Struct Builder

struct Builder<'a, 'tcx> {

    tcx: TyCtxt<'tcx>,
    infcx: InferCtxt<'tcx>,
    region_scope_tree: &'tcx ScopeTree,
    param_env: ParamEnv<'tcx>,
    thir: &'a Thir<'tcx>,
    cfg: CFG<'tcx>,
    def_id: LocalDefId,
    hir_id: HirId,
    parent_module: DefId,
    check_overflow: bool,
    fn_span: Span,
    arg_count: usize,
    coroutine: Option<Box<CoroutineInfo<'tcx>>>,
    scopes: Scopes<'tcx>,
    block_context: BlockContext,
    source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
    source_scope: SourceScope,
    guard_context: Vec<GuardFrame>,
    fixed_temps: FxHashMap<ExprId, Local>,
    fixed_temps_scope: Option<Scope>,
    var_indices: FxHashMap<LocalVarId, LocalsForNode>,
    local_decls: IndexVec<Local, LocalDecl<'tcx>>,
    canonical_user_type_annotations: CanonicalUserTypeAnnotations<'tcx>,
    upvars: SortedIndexMultiMap<usize, HirId, Capture<'tcx>>,
    unit_temp: Option<Place<'tcx>>,
    var_debug_info: Vec<VarDebugInfo<'tcx>>,
    lint_level_roots_cache: GrowableBitSet<ItemLocalId>,
    coverage_info: Option<CoverageInfoBuilder>,
}

Fields

tcx: TyCtxt<'tcx>

infcx: InferCtxt<'tcx>

region_scope_tree: &'tcx ScopeTree

param_env: ParamEnv<'tcx>

thir: &'a Thir<'tcx>

cfg: CFG<'tcx>

def_id: LocalDefId

hir_id: HirId

parent_module: DefId

check_overflow: bool

fn_span: Span

arg_count: usize

coroutine: Option<Box<CoroutineInfo<'tcx>>>

scopes: Scopes<'tcx>

The current set of scopes, updated as we traverse; see the scope module for more details.

block_context: BlockContext

The block-context: each time we build the code within an thir::Block, we push a frame here tracking whether we are building a statement or if we are pushing the tail expression of the block. This is used to embed information in generated temps about whether they were created for a block tail expression or not.

It would be great if we could fold this into self.scopes somehow, but right now I think that is very tightly tied to the code generation in ways that we cannot (or should not) start just throwing new entries onto that vector in order to distinguish the context of EXPR1 from the context of EXPR2 in { STMTS; EXPR1 } + EXPR2.

source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>

The vector of all scopes that we have created thus far; we track this for debuginfo later.

source_scope: SourceScope

guard_context: Vec<GuardFrame>

The guard-context: each time we build the guard expression for a match arm, we push onto this stack, and then pop when we finish building it.

fixed_temps: FxHashMap<ExprId, Local>

Temporaries with fixed indexes. Used so that if-let guards on arms with an or-pattern are only created once.

fixed_temps_scope: Option<Scope>

Scope of temporaries that should be deduplicated using Self::fixed_temps.

var_indices: FxHashMap<LocalVarId, LocalsForNode>

Maps HirIds of variable bindings to the Locals created for them. (A match binding can have two locals; the 2nd is for the arm’s guard.)

local_decls: IndexVec<Local, LocalDecl<'tcx>>

canonical_user_type_annotations: CanonicalUserTypeAnnotations<'tcx>

upvars: SortedIndexMultiMap<usize, HirId, Capture<'tcx>>

unit_temp: Option<Place<'tcx>>

var_debug_info: Vec<VarDebugInfo<'tcx>>

lint_level_roots_cache: GrowableBitSet<ItemLocalId>

coverage_info: Option<CoverageInfoBuilder>

Collects additional coverage information during MIR building. Only present if coverage is enabled and this function is eligible.

Implementations

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn ast_block( &mut self, destination: Place<'tcx>, block: BasicBlock, ast_block: BlockId, source_info: SourceInfo, ) -> BlockAnd<()>

fn ast_block_stmts( &mut self, destination: Place<'tcx>, block: BasicBlock, span: Span, stmts: &[StmtId], expr: Option<ExprId>, region_scope: Scope, ) -> BlockAnd<()>

impl Builder<'_, '_>

pub(crate) fn visit_coverage_branch_operation( &mut self, logical_op: LogicalOp, span: Span, )

pub(crate) fn mcdc_increment_depth_if_enabled(&mut self)

pub(crate) fn mcdc_decrement_depth_if_enabled(&mut self)

impl<'tcx> Builder<'_, 'tcx>

pub(crate) fn visit_coverage_standalone_condition( &mut self, expr_id: ExprId, place: Place<'tcx>, block: &mut BasicBlock, )

If condition coverage is enabled, inject extra blocks and marker statements that will let us track the value of the condition in place.

pub(crate) fn visit_coverage_branch_condition( &mut self, expr_id: ExprId, then_block: BasicBlock, else_block: BasicBlock, )

If branch coverage is enabled, inject marker statements into then_block and else_block, and record their IDs in the table of branch spans.

pub(crate) fn visit_coverage_conditional_let( &mut self, pattern: &Pat<'tcx>, true_block: BasicBlock, false_block: BasicBlock, )

If branch coverage is enabled, inject marker statements into true_block and false_block, and record their IDs in the table of branches.

