| //! 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::FxIndexMap, stack::ensure_sufficient_stack}; |
| use rustc_hir::{BindingMode, ByRef}; |
| use rustc_middle::bug; |
| use rustc_middle::middle::region; |
| use rustc_middle::mir::{self, *}; |
| 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; |
| // helper functions, broken out by category: |
| mod simplify; |
| mod test; |
| mod util; |
| |
| use std::borrow::Borrow; |
| use std::mem; |
| |
| /// Arguments to [`Builder::then_else_break_inner`] that are usually forwarded |
| /// to recursive invocations. |
| #[derive(Clone, Copy)] |
| struct ThenElseArgs { |
| /// Used as the temp scope for lowering `expr`. If absent (for match guards), |
| /// `self.local_scope()` is used. |
| temp_scope_override: Option<region::Scope>, |
| variable_source_info: SourceInfo, |
| /// Forwarded to [`Builder::lower_let_expr`] when lowering [`ExprKind::Let`]. |
| /// When false (for match guards), `let` bindings won't be declared. |
| declare_let_bindings: bool, |
| } |
| |
| impl<'a, 'tcx> Builder<'a, 'tcx> { |
| /// Lowers a condition in a way that ensures that variables bound in any let |
| /// expressions are definitely initialized in the if body. |
| /// |
| /// If `declare_let_bindings` is false then variables created in `let` |
| /// expressions will not be declared. This is for if let guards on arms with |
| /// an or pattern, where the guard is lowered multiple times. |
| pub(crate) fn then_else_break( |
| &mut self, |
| block: BasicBlock, |
| expr_id: ExprId, |
| temp_scope_override: Option<region::Scope>, |
| variable_source_info: SourceInfo, |
| declare_let_bindings: bool, |
| ) -> BlockAnd<()> { |
| self.then_else_break_inner( |
| block, |
| expr_id, |
| ThenElseArgs { temp_scope_override, variable_source_info, declare_let_bindings }, |
| ) |
| } |
| |
| fn then_else_break_inner( |
| &mut self, |
| block: BasicBlock, // Block that the condition and branch will be lowered into |
| expr_id: ExprId, // Condition expression to lower |
| args: ThenElseArgs, |
| ) -> BlockAnd<()> { |
| let this = self; |
| let expr = &this.thir[expr_id]; |
| let expr_span = expr.span; |
| |
| match expr.kind { |
| ExprKind::LogicalOp { op: op @ LogicalOp::And, lhs, rhs } => { |
| this.visit_coverage_branch_operation(op, expr_span); |
| let lhs_then_block = unpack!(this.then_else_break_inner(block, lhs, args)); |
| let rhs_then_block = unpack!(this.then_else_break_inner(lhs_then_block, rhs, args)); |
| rhs_then_block.unit() |
| } |
| ExprKind::LogicalOp { op: op @ LogicalOp::Or, lhs, rhs } => { |
| this.visit_coverage_branch_operation(op, expr_span); |
| let local_scope = this.local_scope(); |
| let (lhs_success_block, failure_block) = |
| this.in_if_then_scope(local_scope, expr_span, |this| { |
| this.then_else_break_inner( |
| block, |
| lhs, |
| ThenElseArgs { declare_let_bindings: true, ..args }, |
| ) |
| }); |
| let rhs_success_block = unpack!(this.then_else_break_inner( |
| failure_block, |
| rhs, |
| ThenElseArgs { declare_let_bindings: true, ..args }, |
| )); |
| |
| // Make the LHS and RHS success arms converge to a common block. |
| // (We can't just make LHS goto RHS, because `rhs_success_block` |
| // might contain statements that we don't want on the LHS path.) |
| let success_block = this.cfg.start_new_block(); |
| this.cfg.goto(lhs_success_block, args.variable_source_info, success_block); |
| this.cfg.goto(rhs_success_block, args.variable_source_info, success_block); |
| success_block.unit() |
| } |
| ExprKind::Unary { op: UnOp::Not, arg } => { |
| // Improve branch coverage instrumentation by noting conditions |
| // nested within one or more `!` expressions. |
| // (Skipped if branch coverage is not enabled.) |
| if let Some(branch_info) = this.coverage_branch_info.as_mut() { |
| branch_info.visit_unary_not(this.thir, expr_id); |
| } |
| |
| let local_scope = this.local_scope(); |
| let (success_block, failure_block) = |
| this.in_if_then_scope(local_scope, expr_span, |this| { |
| // Help out coverage instrumentation by injecting a dummy statement with |
| // the original condition's span (including `!`). This fixes #115468. |
| if this.tcx.sess.instrument_coverage() { |
| this.cfg.push_coverage_span_marker(block, this.source_info(expr_span)); |
| } |
| this.then_else_break_inner( |
| block, |
| arg, |
| ThenElseArgs { declare_let_bindings: true, ..args }, |
| ) |
| }); |
| this.break_for_else(success_block, args.variable_source_info); |
| failure_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_inner(block, value, args) |
| }) |
| } |
| ExprKind::Use { source } => this.then_else_break_inner(block, source, args), |
| ExprKind::Let { expr, ref pat } => this.lower_let_expr( |
| block, |
| expr, |
| pat, |
| Some(args.variable_source_info.scope), |
| args.variable_source_info.span, |
| args.declare_let_bindings, |
| ), |
| _ => { |
| let mut block = block; |
| let temp_scope = args.temp_scope_override.unwrap_or_else(|| this.local_scope()); |
| let mutability = Mutability::Mut; |
| |
| // Increment the decision depth, in case we encounter boolean expressions |
| // further down. |
| this.mcdc_increment_depth_if_enabled(); |
| let place = |
| unpack!(block = this.as_temp(block, Some(temp_scope), expr_id, mutability)); |
| this.mcdc_decrement_depth_if_enabled(); |
| |
| 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); |
| |
| // Record branch coverage info for this condition. |
| // (Does nothing if branch coverage is not enabled.) |
| this.visit_coverage_branch_condition(expr_id, 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, 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) ] |
| /// [ (PlaceMention 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 PlaceMention 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 |
| /// ([util::collect_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. Specifically we want borrowck to think that: |
| /// - at any point, any or none of the patterns and guards seen so far may have been tested; |
