unreachable_candidates, ); for candidate in unreachable_candidates { assert!(candidate.pre_binding_block.is_none()); candidate.pre_binding_block = Some(self.cfg.start_new_block()); } reachable_candidates.last_mut().unwrap().otherwise_block } /// Tests a candidate where there are only or-patterns left to test, or /// forwards to [Builder::test_candidates]. /// /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like /// so: /// /// ```text /// [ start ] /// | /// [ match P, Q ] /// | /// +----------------------------------------+------------------------------------+ /// | | | /// V V V /// [ P matches ] [ Q matches ] [ otherwise ] /// | | | /// V V | /// [ match R, S ] [ match R, S ] | /// | | | /// +--------------+------------+ +--------------+------------+ | /// | | | | | | | /// V V V V V V | /// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] | /// | | | | | | | /// +--------------+------------|------------+--------------+ | | /// | | | | /// | +----------------------------------------+--------+ /// | | /// V V /// [ Success ] [ Failure ] /// ``` /// /// In practice there are some complications: /// /// * If there's a guard, then the otherwise branch of the first match on /// `R | S` goes to a test for whether `Q` matches, and the control flow /// doesn't merge into a single success block until after the guard is /// tested. /// * If neither `P` or `Q` has any bindings or type ascriptions and there /// isn't a match guard, then we create a smaller CFG like: /// /// ```text /// ... /// +---------------+------------+ /// | | | /// [ P matches ] [ Q matches ] [ otherwise ] /// | | | /// +---------------+ | /// | ... /// [ match R, S ] /// | /// ... /// ``` fn test_candidates_with_or( &mut self, span: Span, scrutinee_span: Span, candidates: &mut [&mut Candidate<'_, 'tcx>], block: BasicBlock, otherwise_block: &mut Option, fake_borrows: &mut Option>>, ) { let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap(); // All of the or-patterns have been sorted to the end, so if the first // pattern is an or-pattern we only have or-patterns. match first_candidate.match_pairs[0].pattern.kind { PatKind::Or { .. } => (), _ => { self.test_candidates( span, scrutinee_span, candidates, block, otherwise_block, fake_borrows, ); return; } } let match_pairs = mem::take(&mut first_candidate.match_pairs); first_candidate.pre_binding_block = Some(block); let mut otherwise = None; for match_pair in match_pairs { let PatKind::Or { ref pats } = &match_pair.pattern.kind else { bug!("Or-patterns should have been sorted to the end"); }; let or_span = match_pair.pattern.span; first_candidate.visit_leaves(|leaf_candidate| { self.test_or_pattern( leaf_candidate, &mut otherwise, pats, or_span, &match_pair.place, fake_borrows, ); }); } let remainder_start = otherwise.unwrap_or_else(|| self.cfg.start_new_block()); self.match_candidates( span, scrutinee_span, remainder_start, otherwise_block, remaining_candidates, fake_borrows, ) } #[instrument( skip(self, otherwise, or_span, place, fake_borrows, candidate, pats), level = "debug" )] fn test_or_pattern<'pat>( &mut self, candidate: &mut Candidate<'pat, 'tcx>, otherwise: &mut Option, pats: &'pat [Box>], or_span: Span, place: &PlaceBuilder<'tcx>, fake_borrows: &mut Option>>, ) { debug!("candidate={:#?}\npats={:#?}", candidate, pats); let mut or_candidates: Vec<_> = pats .iter() .map(|pat| Candidate::new(place.clone(), pat, candidate.has_guard, self)) .collect(); let mut or_candidate_refs: Vec<_> = or_candidates.iter_mut().collect(); let otherwise = if candidate.otherwise_block.is_some() { &mut candidate.otherwise_block } else { otherwise }; self.match_candidates( or_span, or_span, candidate.pre_binding_block.unwrap(), otherwise, &mut or_candidate_refs, fake_borrows, ); candidate.subcandidates = or_candidates; self.merge_trivial_subcandidates(candidate, self.source_info(or_span)); } /// Try to merge all of the subcandidates of the given candidate into one. /// This avoids exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. fn merge_trivial_subcandidates( &mut self, candidate: &mut Candidate<'_, 'tcx>, source_info: SourceInfo, ) { if candidate.subcandidates.is_empty() || candidate.has_guard { // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard. return; } let mut can_merge = true; // Not `Iterator::all` because we don't want to short-circuit. for subcandidate in &mut candidate.subcandidates { self.merge_trivial_subcandidates(subcandidate, source_info); // FIXME(or_patterns; matthewjasper) Try to be more aggressive here. can_merge &= subcandidate.subcandidates.is_empty() && subcandidate.bindings.is_empty() && subcandidate.ascriptions.is_empty(); } if can_merge { let any_matches = self.cfg.start_new_block(); for subcandidate in mem::take(&mut candidate.subcandidates) { let or_block = subcandidate.pre_binding_block.unwrap(); self.cfg.goto(or_block, source_info, any_matches); } candidate.pre_binding_block = Some(any_matches); } } /// 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 perform, we take the highest /// priority candidate (the first one in the list, as of January 2021) /// and extract the