Used to instrument let-else and if-let (including let-chains) for branch coverage.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn as_constant(&mut self, expr: &Expr<'tcx>) -> ConstOperand<'tcx>

Compile expr, yielding a compile-time constant. Assumes that expr is a valid compile-time constant!

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn as_local_operand( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<Operand<'tcx>>

Returns an operand suitable for use until the end of the current scope expression.

The operand returned from this function will not be valid after the current enclosing ExprKind::Scope has ended, so please do not return it from functions to avoid bad miscompiles.

pub(crate) fn as_local_call_operand( &mut self, block: BasicBlock, expr: ExprId, ) -> BlockAnd<Operand<'tcx>>

Returns an operand suitable for use until the end of the current scope expression and suitable also to be passed as function arguments.

The operand returned from this function will not be valid after an ExprKind::Scope is passed, so please do not return it from functions to avoid bad miscompiles. Returns an operand suitable for use as a call argument. This is almost always equivalent to as_operand, except for the particular case of passing values of (potentially) unsized types “by value” (see details below).

The operand returned from this function will not be valid after the current enclosing ExprKind::Scope has ended, so please do not return it from functions to avoid bad miscompiles.

Parameters of unsized types

We tweak the handling of parameters of unsized type slightly to avoid the need to create a local variable of unsized type. For example, consider this program:

#![feature(unsized_locals, unsized_fn_params)]
fn foo(p: dyn Debug) { dbg!(p); }

fn bar(box_p: Box<dyn Debug>) { foo(*box_p); }

Ordinarily, for sized types, we would compile the call foo(*p) like so:

let tmp0 = *box_p; // tmp0 would be the operand returned by this function call
foo(tmp0)

But because the parameter to foo is of the unsized type dyn Debug, and because it is being moved the deref of a box, we compile it slightly differently. The temporary tmp0 that we create stores the entire box, and the parameter to the call itself will be *tmp0:

let tmp0 = box_p; call foo(*tmp0)

This way, the temporary tmp0 that we create has type Box<dyn Debug>, which is sized. The value passed to the call (*tmp0) still has the dyn Debug type – but the way that calls are compiled means that this parameter will be passed “by reference”, meaning that we will actually provide a pointer to the interior of the box, and not move the dyn Debug value to the stack.

See #68304 for more details.

pub(crate) fn as_operand( &mut self, block: BasicBlock, scope: Option<Scope>, expr_id: ExprId, local_info: LocalInfo<'tcx>, needs_temporary: NeedsTemporary, ) -> BlockAnd<Operand<'tcx>>

Compile expr into a value that can be used as an operand. If expr is a place like x, this will introduce a temporary tmp = x, so that we capture the value of x at this time.

If we end up needing to create a temporary, then we will use local_info as its LocalInfo, unless as_temporary has already assigned it a non-None LocalInfo. Normally, you should use None for local_info

The operand is known to be live until the end of scope.

Like as_local_call_operand, except that the argument will not be valid once scope ends.

pub(crate) fn as_call_operand( &mut self, block: BasicBlock, scope: Option<Scope>, expr_id: ExprId, ) -> BlockAnd<Operand<'tcx>>

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn as_place( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<Place<'tcx>>

Compile expr, yielding a place that we can move from etc.

WARNING: Any user code might:

• Invalidate any slice bounds checks performed.
• Change the address that this Place refers to.
• Modify the memory that this place refers to.
• Invalidate the memory that this place refers to, this will be caught by borrow checking.