| /// - after the match, any of the patterns may have matched. |
| /// |
| /// For example, all of these would fail to error if borrowck could see the real CFG (examples |
| /// taken from `tests/ui/nll/match-cfg-fake-edges.rs`): |
| /// ```ignore (too many errors, this is already in the test suite) |
| /// let x = String::new(); |
| /// let _ = match true { |
| /// _ => {}, |
| /// _ => drop(x), |
| /// }; |
| /// // Borrowck must not know the second arm is never run. |
| /// drop(x); //~ ERROR use of moved value |
| /// |
| /// let x; |
| /// # let y = true; |
| /// match y { |
| /// _ if { x = 2; true } => {}, |
| /// // Borrowck must not know the guard is always run. |
| /// _ => drop(x), //~ ERROR used binding `x` is possibly-uninitialized |
| /// }; |
| /// |
| /// let x = String::new(); |
| /// # let y = true; |
| /// match y { |
| /// false if { drop(x); true } => {}, |
| /// // Borrowck must not know the guard is not run in the `true` case. |
| /// true => drop(x), //~ ERROR use of moved value: `x` |
| /// false => {}, |
| /// }; |
| /// |
| /// # let mut y = (true, true); |
| /// let r = &mut y.1; |
| /// match y { |
| /// //~^ ERROR cannot use `y.1` because it was mutably borrowed |
| /// (false, true) => {} |
| /// // Borrowck must not know we don't test `y.1` when `y.0` is `true`. |
| /// (true, _) => drop(r), |
| /// (false, _) => {} |
| /// }; |
| /// ``` |
| /// |
| /// We add false edges to act as if we were naively matching each arm in order. What we need is |
| /// a (fake) path from each candidate to the next, specifically from candidate C's pre-binding |
| /// block to next candidate D's pre-binding block. For maximum precision (needed for deref |
| /// patterns), we choose the earliest node on D's success path that doesn't also lead to C (to |
| /// avoid loops). |
| /// |
| /// This turns out to be easy to compute: that block is the `start_block` of the first call to |
| /// `match_candidates` where D is the first candidate in the list. |
| /// |
| /// For example: |
| /// ```rust |
| /// # let (x, y) = (true, true); |
| /// match (x, y) { |
| /// (true, true) => 1, |
| /// (false, true) => 2, |
| /// (true, false) => 3, |
| /// _ => 4, |
| /// } |
| /// # ; |
| /// ``` |
| /// In this example, the pre-binding block of arm 1 has a false edge to the block for result |
| /// `false` of the first test on `x`. The other arms have false edges to the pre-binding blocks |
| /// of the next arm. |
| /// |
| /// On top of this, we also add a false edge from the otherwise_block of each guard to the |
| /// aforementioned start block of the next candidate, to ensure borrock doesn't rely on which |
| /// guards may have run. |
| #[instrument(level = "debug", skip(self, arms))] |
| pub(crate) fn match_expr( |
| &mut self, |
| destination: Place<'tcx>, |
| mut block: BasicBlock, |
| scrutinee_id: ExprId, |
| arms: &[ArmId], |
| span: Span, |
| scrutinee_span: Span, |
| ) -> BlockAnd<()> { |
| let scrutinee_place = |
| unpack!(block = self.lower_scrutinee(block, scrutinee_id, 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::<Vec<_>>(); |
| |
| let match_start_span = span.shrink_to_lo().to(scrutinee_span); |
| |
| let fake_borrow_temps = self.lower_match_tree( |
| block, |
| scrutinee_span, |
| &scrutinee_place, |
| 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 PlaceMention for it. |
| fn lower_scrutinee( |
| &mut self, |
| mut block: BasicBlock, |
| scrutinee_id: ExprId, |
| scrutinee_span: Span, |
| ) -> BlockAnd<PlaceBuilder<'tcx>> { |
| let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee_id)); |
| if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) { |
| let source_info = self.source_info(scrutinee_span); |
| self.cfg.push_place_mention(block, source_info, 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, |
| scrutinee_place_builder: &PlaceBuilder<'tcx>, |
| match_start_span: Span, |
| match_has_guard: bool, |
| candidates: &mut [&mut Candidate<'pat, 'tcx>], |
| ) -> Vec<(Place<'tcx>, Local, FakeBorrowKind)> { |
| // 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 fake_borrows: Vec<(Place<'tcx>, Local, FakeBorrowKind)> = if match_has_guard { |
| util::collect_fake_borrows( |
| self, |
| candidates, |
| scrutinee_span, |
| scrutinee_place_builder.base(), |
| ) |
| } else { |
| Vec::new() |
| }; |
| |
| // See the doc comment on `match_candidates` for why we have an |
| // otherwise block. Match checking will ensure this is actually |
| // unreachable. |
| let otherwise_block = self.cfg.start_new_block(); |
| |
| // This will generate code to test scrutinee_place and |
| // branch to the appropriate arm block |
| self.match_candidates(match_start_span, scrutinee_span, block, otherwise_block, candidates); |
| |
| let source_info = self.source_info(scrutinee_span); |
| |
| // 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); |
| |
| if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) { |
| self.cfg.push_fake_read( |
| otherwise_block, |
| source_info, |
| cause_matched_place, |
| scrutinee_place, |
| ); |
| } |
| |
| 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 { |
| assert!(leaf_candidate.false_edge_start_block.is_some()); |
| prev.next_candidate_start_block = leaf_candidate.false_edge_start_block; |
| } |
| previous_candidate = Some(leaf_candidate); |
| }); |
| } |
| |
| fake_borrows |
| } |
| |
| /// 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, FakeBorrowKind)>, |
| ) -> 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| { |
| let old_dedup_scope = |
| mem::replace(&mut this.fixed_temps_scope, Some(arm.scope)); |
| |
| // `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, |
| opt_scrutinee_place, |
| ); |
| |
| let arm_block = this.bind_pattern( |
| outer_source_info, |
| candidate, |
| &fake_borrow_temps, |
| scrutinee_span, |
| Some((arm, match_scope)), |
| false, |
| ); |
| |
| this.fixed_temps_scope = old_dedup_scope; |
| |
| if let Some(source_scope) = scope { |
| this.source_scope = source_scope; |
| } |
| |