first match-pair from the list. From this we decide /// what kind of test is needed using [`Builder::test`], defined in the /// [`test` module](mod@test). /// /// *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`] to decide what value `x` has /// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in /// 4. etc. /// /// [`Switch`]: TestKind::Switch /// [`SwitchInt`]: TestKind::SwitchInt /// [`Range`]: TestKind::Range /// /// Once we know what sort of test we are going to perform, this /// test may also help us winnow down our 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 `x.0`'s variant, /// 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: /// /// ```ignore (illustrative) /// 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]: /// /// ```ignore (illustrative) /// match x { /// "foo" if foo_guard => ..., /// "bar" if bar_guard => ..., /// "baz" if baz_guard => ..., /// ... /// } /// ``` /// /// [#29740]: https://github.com/rust-lang/rust/issues/29740 /// /// Here we first test the match-pair `x @ "foo"`, which is an [`Eq` test]. /// /// [`Eq` test]: TestKind::Eq /// /// It might seem that we would end up with 2 disjoint candidate /// sets, consisting of the first candidate or the other two, 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 number /// 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 non-exhaustive - as we discarded our otherwise set - so /// we set their continuation to do `match_candidates` on the /// "unmatched" set (which is again non-exhaustive). /// /// 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 the nice property that each guard and arm is only generated /// once. fn test_candidates<'pat, 'b, 'c>( &mut self, span: Span, scrutinee_span: Span, mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], block: BasicBlock, otherwise_block: &mut Option, fake_borrows: &mut Option>>, ) { // 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 } => { for candidate in candidates.iter() { if !self.add_cases_to_switch(&match_place, candidate, switch_ty, options) { 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. if let Some(fb) = fake_borrows && let Some(resolved_place) = match_place.try_to_place(self) { fb.insert(resolved_place); } // 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!("test_candidates: test={:?} match_pair={:?}", test, match_pair); let mut target_candidates: Vec>> = 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() { let Some(idx) = self.sort_candidate(&match_place, &test, candidate) else { break; }; let (candidate, rest) = candidates.split_first_mut().unwrap(); target_candidates[idx].push(candidate); candidates = rest; } // at least the first candidate ought to be tested assert!( total_candidate_count > candidates.len(), "{}, {:#?}", total_candidate_count, candidates ); 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 { // The block that we should branch to if none of the // `target_candidates` match. This is either the block where we // start matching the untested candidates if there are any, // otherwise it's the `otherwise_block`. let remainder_start = &mut None; let remainder_start = if candidates.is_empty() { &mut *otherwise_block } else { remainder_start }; // 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. let target_blocks: Vec<_> = target_candidates .into_iter() .map(|mut candidates| { if !candidates.is_empty() { let candidate_start = this.cfg.start_new_block(); this.match_candidates( span, scrutinee_span, candidate_start, remainder_start, &mut *candidates, fake_borrows, ); candidate_start } else { *remainder_start.get_or_insert_with(|| this.cfg.start_new_block()) } }) .collect(); if !candidates.is_empty() { let remainder_start = remainder_start.unwrap_or_else(|| this.cfg.start_new_block()); this.match_candidates( span, scrutinee_span, remainder_start, otherwise_block, candidates, fake_borrows, ); }; target_blocks }; self.perform_test(span, scrutinee_span, block, &match_place, &test, make_target_blocks); } /// Determine the fake borrows that are needed from a set of places that /// have to be stable across match guards. /// /// Returns a list of places that need a fake borrow and the temporary /// that's used to store the fake borrow. /// /// Match exhaustiveness checking is not able to handle the case where the /// place being matched on is mutated in the guards. 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 /// reference, 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. fn calculate_fake_borrows<'b>( &mut self, fake_borrows: &'b FxIndexSet>, temp_span: Span, ) -> Vec<(Place<'tcx>, Local)> { let tcx = self.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 { let mut cursor = place.projection.as_ref(); while let [proj_base @ .., elem] = cursor { cursor = proj_base; 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 { local: place.local, projection: proj_base }); } } all_fake_borrows.push(place.as_ref()); } // Deduplicate let mut dedup = FxHashSet::default(); all_fake_borrows.retain(|b| dedup.insert(*b)); debug!("add_fake_borrows all_fake_borrows = {:?