Extra care is needed if any user code is allowed to run between calling this method and using it, as is the case for match and index expressions.

pub(crate) fn as_place_builder( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<PlaceBuilder<'tcx>>

This is used when constructing a compound Place, so that we can avoid creating intermediate Place values until we know the full set of projections.

pub(crate) fn as_read_only_place( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<Place<'tcx>>

Compile expr, yielding a place that we can move from etc. Mutability note: The caller of this method promises only to read from the resulting place. The place itself may or may not be mutable:

• If this expr is a place expr like a.b, then we will return that place.
• Otherwise, a temporary is created: in that event, it will be an immutable temporary.

fn as_read_only_place_builder( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<PlaceBuilder<'tcx>>

This is used when constructing a compound Place, so that we can avoid creating intermediate Place values until we know the full set of projections. Mutability note: The caller of this method promises only to read from the resulting place. The place itself may or may not be mutable:

• If this expr is a place expr like a.b, then we will return that place.
• Otherwise, a temporary is created: in that event, it will be an immutable temporary.

fn expr_as_place( &mut self, block: BasicBlock, expr_id: ExprId, mutability: Mutability, fake_borrow_temps: Option<&mut Vec<Local>>, ) -> BlockAnd<PlaceBuilder<'tcx>>

fn lower_captured_upvar( &mut self, block: BasicBlock, closure_def_id: LocalDefId, var_hir_id: LocalVarId, ) -> BlockAnd<PlaceBuilder<'tcx>>

Lower a captured upvar. Note we might not know the actual capture index, so we create a place starting from PlaceBase::Upvar, which will be resolved once all projections that allow us to identify a capture have been applied.

fn lower_index_expression( &mut self, block: BasicBlock, base: ExprId, index: ExprId, mutability: Mutability, fake_borrow_temps: Option<&mut Vec<Local>>, temp_lifetime: Option<Scope>, expr_span: Span, source_info: SourceInfo, ) -> BlockAnd<PlaceBuilder<'tcx>>

Lower an index expression

This has two complications;

• We need to do a bounds check.
• We need to ensure that the bounds check can’t be invalidated using an expression like x[1][{x = y; 2}]. We use fake borrows here to ensure that this is the case.

fn bounds_check( &mut self, block: BasicBlock, slice: &PlaceBuilder<'tcx>, index: Local, expr_span: Span, source_info: SourceInfo, ) -> BasicBlock

fn add_fake_borrows_of_base( &mut self, base_place: Place<'tcx>, block: BasicBlock, fake_borrow_temps: &mut Vec<Local>, expr_span: Span, source_info: SourceInfo, )

fn read_fake_borrows( &mut self, bb: BasicBlock, fake_borrow_temps: &mut Vec<Local>, source_info: SourceInfo, )

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn as_local_rvalue( &mut self, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<Rvalue<'tcx>>

Returns an rvalue suitable for use until the end of the current scope expression.

The operand returned from this function will not be valid after an ExprKind::Scope is passed, so please do not return it from functions to avoid bad miscompiles.

pub(crate) fn as_rvalue( &mut self, block: BasicBlock, scope: Option<Scope>, expr_id: ExprId, ) -> BlockAnd<Rvalue<'tcx>>

Compile expr, yielding an rvalue.

pub(crate) fn build_binary_op( &mut self, block: BasicBlock, op: BinOp, span: Span, ty: Ty<'tcx>, lhs: Operand<'tcx>, rhs: Operand<'tcx>, ) -> BlockAnd<Rvalue<'tcx>>

fn build_zero_repeat( &mut self, block: BasicBlock, value: ExprId, scope: Option<Scope>, outer_source_info: SourceInfo, ) -> BlockAnd<Rvalue<'tcx>>

fn limit_capture_mutability( &mut self, upvar_span: Span, upvar_ty: Ty<'tcx>, temp_lifetime: Option<Scope>, block: BasicBlock, arg: ExprId, ) -> BlockAnd<Operand<'tcx>>

fn neg_1_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx>

fn minval_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx>

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn as_temp( &mut self, block: BasicBlock, temp_lifetime: Option<Scope>, expr_id: ExprId, mutability: Mutability, ) -> BlockAnd<Local>

Compile expr into a fresh temporary. This is used when building up rvalues so as to freeze the value that will be consumed.

fn as_temp_inner( &mut self, block: BasicBlock, temp_lifetime: Option<Scope>, expr_id: ExprId, mutability: Mutability, ) -> BlockAnd<Local>

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn expr_into_dest( &mut self, destination: Place<'tcx>, block: BasicBlock, expr_id: ExprId, ) -> BlockAnd<()>

Compile expr, storing the result into destination, which is assumed to be uninitialized.

fn is_let(&self, expr: ExprId) -> bool

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn stmt_expr( &mut self, block: BasicBlock, expr_id: ExprId, statement_scope: Option<Scope>, ) -> BlockAnd<()>

Builds a block of MIR statements to evaluate the THIR expr.

The statement_scope is used if a statement temporary must be dropped.

impl<'a, 'tcx> Builder<'a, 'tcx>

fn field_match_pairs<'pat>( &mut self, place: PlaceBuilder<'tcx>, subpatterns: &'pat [FieldPat<'tcx>], ) -> Vec<MatchPairTree<'pat, 'tcx>>

Builds and returns MatchPairTree subtrees, one for each pattern in subpatterns, representing the fields of a PatKind::Variant or PatKind::Leaf.