| this.expr_into_dest(destination, arm_block, 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, FakeBorrowKind)], |
| 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_data| { |
| if let Some(arm) = arm { |
| self.clear_top_scope(arm.scope); |
| } |
| let binding_end = self.bind_and_guard_matched_candidate( |
| leaf_candidate, |
| parent_data, |
| 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_data| { |
| parent_data.push(inner_candidate.extra_data); |
| inner_candidate.subcandidates.into_iter() |
| }, |
| |parent_data| { |
| parent_data.pop(); |
| }, |
| ); |
| |
| target_block |
| } |
| } |
| |
| pub(super) fn expr_into_pattern( |
| &mut self, |
| mut block: BasicBlock, |
| irrefutable_pat: &Pat<'tcx>, |
| initializer_id: ExprId, |
| ) -> BlockAnd<()> { |
| match irrefutable_pat.kind { |
| // Optimize the case of `let x = ...` to write directly into `x` |
| PatKind::Binding { mode: BindingMode(ByRef::No, _), 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_id)); |
| |
| // 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(ByRef::No, _), |
| 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_id)); |
| |
| // 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 = <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 initializer = &self.thir[initializer_id]; |
| let place_builder = |
| unpack!(block = self.lower_scrutinee(block, initializer_id, initializer.span)); |
| 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, |
| &initializer, |
| 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 next = Some(&candidate); |
| while let Some(candidate_ref) = next.take() { |
| for binding in &candidate_ref.extra_data.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 LocalInfo::User(BindingForm::Var(VarBindingForm { |
| opt_match_place: Some((ref mut match_place, _)), |
| .. |
| })) = **self.local_decls[local].local_info.as_mut().assert_crate_local() |
| 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. |
| next = candidate_ref.subcandidates.get(0) |
| } |
| } |
| |
| self.bind_pattern( |
| self.source_info(irrefutable_pat.span), |
| candidate, |
| fake_borrow_temps.as_slice(), |
| 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<SourceScope>, |
| scope_span: Span, |
| pattern: &Pat<'tcx>, |
| guard: Option<ExprId>, |
| opt_match_place: Option<(Option<&Place<'tcx>>, Span)>, |
| ) -> Option<SourceScope> { |
| self.visit_primary_bindings( |
| pattern, |
| UserTypeProjections::none(), |
| &mut |this, name, mode, var, span, ty, user_ty| { |
| if visibility_scope.is_none() { |
| visibility_scope = |
| Some(this.new_source_scope(scope_span, LintLevel::Inherited)); |
| } |
| let source_info = SourceInfo { span, scope: this.source_scope }; |
| let visibility_scope = visibility_scope.unwrap(); |
| this.declare_binding( |
| source_info, |
| visibility_scope, |
| 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_expr) = guard { |
| self.declare_guard_bindings(guard_expr, scope_span, visibility_scope); |
| } |
| visibility_scope |
| } |
| |
| /// Declare bindings in a guard. This has to be done when declaring bindings |
| /// for an arm to ensure that or patterns only have one version of each |
| /// variable. |
| pub(crate) fn declare_guard_bindings( |
| &mut self, |
| guard_expr: ExprId, |
| scope_span: Span, |
| visibility_scope: Option<SourceScope>, |
| ) { |
| match self.thir.exprs[guard_expr].kind { |
| ExprKind::Let { expr: _, pat: ref guard_pat } => { |
| // FIXME: pass a proper `opt_match_place` |
| self.declare_bindings(visibility_scope, scope_span, guard_pat, None, None); |
| } |
| ExprKind::Scope { value, .. } => { |
| self.declare_guard_bindings(value, scope_span, visibility_scope); |
| } |
| ExprKind::Use { source } => { |
| self.declare_guard_bindings(source, scope_span, visibility_scope); |
| } |
| ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => { |
| self.declare_guard_bindings(lhs, scope_span, visibility_scope); |
| self.declare_guard_bindings(rhs, scope_span, 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, |
| Symbol, |
| BindingMode, |
| LocalVarId, |
| Span, |
| Ty<'tcx>, |
| UserTypeProjections, |
| ), |
| ) { |
| debug!( |
| "visit_primary_bindings: pattern={:?} pattern_user_ty={:?}", |
| pattern, pattern_user_ty |
| ); |
| match pattern.kind { |
| PatKind::Binding { name, mode, var, ty, ref subpattern, is_primary, .. } => { |
| if is_primary { |
| f(self, 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); |
| } |
| if let Some(subpattern) = 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::Never |
| | PatKind::Error(_) => {} |
| |
| PatKind::Deref { ref subpattern } => { |
| self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f); |
| } |
| |
| PatKind::DerefPattern { ref subpattern, .. } => { |
| self.visit_primary_bindings(subpattern, UserTypeProjections::none(), 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::InlineConstant { ref subpattern, .. } => { |
| self.visit_primary_bindings(subpattern, pattern_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, args: _, 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); |
| } |
| } |
| } |
| } |
| } |
| |
| /// Data extracted from a pattern that doesn't affect which branch is taken. Collected during |
| /// pattern simplification and not mutated later. |
| #[derive(Debug, Clone)] |
| struct PatternExtraData<'tcx> { |
| /// [`Span`] of the original pattern. |
| span: Span, |
| |
| /// Bindings that must be established. |
| bindings: Vec<Binding<'tcx>>, |
| |
| /// Types that must be asserted. |
| ascriptions: Vec<Ascription<'tcx>>, |
| |
| /// Whether this corresponds to a never pattern. |
| is_never: bool, |
| } |
| |
| impl<'tcx> PatternExtraData<'tcx> { |
| fn is_empty(&self) -> bool { |
| self.bindings.is_empty() && self.ascriptions.is_empty() |
| } |
| } |
| |
| /// A pattern in a form suitable for generating code. |
| #[derive(Debug, Clone)] |
| struct FlatPat<'pat, 'tcx> { |
| /// To match the pattern, all of these must be satisfied... |
| // Invariant: all the `MatchPair`s are recursively simplified. |
| // Invariant: or-patterns must be sorted to the end. |
