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//! 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<region::Scope>,
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::LogicalOp { op: LogicalOp::Or, lhs, rhs } => {
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(
block,
&this.thir[lhs],
temp_scope_override,
local_scope,
variable_source_info,
)
});
let rhs_success_block = unpack!(this.then_else_break(
failure_block,
&this.thir[rhs],
temp_scope_override,
break_scope,
variable_source_info,
));
this.cfg.goto(lhs_success_block, variable_source_info, rhs_success_block);
rhs_success_block.unit()
}
ExprKind::Unary { op: UnOp::Not, arg } => {
let local_scope = this.local_scope();
let (success_block, failure_block) =
this.in_if_then_scope(local_scope, expr_span, |this| {
this.then_else_break(
block,
&this.thir[arg],
temp_scope_override,
local_scope,
variable_source_info,
)
});
this.break_for_else(success_block, break_scope, 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(
block,
&this.thir[value],
temp_scope_override,
break_scope,
variable_source_info,
)
})
}
ExprKind::Use { source } => this.then_else_break(
block,
&this.thir[source],
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::<Vec<_>>();
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<PlaceBuilder<'tcx>> {
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 = <expr>
// ```
//
// We are creating an ascription that defines the type of `x` to be
// exactly `T` (i.e., with invariance). The variance field, in
// contrast, is intended to be used to relate `T` to the type of
// `<expr>`.
ty::Variance::Invariant,
),
},
);
self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
block.unit()
}
_ => {
let place_builder = unpack!(block = self.as_place_builder(block, initializer));
if let Some(place) = place_builder.try_to_place(self) {
let source_info = self.source_info(initializer.span);
self.cfg.push_place_mention(block, source_info, place);
}
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 next = Some(&candidate);
while let Some(candidate_ref) = next.take() {
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 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,
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<&Guard<'tcx>>,
opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
) -> Option<SourceScope> {
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, 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);
}
}
}
}
}
#[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<Binding<'tcx>>,
/// ...and these types asserted...
ascriptions: Vec<Ascription<'tcx>>,
/// ...and if this is non-empty, one of these subcandidates also has to match...
subcandidates: Vec<Candidate<'pat, 'tcx>>,
/// ...and the guard must be evaluated; if it's `false` then branch to `otherwise_block`.
otherwise_block: Option<BasicBlock>,
/// The block before the `bindings` have been established.
pre_binding_block: Option<BasicBlock>,
/// The pre-binding block of the next candidate.
next_candidate_pre_binding_block: Option<BasicBlock>,
}
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<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(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<VariantIdx>,
},
/// Test what value an integer, `bool`, or `char` has.
SwitchInt {
/// The type of the value that we're testing.
switch_ty: Ty<'tcx>,
/// The (ordered) set of values that we test for.
///
/// For integers and `char`s we create a branch to each of the values in
/// `options`, as well as an "otherwise" branch for all other values, even
/// in the (rare) case that `options` is exhaustive.
///
/// For `bool` we always generate two edges, one for `true` and one for
/// `false`.
options: FxIndexMap<Const<'tcx>, u128>,
},
/// 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 },
}
/// 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<BasicBlock>,
candidates: &mut [&mut Candidate<'pat, 'tcx>],
fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
) {
// 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<BasicBlock>,
candidates: &mut [&mut Candidate<'_, 'tcx>],
fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
) {
// 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<FxIndexSet<Place<'tcx>>>,
) -> Option<BasicBlock> {
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<BasicBlock>,
fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
) {
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<BasicBlock>,
pats: &'pat [Box<Pat<'tcx>>],
or_span: Span,
place: &PlaceBuilder<'tcx>,
fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
) {
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<BasicBlock>,
fake_borrows: &mut Option<FxIndexSet<Place<'tcx>>>,
) {
// extract the match-pair from the highest priority candidate
let match_pair = &candidates.first().unwrap().match_pairs[0];
let mut test = self.test(match_pair);
let match_place = match_pair.place.clone();
// most of the time, the test to perform is simply a function
// of the main candidate; but for a test like SwitchInt, we
// may want to add cases based on the candidates that are
// available
match test.kind {
TestKind::SwitchInt { switch_ty: _, ref mut options } => {
for candidate in candidates.iter() {
if !self.add_cases_to_switch(&match_place, candidate, 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<&mut Candidate<'pat, 'tcx>>> = vec![];
target_candidates.resize_with(test.targets(), Default::default);
let total_candidate_count = candidates.len();
// Sort the candidates into the appropriate vector in
// `target_candidates`. Note that at some point we may
// encounter a candidate where the test is not relevant; at
// that point, we stop sorting.
while let Some(candidate) = candidates.first_mut() {
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<BasicBlock> {
// 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<Place<'tcx>>,
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 =
Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
let mut fake_borrow_temp = LocalDecl::new(fake_borrow_ty, temp_span);
fake_borrow_temp.local_info = ClearCrossCrate::Set(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<SourceScope>,
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<Binding<'tcx>>, Vec<Ascription<'tcx>>)],
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<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 {
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<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 {
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<Place<'tcx>>, 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,
local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(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()),
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: &Expr<'tcx>,
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, 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)
}
}