Used internally by MatchPairTree::for_pattern.

fn prefix_slice_suffix<'pat>( &mut self, match_pairs: &mut Vec<MatchPairTree<'pat, 'tcx>>, place: &PlaceBuilder<'tcx>, prefix: &'pat [Box<Pat<'tcx>>], opt_slice: &'pat Option<Box<Pat<'tcx>>>, suffix: &'pat [Box<Pat<'tcx>>], )

Builds MatchPairTree subtrees for the prefix/middle/suffix parts of an array pattern or slice pattern, and adds those trees to match_pairs.

Used internally by MatchPairTree::for_pattern.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(super) fn simplify_match_pairs<'pat>( &mut self, match_pairs: &mut Vec<MatchPairTree<'pat, 'tcx>>, extra_data: &mut PatternExtraData<'tcx>, )

Simplify a list of match pairs so they all require a test. Stores relevant bindings and ascriptions in extra_data.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(super) fn pick_test_for_match_pair<'pat>( &mut self, match_pair: &MatchPairTree<'pat, 'tcx>, ) -> Test<'tcx>

Identifies what test is needed to decide if match_pair is applicable.

It is a bug to call this with a not-fully-simplified pattern.

pub(super) fn perform_test( &mut self, match_start_span: Span, scrutinee_span: Span, block: BasicBlock, otherwise_block: BasicBlock, place: Place<'tcx>, test: &Test<'tcx>, target_blocks: FxIndexMap<TestBranch<'tcx>, BasicBlock>, )

pub(super) fn call_deref( &mut self, block: BasicBlock, target_block: BasicBlock, place: Place<'tcx>, mutability: Mutability, ty: Ty<'tcx>, temp: Place<'tcx>, span: Span, )

Perform let temp = <ty as Deref>::deref(&place). or let temp = <ty as DerefMut>::deref_mut(&mut place).

fn compare( &mut self, block: BasicBlock, success_block: BasicBlock, fail_block: BasicBlock, source_info: SourceInfo, op: BinOp, left: Operand<'tcx>, right: Operand<'tcx>, )

Compare using the provided built-in comparison operator

fn non_scalar_compare( &mut self, block: BasicBlock, success_block: BasicBlock, fail_block: BasicBlock, source_info: SourceInfo, value: Const<'tcx>, val: Place<'tcx>, ty: Ty<'tcx>, )

Compare two values using <T as std::compare::PartialEq>::eq. If the values are already references, just call it directly, otherwise take a reference to the values first and then call it.

pub(super) fn sort_candidate( &mut self, test_place: Place<'tcx>, test: &Test<'tcx>, candidate: &mut Candidate<'_, 'tcx>, sorted_candidates: &FxIndexMap<TestBranch<'tcx>, Vec<&mut Candidate<'_, 'tcx>>>, ) -> Option<TestBranch<'tcx>>

Given that we are performing test against test_place, this job sorts out what the status of candidate will be after the test. See test_candidates for the usage of this function. The candidate may be modified to update its match_pairs.

So, for example, if this candidate is x @ Some(P0) and the Test is a variant test, then we would modify the candidate to be (x as Option).0 @ P0 and return the index corresponding to the variant Some.

However, in some cases, the test may just not be relevant to candidate. For example, suppose we are testing whether foo.x == 22, but in one match arm we have Foo { x: _, ... }… in that case, the test for the value of x has no particular relevance to this candidate. In such cases, this function just returns None without doing anything. This is used by the overall match_candidates algorithm to structure the match as a whole. See match_candidates for more details.

FIXME(#29623). In some cases, we have some tricky choices to make. for example, if we are testing that x == 22, but the candidate is x @ 13..55, what should we do? In the event that the test is true, we know that the candidate applies, but in the event of false, we don’t know that it doesn’t apply. For now, we return false, indicate that the test does not apply to this candidate, but it might be we can get tighter match code if we do something a bit different.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn false_edges( &mut self, from_block: BasicBlock, real_target: BasicBlock, imaginary_target: BasicBlock, source_info: SourceInfo, )

Creates a false edge to imaginary_target and a real edge to real_target. If imaginary_target is none, or is the same as the real target, a Goto is generated instead to simplify the generated MIR.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn then_else_break( &mut self, block: BasicBlock, expr_id: ExprId, temp_scope_override: Option<Scope>, variable_source_info: SourceInfo, declare_let_bindings: DeclareLetBindings, ) -> BlockAnd<()>

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.

fn then_else_break_inner( &mut self, block: BasicBlock, expr_id: ExprId, args: ThenElseArgs, ) -> BlockAnd<()>

pub(crate) fn match_expr( &mut self, destination: Place<'tcx>, block: BasicBlock, scrutinee_id: ExprId, arms: &[ArmId], span: Span, scrutinee_span: Span, ) -> BlockAnd<()>

Generates MIR for a match expression.