| match_pairs: Vec<MatchPair<'pat, 'tcx>>, |
| |
| extra_data: PatternExtraData<'tcx>, |
| } |
| |
| impl<'tcx, 'pat> FlatPat<'pat, 'tcx> { |
| fn new( |
| place: PlaceBuilder<'tcx>, |
| pattern: &'pat Pat<'tcx>, |
| cx: &mut Builder<'_, 'tcx>, |
| ) -> Self { |
| let is_never = pattern.is_never_pattern(); |
| let mut flat_pat = FlatPat { |
| match_pairs: vec![MatchPair::new(place, pattern, cx)], |
| extra_data: PatternExtraData { |
| span: pattern.span, |
| bindings: Vec::new(), |
| ascriptions: Vec::new(), |
| is_never, |
| }, |
| }; |
| cx.simplify_match_pairs(&mut flat_pat.match_pairs, &mut flat_pat.extra_data); |
| flat_pat |
| } |
| } |
| |
| #[derive(Debug)] |
| struct Candidate<'pat, 'tcx> { |
| /// For the candidate to match, all of these must be satisfied... |
| // Invariant: all the `MatchPair`s are recursively simplified. |
| // Invariant: or-patterns must be sorted at the end. |
| match_pairs: Vec<MatchPair<'pat, 'tcx>>, |
| |
| /// ...and if this is non-empty, one of these subcandidates also has to match... |
| // Invariant: at the end of the algorithm, this must never contain a `is_never` candidate |
| // because that would break binding consistency. |
| subcandidates: Vec<Candidate<'pat, 'tcx>>, |
| |
| /// ...and the guard must be evaluated if there is one. |
| has_guard: bool, |
| |
| /// If the guard is `false` then branch to `otherwise_block`. |
| otherwise_block: Option<BasicBlock>, |
| |
| /// If the candidate matches, bindings and ascriptions must be established. |
| extra_data: PatternExtraData<'tcx>, |
| |
| /// If we filled `self.subcandidate`, we store here the span of the or-pattern they came from. |
| // Invariant: it is `None` iff `subcandidates.is_empty()`. |
| or_span: Option<Span>, |
| |
| /// The block before the `bindings` have been established. |
| pre_binding_block: Option<BasicBlock>, |
| |
| /// The earliest block that has only candidates >= this one as descendents. Used for false |
| /// edges, see the doc for [`Builder::match_expr`]. |
| false_edge_start_block: Option<BasicBlock>, |
| /// The `false_edge_start_block` of the next candidate. |
| next_candidate_start_block: Option<BasicBlock>, |
| } |
| |
| impl<'tcx, 'pat> Candidate<'pat, 'tcx> { |
| fn new( |
| place: PlaceBuilder<'tcx>, |
| pattern: &'pat Pat<'tcx>, |
| has_guard: bool, |
| cx: &mut Builder<'_, 'tcx>, |
| ) -> Self { |
| Self::from_flat_pat(FlatPat::new(place, pattern, cx), has_guard) |
| } |
| |
| fn from_flat_pat(flat_pat: FlatPat<'pat, 'tcx>, has_guard: bool) -> Self { |
| Candidate { |
| match_pairs: flat_pat.match_pairs, |
| extra_data: flat_pat.extra_data, |
| has_guard, |
| subcandidates: Vec::new(), |
| or_span: None, |
| otherwise_block: None, |
| pre_binding_block: None, |
| false_edge_start_block: None, |
| next_candidate_start_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<Candidate<'pat, 'tcx>>, |
| I: Iterator<Item = C>, |
| { |
| 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(Debug, Clone)] |
| enum TestCase<'pat, 'tcx> { |
| Irrefutable { binding: Option<Binding<'tcx>>, ascription: Option<Ascription<'tcx>> }, |
| Variant { adt_def: ty::AdtDef<'tcx>, variant_index: VariantIdx }, |
| Constant { value: mir::Const<'tcx> }, |
| Range(&'pat PatRange<'tcx>), |
| Slice { len: usize, variable_length: bool }, |
| Deref { temp: Place<'tcx>, mutability: Mutability }, |
| Never, |
| Or { pats: Box<[FlatPat<'pat, 'tcx>]> }, |
| } |
| |
| impl<'pat, 'tcx> TestCase<'pat, 'tcx> { |
| fn as_range(&self) -> Option<&'pat PatRange<'tcx>> { |
| if let Self::Range(v) = self { Some(*v) } else { None } |
| } |
| } |
| |
| #[derive(Debug, Clone)] |
| pub(crate) struct MatchPair<'pat, 'tcx> { |
| /// This place... |
| // This can be `None` if it referred to a non-captured place in a closure. |
| // Invariant: place.is_none() => test_case is Irrefutable |
| // In other words this must be `Some(_)` after simplification. |
| place: Option<Place<'tcx>>, |
| |
| /// ... must pass this test... |
| // Invariant: after creation and simplification in `Candidate::new()`, this must not be |
| // `Irrefutable`. |
| test_case: TestCase<'pat, 'tcx>, |
| |
| /// ... and these subpairs must match. |
| subpairs: Vec<Self>, |
| |
| /// The pattern this was created from. |
| 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>, |
| }, |
| |
| /// Test what value an integer or `char` has. |
| SwitchInt, |
| |
| /// Test what value a `bool` has. |
| If, |
| |
| /// Test for equality with value, possibly after an unsizing coercion to |
| /// `ty`, |
| Eq { |
| value: 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(Box<PatRange<'tcx>>), |
| |
| /// Test that the length of the slice is equal to `len`. |
| Len { len: u64, op: BinOp }, |
| |
| /// Call `Deref::deref[_mut]` on the value. |
| Deref { |
| /// Temporary to store the result of `deref()`/`deref_mut()`. |
| temp: Place<'tcx>, |
| mutability: Mutability, |
| }, |
| |
| /// Assert unreachability of never patterns. |
| Never, |
| } |
| |
| /// 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>, |
| } |
| |
| /// The branch to be taken after a test. |
| #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)] |
| enum TestBranch<'tcx> { |
| /// Success branch, used for tests with two possible outcomes. |
| Success, |
| /// Branch corresponding to this constant. |
| Constant(Const<'tcx>, u128), |
| /// Branch corresponding to this variant. |
| Variant(VariantIdx), |
| /// Failure branch for tests with two possible outcomes, and "otherwise" branch for other tests. |
| Failure, |
| } |
| |
| impl<'tcx> TestBranch<'tcx> { |
| fn as_constant(&self) -> Option<&Const<'tcx>> { |
| if let Self::Constant(v, _) = self { Some(v) } else { None } |
| } |
| } |
| |
| /// `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 `otherwise_block`. |
| /// |
| /// 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 how our lowering approach (called "backtracking |
| /// automaton" in the literature) works. |
| /// 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 how we use `otherwise_block`, consider: |