}", all_fake_borrows); all_fake_borrows .into_iter() .map(|matched_place_ref| { let matched_place = Place { local: matched_place_ref.local, projection: tcx.mk_place_elems(matched_place_ref.projection), }; let fake_borrow_deref_ty = matched_place.ty(&self.local_decls, tcx).ty; let fake_borrow_ty = tcx.mk_imm_ref(tcx.lifetimes.re_erased, fake_borrow_deref_ty); let mut fake_borrow_temp = LocalDecl::new(fake_borrow_ty, temp_span); fake_borrow_temp.internal = self.local_decls[matched_place.local].internal; fake_borrow_temp.local_info = Some(Box::new(LocalInfo::FakeBorrow)); let fake_borrow_temp = self.local_decls.push(fake_borrow_temp); (matched_place, fake_borrow_temp) }) .collect() } } /////////////////////////////////////////////////////////////////////////// // Pat binding - used for `let` and function parameters as well. impl<'a, 'tcx> Builder<'a, 'tcx> { /// If the bindings have already been declared, set `declare_bindings` to /// `false` to avoid duplicated bindings declaration. Used for if-let guards. pub(crate) fn lower_let_expr( &mut self, mut block: BasicBlock, expr: &Expr<'tcx>, pat: &Pat<'tcx>, else_target: region::Scope, source_scope: Option, span: Span, declare_bindings: bool, ) -> BlockAnd<()> { let expr_span = expr.span; let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr, expr_span)); let wildcard = Pat::wildcard_from_ty(pat.ty); let mut guard_candidate = Candidate::new(expr_place_builder.clone(), &pat, false, self); let mut otherwise_candidate = Candidate::new(expr_place_builder.clone(), &wildcard, false, self); let fake_borrow_temps = self.lower_match_tree( block, pat.span, pat.span, false, &mut [&mut guard_candidate, &mut otherwise_candidate], ); let expr_place = expr_place_builder.try_to_place(self); let opt_expr_place = expr_place.as_ref().map(|place| (Some(place), expr_span)); let otherwise_post_guard_block = otherwise_candidate.pre_binding_block.unwrap(); self.break_for_else(otherwise_post_guard_block, else_target, self.source_info(expr_span)); if declare_bindings { self.declare_bindings(source_scope, pat.span.to(span), pat, None, opt_expr_place); } let post_guard_block = self.bind_pattern( self.source_info(pat.span), guard_candidate, &fake_borrow_temps, expr.span, None, false, ); post_guard_block.unit() } /// 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 succeeds. /// /// 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>, parent_bindings: &[(Vec>, Vec>)], fake_borrows: &[(Place<'tcx>, Local)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>, schedule_drops: bool, storages_alive: bool, ) -> 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.unwrap(); if candidate.next_candidate_pre_binding_block.is_some() { 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, parent_bindings .iter() .flat_map(|(_, ascriptions)| ascriptions) .cloned() .chain(candidate.ascriptions), ); // 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((arm, match_scope)) = arm_match_scope && let Some(guard) = &arm.guard { let tcx = self.tcx; let bindings = parent_bindings .iter() .flat_map(|(bindings, _)| bindings) .chain(&candidate.bindings); self.bind_matched_candidate_for_guard(block, schedule_drops, bindings.clone()); let guard_frame = GuardFrame { locals: bindings.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); self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow); } let mut guard_span = rustc_span::DUMMY_SP; let (post_guard_block, otherwise_post_guard_block) = self.in_if_then_scope(match_scope, guard_span, |this| match *guard { Guard::If(e) => { let e = &this.thir[e]; guard_span = e.span; this.then_else_break( block, e, None, match_scope, this.source_info(arm.span), ) } Guard::IfLet(ref pat, scrutinee) => { let s = &this.thir[scrutinee]; guard_span = s.span; this.lower_let_expr(block, s, pat, match_scope, None, arm.span, false) } }); let source_info = self.source_info(guard_span); let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span)); let guard_frame = self.guard_context.pop().unwrap(); debug!("Exiting guard building context with locals: {:?}", guard_frame); for &(_, temp) in fake_borrows { let cause = FakeReadCause::ForMatchGuard; self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp)); } let otherwise_block = candidate.otherwise_block.unwrap_or_else(|| { let unreachable = self.cfg.start_new_block(); self.cfg.terminate(unreachable, source_info, TerminatorKind::Unreachable); unreachable }); self.false_edges( otherwise_post_guard_block, otherwise_block, candidate.next_candidate_pre_binding_block, source_info, ); // 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 an `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 = parent_bindings .iter() .flat_map(|(bindings, _)| bindings) .chain(&candidate.bindings) .filter(|binding| matches!(binding.binding_mode, BindingMode::ByValue)); // 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 cause = FakeReadCause::ForGuardBinding; self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id)); } assert!