The MIR that we generate for a match looks like this.

[ 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):

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;
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();
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 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:

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.

fn lower_scrutinee( &mut self, block: BasicBlock, scrutinee_id: ExprId, scrutinee_span: Span, ) -> BlockAnd<PlaceBuilder<'tcx>>

Evaluate the scrutinee and add the PlaceMention for it.

fn lower_match_arms<'pat>( &mut self, destination: Place<'tcx>, scrutinee_place_builder: PlaceBuilder<'tcx>, scrutinee_span: Span, arms: impl IntoIterator<Item = &'pat Arm<'tcx>>, built_match_tree: BuiltMatchTree<'tcx>, outer_source_info: SourceInfo, ) -> BlockAnd<()>

where 'tcx: 'pat,

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 bind_pattern( &mut self, outer_source_info: SourceInfo, branch: MatchTreeBranch<'tcx>, fake_borrow_temps: &[(Place<'tcx>, Local, FakeBorrowKind)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, Scope)>, emit_storage_live: EmitStorageLive, ) -> BasicBlock

For a top-level match arm or a let binding, binds the variables and ascribes types, and also checks the match arm guard (if present).

arm_scope should be Some if and only if this is called for a match arm.

In the presence of or-patterns, a match arm might have multiple sub-branches representing different ways to match, with each sub-branch requiring its own bindings and its own copy of the guard. This method handles those sub-branches individually, and then has them jump together to a common block.

Returns a single block that the match arm can be lowered into. (For let bindings, this is the code that can use the bindings.)

pub(super) fn expr_into_pattern( &mut self, block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer_id: ExprId, ) -> BlockAnd<()>

pub(crate) fn place_into_pattern( &mut self, block: BasicBlock, irrefutable_pat: &Pat<'tcx>, initializer: PlaceBuilder<'tcx>, set_match_place: bool, ) -> BlockAnd<()>

pub(crate) fn declare_bindings( &mut self, visibility_scope: Option<SourceScope>, scope_span: Span, pattern: &Pat<'tcx>, guard: Option<ExprId>, opt_match_place: Option<(Option<&Place<'tcx>>, Span)>, ) -> Option<SourceScope>

Declares the bindings of the given patterns and returns the visibility scope for the bindings in these patterns, if such a scope had to be created. NOTE: Declaring the bindings should always be done in their drop scope.

pub(crate) fn declare_guard_bindings( &mut self, guard_expr: ExprId, scope_span: Span, visibility_scope: Option<SourceScope>, )

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 storage_live_binding( &mut self, block: BasicBlock, var: LocalVarId, span: Span, for_guard: ForGuard, schedule_drop: ScheduleDrops, ) -> Place<'tcx>

Emits a StatementKind::StorageLive for the given var, and also schedules a drop if requested (and possible).

pub(crate) fn schedule_drop_for_binding( &mut self, var: LocalVarId, span: Span, for_guard: ForGuard, )

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), )

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.

impl<'a, 'tcx> Builder<'a, 'tcx>

fn lower_match_tree<'pat>( &mut self, block: BasicBlock, scrutinee_span: Span, scrutinee_place_builder: &PlaceBuilder<'tcx>, match_start_span: Span, patterns: Vec<(&'pat Pat<'tcx>, HasMatchGuard)>, refutable: bool, ) -> BuiltMatchTree<'tcx>

where 'tcx: 'pat,

The entrypoint of the matching algorithm. Create the decision tree for the match expression, starting from block.

patterns is a list of patterns, one for each arm. The associated boolean indicates whether the arm has a guard.

refutable indicates whether the candidate list is refutable (for if let and let else) or not (for let and match). In the refutable case we return the block to which we branch on failure.

fn match_candidates( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, candidates: &mut [&mut Candidate<'_, 'tcx>], ) -> BasicBlock

The main match algorithm. It begins with a set of candidates candidates and has the job of generating code that branches to an appropriate block if the scrutinee matches one of these candidates. The candidates are ordered 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 none of the candidates apply, we continue to the returned otherwise_block.

Note that while match expressions in the Rust language are exhaustive, candidate lists passed to this method are often non-exhaustive. For example, the match lowering process will frequently divide up the list of candidates, and recursively call this method with a non-exhaustive subset of candidates. See Builder::test_candidates for more details on this “backtracking automata” approach.