| /// ``` |
| /// # fn foo((x, y): (bool, bool)) -> u32 { |
| /// match (x, y) { |
| /// (true, true) => 1, |
| /// (_, false) => 2, |
| /// (false, true) => 3, |
| /// } |
| /// # } |
| /// ``` |
| /// For this match, we generate something like: |
| /// ``` |
| /// # fn foo((x, y): (bool, bool)) -> u32 { |
| /// if x { |
| /// if y { |
| /// return 1 |
| /// } else { |
| /// // continue |
| /// } |
| /// } else { |
| /// // continue |
| /// } |
| /// if y { |
| /// if x { |
| /// // This is actually unreachable because the `(true, true)` case was handled above. |
| /// // continue |
| /// } else { |
| /// return 3 |
| /// } |
| /// } else { |
| /// return 2 |
| /// } |
| /// // this is the final `otherwise_block`, which is unreachable because the match was exhaustive. |
| /// unreachable!() |
| /// # } |
| /// ``` |
| /// |
| /// Every `continue` is an instance of branching to some `otherwise_block` somewhere deep within |
| /// the algorithm. For more details on why we lower like this, see [`Builder::test_candidates`]. |
| /// |
| /// Note how we test `x` twice. This is the tradeoff of backtracking automata: we prefer smaller |
| /// code size at the expense of non-optimal code paths. |
| #[instrument(skip(self), level = "debug")] |
| fn match_candidates<'pat>( |
| &mut self, |
| span: Span, |
| scrutinee_span: Span, |
| start_block: BasicBlock, |
| otherwise_block: BasicBlock, |
| candidates: &mut [&mut Candidate<'pat, 'tcx>], |
| ) { |
| let mut split_or_candidate = false; |
| for candidate in &mut *candidates { |
| if let [MatchPair { test_case: TestCase::Or { .. }, .. }] = &*candidate.match_pairs { |
| // Split a candidate in which the only match-pair is an or-pattern into multiple |
| // candidates. This is so that |
| // |
| // match x { |
| // 0 | 1 => { ... }, |
| // 2 | 3 => { ... }, |
| // } |
| // |
| // only generates a single switch. |
| let match_pair = candidate.match_pairs.pop().unwrap(); |
| self.create_or_subcandidates(candidate, match_pair); |
| split_or_candidate = true; |
| } |
| } |
| |
| 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.iter_mut() { |
| candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate)); |
| } |
| self.match_candidates( |
| span, |
| scrutinee_span, |
| start_block, |
| otherwise_block, |
| &mut *new_candidates, |
| ); |
| |
| for candidate in candidates { |
| self.merge_trivial_subcandidates(candidate); |
| } |
| } else { |
| self.match_simplified_candidates( |
| span, |
| scrutinee_span, |
| start_block, |
| otherwise_block, |
| candidates, |
| ); |
| } |
| }); |
| } |
| |
| fn match_simplified_candidates( |
| &mut self, |
| span: Span, |
| scrutinee_span: Span, |
| mut start_block: BasicBlock, |
| otherwise_block: BasicBlock, |
| candidates: &mut [&mut Candidate<'_, 'tcx>], |
| ) { |
| if let [first, ..] = candidates { |
| if first.false_edge_start_block.is_none() { |
| first.false_edge_start_block = Some(start_block); |
| } |
| } |
| |
| match candidates { |
| [] => { |
| // If there are no candidates that still need testing, we're done. Since all matches are |
| // exhaustive, execution should never reach this point. |
| let source_info = self.source_info(span); |
| self.cfg.goto(start_block, source_info, otherwise_block); |
| } |
| [first, remaining @ ..] if first.match_pairs.is_empty() => { |
| // The first candidate has satisfied all its match pairs; we link it up and continue |
| // with the remaining candidates. |
| start_block = self.select_matched_candidate(first, start_block); |
| self.match_simplified_candidates( |
| span, |
| scrutinee_span, |
| start_block, |
| otherwise_block, |
| remaining, |
| ) |
| } |
| candidates => { |
| // The first candidate has some unsatisfied match pairs; we proceed to do more tests. |
| self.test_candidates_with_or( |
| span, |
| scrutinee_span, |
| candidates, |
| start_block, |
| otherwise_block, |
| ); |
| } |
| } |
| } |
| |
| /// 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 later 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_candidate( |
| &mut self, |
| candidate: &mut Candidate<'_, 'tcx>, |
| start_block: BasicBlock, |
| ) -> BasicBlock { |
| assert!(candidate.otherwise_block.is_none()); |
| assert!(candidate.pre_binding_block.is_none()); |
| assert!(candidate.subcandidates.is_empty()); |
| |
| candidate.pre_binding_block = Some(start_block); |
| let otherwise_block = self.cfg.start_new_block(); |
| if candidate.has_guard { |
| // Create the otherwise block for this candidate, which is the |
| // pre-binding block for the next candidate. |
| candidate.otherwise_block = Some(otherwise_block); |
| } |
| 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>], |
| start_block: BasicBlock, |
| otherwise_block: BasicBlock, |
| ) { |
| let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap(); |
| assert!(first_candidate.subcandidates.is_empty()); |
| if !matches!(first_candidate.match_pairs[0].test_case, TestCase::Or { .. }) { |
| self.test_candidates(span, scrutinee_span, candidates, start_block, otherwise_block); |
| return; |
| } |
| |
| let first_match_pair = first_candidate.match_pairs.remove(0); |
| let remaining_match_pairs = mem::take(&mut first_candidate.match_pairs); |
| let remainder_start = self.cfg.start_new_block(); |
| // Test the alternatives of this or-pattern. |
| self.test_or_pattern(first_candidate, start_block, remainder_start, first_match_pair); |
| |
| if !remaining_match_pairs.is_empty() { |
| // If more match pairs remain, test them after each subcandidate. |
| // We could add them to the or-candidates before the call to `test_or_pattern` but this |
| // would make it impossible to detect simplifiable or-patterns. That would guarantee |
| // exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`. |
| first_candidate.visit_leaves(|leaf_candidate| { |
| assert!(leaf_candidate.match_pairs.is_empty()); |
| leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned()); |
| let or_start = leaf_candidate.pre_binding_block.unwrap(); |