(schedule_drops, "patterns with guards must schedule drops"); self.bind_matched_candidate_for_arm_body( post_guard_block, true, by_value_bindings, storages_alive, ); post_guard_block } else { // (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, schedule_drops, parent_bindings .iter() .flat_map(|(bindings, _)| bindings) .chain(&candidate.bindings), storages_alive, ); block } } /// Append `AscribeUserType` statements onto the end of `block` /// for each ascription fn ascribe_types( &mut self, block: BasicBlock, ascriptions: impl IntoIterator>, ) { for ascription in ascriptions { let source_info = self.source_info(ascription.annotation.span); let base = self.canonical_user_type_annotations.push(ascription.annotation); self.cfg.push( block, Statement { source_info, kind: StatementKind::AscribeUserType( Box::new(( ascription.source, UserTypeProjection { base, projs: Vec::new() }, )), ascription.variance, ), }, ); } } fn bind_matched_candidate_for_guard<'b>( &mut self, block: BasicBlock, schedule_drops: bool, bindings: impl IntoIterator>, ) where 'tcx: 'b, { debug!("bind_matched_candidate_for_guard(block={:?})", block); // 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.tcx.lifetimes.re_erased; for binding in bindings { debug!("bind_matched_candidate_for_guard(binding={:?})", binding); 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, schedule_drops, ); match binding.binding_mode { BindingMode::ByValue => { let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source); 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, schedule_drops, ); let rvalue = Rvalue::Ref(re_erased, borrow_kind, binding.source); 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, schedule_drops: bool, bindings: impl IntoIterator>, storages_alive: bool, ) where 'tcx: 'b, { debug!("bind_matched_candidate_for_arm_body(block={:?})", block); let re_erased = self.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 = if storages_alive { // Here storages are already alive, probably because this is a binding // from let-else. // We just need to schedule drop for the value. self.var_local_id(binding.var_id, OutsideGuard).into() } else { self.storage_live_binding( block, binding.var_id, binding.span, OutsideGuard, schedule_drops, ) }; if schedule_drops { 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)), BindingMode::ByRef(borrow_kind) => { Rvalue::Ref(re_erased, borrow_kind, binding.source) } }; 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`. #[instrument(skip(self), level = "debug")] fn declare_binding( &mut self, source_info: SourceInfo, visibility_scope: SourceScope, mutability: Mutability, name: Symbol, mode: BindingMode, var_id: LocalVarId, var_ty: Ty<'tcx>, user_ty: UserTypeProjections, has_guard: ArmHasGuard, opt_match_place: Option<(Option>, Span)>, pat_span: Span, ) { let tcx = self.tcx; let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope }; let binding_mode = match mode { BindingMode::ByValue => ty::BindingMode::BindByValue(mutability), BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability), }; let local = LocalDecl { mutability, ty: var_ty, user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) }, source_info, internal: false, is_block_tail: None, local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var( VarBindingForm { binding_mode, // hypothetically, `visit_primary_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); self.var_debug_info.push(VarDebugInfo { name, source_info: debug_source_info, value: VarDebugInfoContents::Place(for_arm_body.into()), }); 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: None, source_info, internal: false, is_block_tail: None, local_info: Some(Box::new(LocalInfo::User(ClearCrossCrate::Set( BindingForm::RefForGuard, )))), }); self.var_debug_info.push(VarDebugInfo { name, source_info: debug_source_info, value: VarDebugInfoContents::Place(ref_for_guard.into()), }); LocalsForNode::ForGuard { ref_for_guard, for_arm_body } } else { LocalsForNode::One(for_arm_body) }; debug!(?locals); self.var_indices.insert(var_id, locals); } pub(crate) fn ast_let_else( &mut self, mut block: BasicBlock, init: &Expr<'tcx>, initializer_span: Span, else_block: BlockId, let_else_scope: ®ion::Scope, pattern: &Pat<'tcx>, ) -> BlockAnd { let else_block_span = self.thir[else_block].span; let (matching, failure) = self.in_if_then_scope(*let_else_scope, else_block_span, |this| { let scrutinee = unpack!(block = this.lower_scrutinee(block, init, initializer_span)); let pat = Pat { ty: init.ty, span: else_block_span, kind: PatKind::Wild }; let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false, this); let mut candidate = Candidate::new(scrutinee.clone(), pattern, false, this); let fake_borrow_temps = this.lower_match_tree( block, initializer_span, pattern.span, false, &mut [&mut candidate, &mut wildcard], ); // This block is for the matching case let matching = this.bind_pattern( this.source_info(pattern.span), candidate, &fake_borrow_temps, initializer_span, None, true, ); // This block is for the failure case let failure = this.bind_pattern( this.source_info(else_block_span), wildcard, &fake_borrow_temps, initializer_span, None, true, ); this.break_for_else(failure, *let_else_scope, this.source_info(initializer_span)); matching.unit() }); matching.and(failure) } }