For an example of how we use otherwise_block, consider:

match (x, y) {
    (true, true) => 1,
    (_, false) => 2,
    (false, true) => 3,
}

For this match, we generate something like:

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,
        // but we don't know that from within the lowering algorithm.
        // 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 so we accept non-optimal code paths.

fn match_candidates_inner( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, candidates: &mut [&mut Candidate<'_, 'tcx>], ) -> BasicBlock

Construct the decision tree for candidates. Don’t call this, call match_candidates instead to reserve sufficient stack space.

fn select_matched_candidate( &mut self, candidate: &mut Candidate<'_, 'tcx>, start_block: BasicBlock, ) -> BasicBlock

Link up matched candidates.

For example, if we have something like this:

...
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.

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.

fn expand_and_match_or_candidates<'pat, 'b, 'c>( &mut self, span: Span, scrutinee_span: Span, start_block: BasicBlock, candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], ) -> BlockAnd<&'b mut [&'c mut Candidate<'pat, 'tcx>]>

Takes a list of candidates such that some of the candidates’ first match pairs are or-patterns. This expands as many or-patterns as possible and processes the resulting candidates. Returns the unprocessed candidates if any.

fn create_or_subcandidates<'pat>( &mut self, candidate: &mut Candidate<'pat, 'tcx>, match_pair: MatchPairTree<'pat, 'tcx>, )

Given a match-pair that corresponds to an or-pattern, expand each subpattern into a new subcandidate. Any candidate that has been expanded this way should also be postprocessed at the end of Self::expand_and_match_or_candidates.

fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>)

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 candidate should have been expanded with create_or_subcandidates.

Given a pattern (P | Q, R | S) we (in principle) generate a CFG like so:

[ 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:

    ...
     +---------------+------------+
     |               |            |
[ P matches ] [ Q matches ] [ otherwise ]
     |               |            |
     +---------------+            |
     |                           ...
[ match R, S ]
     |
    ...

Note that this takes place after the subcandidates have participated in match tree lowering.

fn remove_never_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>)

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.

fn test_remaining_match_pairs_after_or( &mut self, span: Span, scrutinee_span: Span, candidate: &mut Candidate<'_, 'tcx>, )

If more match pairs remain, test them after each subcandidate. We could have added them to the or-candidates during or-pattern expansion, but that would make it impossible to detect simplifiable or-patterns. That would guarantee exponentially large CFGs for cases like (1 | 2, 3 | 4, ...).

fn pick_test( &mut self, candidates: &[&mut Candidate<'_, 'tcx>], ) -> (Place<'tcx>, Test<'tcx>)

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.

fn sort_candidates<'b, 'c, 'pat>( &mut self, match_place: Place<'tcx>, test: &Test<'tcx>, candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], ) -> (&'b mut [&'c mut Candidate<'pat, 'tcx>], FxIndexMap<TestBranch<'tcx>, Vec<&'b mut Candidate<'pat, 'tcx>>>)

Given a test, we partition the input candidates into several buckets. If a candidate matches in exactly one of the branches of test (and no other branches), we put it into the corresponding bucket. If it could match in more than one of the branches of test, the test doesn’t usefully apply to it, and we stop partitioning candidates.

Importantly, we also mutate the branched candidates to remove match pairs that are entailed by the outcome of the test, and add any sub-pairs of the removed pairs.

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.

For example:

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. Conceptually, there are 2 overlapping candidate sets:

• If the outcome is that x is true, candidates {0, 2, 3} are possible
• If the outcome is that x is false, candidates {1, 2} are possible

Following our algorithm:

• Candidate 0 is sorted into outcome x == true
• Candidate 1 is sorted into outcome x == false
• Candidate 2 remains unsorted, because testing x has no effect on it
• Candidate 3 remains unsorted, because a previous candidate (2) was unsorted
  • This helps preserve the illusion that candidates are tested “in order”

The sorted candidates are mutated to remove entailed match pairs:

• 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 test_candidates<'pat, 'b, 'c>( &mut self, span: Span, scrutinee_span: Span, candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>], start_block: BasicBlock, ) -> BlockAnd<&'b mut [&'c mut Candidate<'pat, 'tcx>]>

This is the most subtle part of the match lowering algorithm. At this point, there are no fully-satisfied candidates, and no or-patterns to expand, so we actually need to perform some sort of test to make progress.

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 list of (modified) candidates. If a candidate matches in exactly one branch of our test, we add it to the corresponding outcome. We also mutate its list of match pairs if appropriate, to reflect 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:

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:

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 Candidates 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:

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:

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!(),
}

We return the unprocessed candidates.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn lower_let_expr( &mut self, block: BasicBlock, expr_id: ExprId, pat: &Pat<'tcx>, source_scope: Option<SourceScope>, scope_span: Span, declare_let_bindings: DeclareLetBindings, emit_storage_live: EmitStorageLive, ) -> BlockAnd<()>

Lowers a let expression that appears in a suitable context (e.g. an if condition or match guard).