| // In a case like `(a | b, c | d)`, if `a` succeeds and `c | d` fails, we know `(b, |
| // c | d)` will fail too. If there is no guard, we skip testing of `b` by branching |
| // directly to `remainder_start`. If there is a guard, we have to try `(b, c | d)`. |
| let or_otherwise = leaf_candidate.otherwise_block.unwrap_or(remainder_start); |
| self.test_candidates_with_or( |
| span, |
| scrutinee_span, |
| &mut [leaf_candidate], |
| or_start, |
| or_otherwise, |
| ); |
| }); |
| } |
| |
| // Test the remaining candidates. |
| self.match_candidates( |
| span, |
| scrutinee_span, |
| remainder_start, |
| otherwise_block, |
| remaining_candidates, |
| ); |
| } |
| |
| #[instrument(skip(self, start_block, otherwise_block, candidate, match_pair), level = "debug")] |
| fn test_or_pattern<'pat>( |
| &mut self, |
| candidate: &mut Candidate<'pat, 'tcx>, |
| start_block: BasicBlock, |
| otherwise_block: BasicBlock, |
| match_pair: MatchPair<'pat, 'tcx>, |
| ) { |
| let or_span = match_pair.pattern.span; |
| self.create_or_subcandidates(candidate, match_pair); |
| let mut or_candidate_refs: Vec<_> = candidate.subcandidates.iter_mut().collect(); |
| self.match_candidates( |
| or_span, |
| or_span, |
| start_block, |
| otherwise_block, |
| &mut or_candidate_refs, |
| ); |
| self.merge_trivial_subcandidates(candidate); |
| } |
| |
| /// Given a match-pair that corresponds to an or-pattern, expand each subpattern into a new |
| /// subcandidate. Any candidate that has been expanded that way should be passed to |
| /// `merge_trivial_subcandidates` after its subcandidates have been processed. |
| fn create_or_subcandidates<'pat>( |
| &mut self, |
| candidate: &mut Candidate<'pat, 'tcx>, |
| match_pair: MatchPair<'pat, 'tcx>, |
| ) { |
| let TestCase::Or { pats } = match_pair.test_case else { bug!() }; |
| debug!("expanding or-pattern: candidate={:#?}\npats={:#?}", candidate, pats); |
| candidate.or_span = Some(match_pair.pattern.span); |
| candidate.subcandidates = pats |
| .into_vec() |
| .into_iter() |
| .map(|flat_pat| Candidate::from_flat_pat(flat_pat, candidate.has_guard)) |
| .collect(); |
| candidate.subcandidates[0].false_edge_start_block = candidate.false_edge_start_block; |
| } |
| |
| /// 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, ...)`. The or-pattern should have |
| /// been expanded with `create_or_subcandidates`. |
| fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) { |
| if candidate.subcandidates.is_empty() || candidate.has_guard { |
| // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard. |
| return; |
| } |
| |
| // FIXME(or_patterns; matthewjasper) Try to be more aggressive here. |
| let can_merge = candidate.subcandidates.iter().all(|subcandidate| { |
| subcandidate.subcandidates.is_empty() && subcandidate.extra_data.is_empty() |
| }); |
| if can_merge { |
| let any_matches = self.cfg.start_new_block(); |
| let or_span = candidate.or_span.take().unwrap(); |
| let source_info = self.source_info(or_span); |
| if candidate.false_edge_start_block.is_none() { |
| candidate.false_edge_start_block = |
| candidate.subcandidates[0].false_edge_start_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); |
| } else { |
| // Never subcandidates may have a set of bindings inconsistent with their siblings, |
| // which would break later code. So we filter them out. Note that we can't filter out |
| // top-level candidates this way. |
| candidate.subcandidates.retain_mut(|candidate| { |
| if candidate.extra_data.is_never { |
| candidate.visit_leaves(|subcandidate| { |
| let block = subcandidate.pre_binding_block.unwrap(); |
| // That block is already unreachable but needs a terminator to make the MIR well-formed. |
| let source_info = self.source_info(subcandidate.extra_data.span); |
| self.cfg.terminate(block, source_info, TerminatorKind::Unreachable); |
| }); |
| false |
| } else { |
| true |
| } |
| }); |
| if candidate.subcandidates.is_empty() { |
| // If `candidate` has become a leaf candidate, ensure it has a `pre_binding_block`. |
| candidate.pre_binding_block = Some(self.cfg.start_new_block()); |
| } |
| } |
| } |
| |
| /// Pick a test to run. Which test doesn't matter as long as it is guaranteed to fully match at |
| /// least one match pair. We currently simply pick the test corresponding to the first match |
| /// pair of the first candidate in the list. |
| /// |
| /// *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 |
| fn pick_test(&mut self, candidates: &[&mut Candidate<'_, 'tcx>]) -> (Place<'tcx>, Test<'tcx>) { |
| // Extract the match-pair from the highest priority candidate |
| let match_pair = &candidates.first().unwrap().match_pairs[0]; |
| let test = self.test(match_pair); |
| // Unwrap is ok after simplification. |
| let match_place = match_pair.place.unwrap(); |
| debug!(?test, ?match_pair); |
| |
| (match_place, test) |
| } |
| |
| /// Given a test, we sort the input candidates into several buckets. If a candidate only matches |
| /// in one of the branches of `test`, we move it there. If it could match in more than one of |
| /// the branches of `test`, we stop sorting candidates. |
| /// |
| /// This returns a pair of |
| /// - the candidates that weren't sorted; |
| /// - for each possible outcome of the test, the candidates that match in that outcome. |
| /// |
| /// Moreover, we transform the branched candidates to reflect the fact that we know which |
| /// outcome of `test` occurred. |
| /// |
| /// For example: |
| /// ``` |
| /// # let (x, y, z) = (true, true, true); |
| /// match (x, y, z) { |
| /// (true , _ , true ) => true, // (0) |
| /// (false, false, _ ) => false, // (1) |
| /// (_ , true , _ ) => true, // (2) |
| /// (true , _ , false) => false, // (3) |
| /// } |
| /// # ; |
| /// ``` |
| /// |
| /// Assume we are testing on `x`. There are 2 overlapping candidate sets: |
| /// - If the outcome is that `x` is true, candidates 0, 2, and 3 |
| /// - If the outcome is that `x` is false, candidates 1 and 2 |
| /// |
| /// Following our algorithm, candidate 0 is sorted into outcome `x == true`, candidate 1 goes |
| /// into outcome `x == false`, and candidate 2 and 3 remain unsorted. |
| /// |
| /// The sorted candidates are transformed: |
| /// - candidate 0 becomes `[z @ true]` since we know that `x` was `true`; |
| /// - candidate 1 becomes `[y @ false]` since we know that `x` was `false`. |
| fn sort_candidates<'b, 'c, 'pat>( |
| &mut self, |
| match_place: Place<'tcx>, |
| test: &Test<'tcx>, |
| mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], |
| ) -> ( |
| &'b mut [&'c mut Candidate<'pat, 'tcx>], |
| FxIndexMap<TestBranch<'tcx>, Vec<&'b mut Candidate<'pat, 'tcx>>>, |
| ) { |
| // For each of the possible outcomes, collect vector of candidates that apply if the test |
| // has that particular outcome. |
| let mut target_candidates: FxIndexMap<_, Vec<&mut Candidate<'_, '_>>> = 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(branch) = |
| self.sort_candidate(match_place, test, candidate, &target_candidates) |
| else { |
| break; |
| }; |
| let (candidate, rest) = candidates.split_first_mut().unwrap(); |
| target_candidates.entry(branch).or_insert_with(Vec::new).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()); |
| |
| (candidates, target_candidates) |
| } |
| |
| /// This is the most subtle part of the match lowering algorithm. At this point, the input |
| /// candidates have been fully simplified, so all remaining match-pairs require some sort of |
| /// test. |
| /// |
| /// Once we pick what sort of test we are going to perform, this test will help us winnow down |
| /// our candidates. So we walk over the candidates (from high to low priority) and check. We |
| /// compute, for each outcome of the test, a transformed list of candidates. If a candidate |
| /// matches in a single branch of our test, we add it to the corresponding outcome. We also |
| /// transform it to record the fact that we know which outcome occurred. |
| /// |
| /// 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)` in the |
| /// branch corresponding to `Some`. To ensure we make progress, we always pick a test that |
| /// results in simplifying the first candidate. |
| /// |
| /// But there may also be candidates that the test doesn't |
| /// apply to. The classical example is wildcards: |
| /// |
| /// ``` |
| /// # let (x, y, z) = (true, true, true); |
| /// match (x, y, z) { |
| /// (true , _ , true ) => true, // (0) |
| /// (false, false, _ ) => false, // (1) |
| /// (_ , true , _ ) => true, // (2) |
| /// (true , _ , false) => false, // (3) |
| /// } |
| /// # ; |
| /// ``` |
| /// |
| /// Here, the traditional "decision tree" method would generate 2 separate code-paths for the 2 |
| /// possible values of `x`. This would however duplicate some candidates, which would need to be |
| /// lowered several times. |
| /// |
| /// 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. |
| /// |
| /// In rustc, we opt instead for the "backtracking automaton" approach. This guarantees we never |
| /// duplicate a candidate (except in the presence of or-patterns). In fact this guarantee is |
| /// ensured by the fact that we carry around `&mut Candidate`s which can't be duplicated. |
| /// |
| /// To make this work, whenever we decide to perform a test, if we encounter a candidate that |
| /// could match in more than one branch of the test, we stop. We generate code for the test and |
| /// for the candidates in its branches; the remaining candidates will be tested if the |
| /// candidates in the branches fail to match. |
| /// |
| /// For example, if we test on `x` in the following: |
| /// ``` |
| /// # fn foo((x, y, z): (bool, bool, bool)) -> u32 { |
| /// match (x, y, z) { |
| /// (true , _ , true ) => 0, |
| /// (false, false, _ ) => 1, |
| /// (_ , true , _ ) => 2, |
| /// (true , _ , false) => 3, |
| /// } |
| /// # } |
| /// ``` |
| /// this function generates code that looks more of less like: |
| /// ``` |
| /// # fn foo((x, y, z): (bool, bool, bool)) -> u32 { |
| /// if x { |
| /// match (y, z) { |
| /// (_, true) => return 0, |
| /// _ => {} // continue matching |
| /// } |
| /// } else { |
| /// match (y, z) { |
| /// (false, _) => return 1, |
| /// _ => {} // continue matching |
| /// } |
| /// } |
| /// // the block here is `remainder_start` |
| /// match (x, y, z) { |
| /// (_ , true , _ ) => 2, |
| /// (true , _ , false) => 3, |
| /// _ => unreachable!(), |
| /// } |
| /// # } |
| /// ``` |
| fn test_candidates<'pat, 'b, 'c>( |
| &mut self, |
| span: Span, |
| scrutinee_span: Span, |
| candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], |
| start_block: BasicBlock, |
| otherwise_block: BasicBlock, |
| ) { |
| // Extract the match-pair from the highest priority candidate and build a test from it. |
| let (match_place, test) = self.pick_test(candidates); |
| |
| // For each of the N possible test outcomes, build the vector of candidates that applies if |
| // the test has that particular outcome. |
| let (remaining_candidates, target_candidates) = |
| self.sort_candidates(match_place, &test, candidates); |
| |
| // The block that we should branch to if none of the |
| // `target_candidates` match. |
| let remainder_start = if !remaining_candidates.is_empty() { |
| let remainder_start = self.cfg.start_new_block(); |
| self.match_candidates( |
| span, |
| scrutinee_span, |
| remainder_start, |
| otherwise_block, |
| remaining_candidates, |
| ); |
| remainder_start |
| } else { |
| otherwise_block |
| }; |
| |
| // For each outcome of test, process the candidates that still apply. |
| let target_blocks: FxIndexMap<_, _> = target_candidates |
| .into_iter() |
| .map(|(branch, mut candidates)| { |
| let candidate_start = self.cfg.start_new_block(); |
| self.match_candidates( |
| span, |
| scrutinee_span, |
| candidate_start, |
| remainder_start, |
| &mut *candidates, |
| ); |
| (branch, candidate_start) |
| }) |
| .collect(); |
| |
| // Perform the test, branching to one of N blocks. |
| self.perform_test( |
| span, |
| scrutinee_span, |
| start_block, |
| remainder_start, |
| match_place, |