Also used for lowering let-else statements, since they have similar needs despite not actually using let expressions.

Use DeclareLetBindings to control whether the let bindings are declared or not.

fn bind_and_guard_matched_candidate( &mut self, sub_branch: MatchTreeSubBranch<'tcx>, fake_borrows: &[(Place<'tcx>, Local, FakeBorrowKind)], scrutinee_span: Span, arm_match_scope: Option<(&Arm<'tcx>, Scope)>, schedule_drops: ScheduleDrops, emit_storage_live: EmitStorageLive, ) -> BasicBlock

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 ascribe_types( &mut self, block: BasicBlock, ascriptions: impl IntoIterator<Item = Ascription<'tcx>>, )

Append AscribeUserType statements onto the end of block for each ascription

fn bind_matched_candidate_for_guard<'b>( &mut self, block: BasicBlock, schedule_drops: ScheduleDrops, bindings: impl IntoIterator<Item = &'b Binding<'tcx>>, )

where 'tcx: 'b,

Binding for guards is a bit different from binding for the arm body, because an extra layer of implicit reference/dereference is added.

The idea is that 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), any occurrence of the identifier in the guard expression will automatically get a deref op applied to it. (See the caller of Self::is_bound_var_in_guard.)

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) }, ... }

---

Implementation notes

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 Self::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 Self::bind_matched_candidate_for_arm_body.

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.

---

See these PRs for some historical context:

• https://github.com/rust-lang/rust/pull/49870 (introduction of autoref)
• https://github.com/rust-lang/rust/pull/59114 (always use autoref)

fn bind_matched_candidate_for_arm_body<'b>( &mut self, block: BasicBlock, schedule_drops: ScheduleDrops, bindings: impl IntoIterator<Item = &'b Binding<'tcx>>, emit_storage_live: EmitStorageLive, )

where 'tcx: 'b,

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, )

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.

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn temp(&mut self, ty: Ty<'tcx>, span: Span) -> Place<'tcx>

Adds a new temporary value of type ty storing the result of evaluating expr.

N.B., No cleanup is scheduled for this temporary. You should call schedule_drop once the temporary is initialized.

pub(crate) fn literal_operand( &mut self, span: Span, const_: Const<'tcx>, ) -> Operand<'tcx>

Convenience function for creating a literal operand, one without any user type annotation.

pub(crate) fn zero_literal(&mut self, span: Span, ty: Ty<'tcx>) -> Operand<'tcx>

Returns a zero literal operand for the appropriate type, works for bool, char and integers.

pub(crate) fn push_usize( &mut self, block: BasicBlock, source_info: SourceInfo, value: u64, ) -> Place<'tcx>

pub(crate) fn consume_by_copy_or_move( &self, place: Place<'tcx>, ) -> Operand<'tcx>

impl<'a, 'tcx> Builder<'a, 'tcx>

pub(crate) fn in_breakable_scope<F>( &mut self, loop_block: Option<BasicBlock>, break_destination: Place<'tcx>, span: Span, f: F, ) -> BlockAnd<()>

where F: FnOnce(&mut Builder<'a, 'tcx>) -> Option<BlockAnd<()>>,

Start a breakable scope, which tracks where continue, break and return should branch to.

pub(crate) fn in_if_then_scope<F>( &mut self, region_scope: Scope, span: Span, f: F, ) -> (BasicBlock, BasicBlock)

where F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<()>,

Start an if-then scope which tracks drop for if expressions and if guards.

For an if-let chain:

if let Some(x) = a && let Some(y) = b && let Some(z) = c { … }

There are three possible ways the condition can be false and we may have to drop x, x and y, or neither depending on which binding fails. To handle this correctly we use a DropTree in a similar way to a loop expression and ‘break’ out on all of the ‘else’ paths.

Notes:

• We don’t need to keep a stack of scopes in the Builder because the ‘else’ paths will only leave the innermost scope.
• This is also used for match guards.

pub(crate) fn in_scope<F, R>( &mut self, region_scope: (Scope, SourceInfo), lint_level: LintLevel, f: F, ) -> BlockAnd<R>

where F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>,

Convenience wrapper that pushes a scope and then executes f to build its contents, popping the scope afterwards.

pub(crate) fn push_scope(&mut self, region_scope: (Scope, SourceInfo))

Push a scope onto the stack. You can then build code in this scope and call pop_scope afterwards. Note that these two calls must be paired; using in_scope as a convenience wrapper maybe preferable.

pub(crate) fn pop_scope( &mut self, region_scope: (Scope, SourceInfo), block: BasicBlock, ) -> BlockAnd<()>

Pops a scope, which should have region scope region_scope, adding any drops onto the end of block that are needed. This must match 1-to-1 with push_scope.

pub(crate) fn break_scope( &mut self, block: BasicBlock, value: Option<ExprId>, target: BreakableTarget, source_info: SourceInfo, ) -> BlockAnd<()>

Sets up the drops for breaking from block to target.

pub(crate) fn break_for_else( &mut self, block: BasicBlock, source_info: SourceInfo, )

Sets up the drops for breaking from block due to an if condition that turned out to be false.