| &test, |
| target_blocks, |
| ); |
| } |
| } |
| |
| /////////////////////////////////////////////////////////////////////////// |
| // 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_id: ExprId, |
| pat: &Pat<'tcx>, |
| source_scope: Option<SourceScope>, |
| span: Span, |
| declare_bindings: bool, |
| ) -> BlockAnd<()> { |
| let expr_span = self.thir[expr_id].span; |
| let expr_place_builder = unpack!(block = self.lower_scrutinee(block, expr_id, 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, |
| &expr_place_builder, |
| 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, 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.as_slice(), |
| expr_span, |
| None, |
| false, |
| ); |
| |
| // If branch coverage is enabled, record this branch. |
| self.visit_coverage_conditional_let(pat, post_guard_block, otherwise_post_guard_block); |
| |
| 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_data: &[PatternExtraData<'tcx>], |
| fake_borrows: &[(Place<'tcx>, Local, FakeBorrowKind)], |
| 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.extra_data.span); |
| |
| let mut block = candidate.pre_binding_block.unwrap(); |
| |
| if candidate.next_candidate_start_block.is_some() { |
| let fresh_block = self.cfg.start_new_block(); |
| self.false_edges( |
| block, |
| fresh_block, |
| candidate.next_candidate_start_block, |
| candidate_source_info, |
| ); |
| block = fresh_block; |
| } |
| |
| if candidate.extra_data.is_never { |
| // This arm has a dummy body, we don't need to generate code for it. `block` is already |
| // unreachable (except via false edge). |
| let source_info = self.source_info(candidate.extra_data.span); |
| self.cfg.terminate(block, source_info, TerminatorKind::Unreachable); |
| return self.cfg.start_new_block(); |
| } |
| |
| self.ascribe_types( |
| block, |
| parent_data |
| .iter() |
| .flat_map(|d| &d.ascriptions) |
| .cloned() |
| .chain(candidate.extra_data.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_data.iter().flat_map(|d| &d.bindings).chain(&candidate.extra_data.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)).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, kind) in fake_borrows { |
| let borrow = Rvalue::Ref(re_erased, BorrowKind::Fake(kind), 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| { |
| guard_span = this.thir[guard].span; |
| this.then_else_break( |
| block, |
| guard, |
| None, // Use `self.local_scope()` as the temp scope |
| this.source_info(arm.span), |
| false, // For guards, `let` bindings are declared separately |
| ) |
| }); |
| |
| 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_start_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_data |
| .iter() |
| .flat_map(|d| &d.bindings) |
| .chain(&candidate.extra_data.bindings) |
| .filter(|binding| matches!(binding.binding_mode.0, ByRef::No)); |
| // 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_data.iter().flat_map(|d| &d.bindings).chain(&candidate.extra_data.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<Item = Ascription<'tcx>>, |
| ) { |
| 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<Item = &'b Binding<'tcx>>, |
| ) 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.0 { |
| ByRef::No => { |
| let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source); |
| self.cfg.push_assign(block, source_info, ref_for_guard, rvalue); |
| } |
| ByRef::Yes(mutbl) => { |
| let value_for_arm = self.storage_live_binding( |
| block, |
| binding.var_id, |
| binding.span, |
| OutsideGuard, |
| schedule_drops, |
| ); |
| |
| let rvalue = |
| Rvalue::Ref(re_erased, util::ref_pat_borrow_kind(mutbl), 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<Item = &'b Binding<'tcx>>, |
| 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.0 { |
| ByRef::No => Rvalue::Use(self.consume_by_copy_or_move(binding.source)), |
| ByRef::Yes(mutbl) => { |
| Rvalue::Ref(re_erased, util::ref_pat_borrow_kind(mutbl), 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, |
| name: Symbol, |
| mode: BindingMode, |
| var_id: LocalVarId, |
| var_ty: Ty<'tcx>, |
| user_ty: UserTypeProjections, |
| has_guard: ArmHasGuard, |
| opt_match_place: Option<(Option<Place<'tcx>>, Span)>, |
| pat_span: Span, |
| ) { |
| let tcx = self.tcx; |
| let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope }; |
| let local = LocalDecl { |
| mutability: mode.1, |
| ty: var_ty, |
| user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) }, |
| source_info, |
| local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(BindingForm::Var( |
| VarBindingForm { |
| binding_mode: 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()), |
| composite: None, |
| argument_index: None, |
| }); |
| 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: Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, var_ty), |
| user_ty: None, |
| source_info, |
| local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User( |
| BindingForm::RefForGuard, |
| ))), |
| }); |
| self.var_debug_info.push(VarDebugInfo { |
| name, |
| source_info: debug_source_info, |
| value: VarDebugInfoContents::Place(ref_for_guard.into()), |
| composite: None, |
| argument_index: None, |
| }); |
| 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_id: ExprId, |
| initializer_span: Span, |
| else_block: BlockId, |
| let_else_scope: ®ion::Scope, |
| pattern: &Pat<'tcx>, |
| ) -> BlockAnd<BasicBlock> { |
| 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_id, initializer_span)); |
| let pat = Pat { ty: pattern.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, |
| &scrutinee, |
| 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.as_slice(), |
| 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.as_slice(), |
| initializer_span, |
| None, |
| true, |
| ); |
| |
| // If branch coverage is enabled, record this branch. |
| this.visit_coverage_conditional_let(pattern, matching, failure); |
| |
| this.break_for_else(failure, this.source_info(initializer_span)); |
| matching.unit() |
| }); |
| matching.and(failure) |
| } |
| } |