Must be called in the context of Builder::in_if_then_scope, so that there is an if-then scope to tell us what the target scope is.

pub(crate) fn break_for_tail_call( &mut self, block: BasicBlock, args: &[Spanned<Operand<'tcx>>], source_info: SourceInfo, ) -> BlockAnd<()>

Sets up the drops for explicit tail calls.

Unlike other kinds of early exits, tail calls do not go through the drop tree. Instead, all scheduled drops are immediately added to the CFG.

fn leave_top_scope(&mut self, block: BasicBlock) -> BasicBlock

pub(crate) fn maybe_new_source_scope( &mut self, span: Span, current_id: HirId, parent_id: HirId, )

Possibly creates a new source scope if current_root and parent_root are different, or if -Zmaximal-hir-to-mir-coverage is enabled.

fn maybe_lint_level_root_bounded(&mut self, orig_id: HirId) -> HirId

Walks upwards from orig_id to find a node which might change lint levels with attributes. It stops at self.hir_id and just returns it if reached.

pub(crate) fn new_source_scope( &mut self, span: Span, lint_level: LintLevel, ) -> SourceScope

Creates a new source scope, nested in the current one.

pub(crate) fn source_info(&self, span: Span) -> SourceInfo

Given a span and the current source scope, make a SourceInfo.

pub(crate) fn local_scope(&self) -> Scope

Returns the scope that we should use as the lifetime of an operand. Basically, an operand must live until it is consumed. This is similar to, but not quite the same as, the temporary scope (which can be larger or smaller).

Consider:

let x = foo(bar(X, Y));

We wish to pop the storage for X and Y after bar() is called, not after the whole let is completed.

As another example, if the second argument diverges:

foo(Box::new(2), panic!())

We would allocate the box but then free it on the unwinding path; we would also emit a free on the ‘success’ path from panic, but that will turn out to be removed as dead-code.

pub(crate) fn schedule_drop_storage_and_value( &mut self, span: Span, region_scope: Scope, local: Local, )

pub(crate) fn schedule_drop( &mut self, span: Span, region_scope: Scope, local: Local, drop_kind: DropKind, )

Indicates that place should be dropped on exit from region_scope.

When called with DropKind::Storage, place shouldn’t be the return place, or a function parameter.

pub(crate) fn record_operands_moved( &mut self, operands: &[Spanned<Operand<'tcx>>], )

Indicates that the “local operand” stored in local is moved at some point during execution (see local_scope for more information about what a “local operand” is – in short, it’s an intermediate operand created as part of preparing some MIR instruction). We use this information to suppress redundant drops on the non-unwind paths. This results in less MIR, but also avoids spurious borrow check errors (c.f. #64391).

Example: when compiling the call to foo here:

foo(bar(), ...)

we would evaluate bar() to an operand _X. We would also schedule _X to be dropped when the expression scope for foo(bar()) is exited. This is relevant, for example, if the later arguments should unwind (it would ensure that _X gets dropped). However, if no unwind occurs, then _X will be unconditionally consumed by the call:

bb {
  ...
  _R = CALL(foo, _X, ...)
}

However, _X is still registered to be dropped, and so if we do nothing else, we would generate a DROP(_X) that occurs after the call. This will later be optimized out by the drop-elaboration code, but in the meantime it can lead to spurious borrow-check errors – the problem, ironically, is not the DROP(_X) itself, but the (spurious) unwind pathways that it creates. See #64391 for an example.

fn diverge_cleanup(&mut self) -> DropIdx

Returns the DropIdx for the innermost drop if the function unwound at this point. The DropIdx will be created if it doesn’t already exist.

fn diverge_cleanup_target(&mut self, target_scope: Scope, span: Span) -> DropIdx

This is similar to diverge_cleanup except its target is set to some ancestor scope instead of the current scope. It is possible to unwind to some ancestor scope if some drop panics as the program breaks out of a if-then scope.

pub(crate) fn diverge_from(&mut self, start: BasicBlock)

Prepares to create a path that performs all required cleanup for a terminator that can unwind at the given basic block.

This path terminates in Resume. The path isn’t created until after all of the non-unwind paths in this item have been lowered.

pub(crate) fn coroutine_drop_cleanup(&mut self, yield_block: BasicBlock)

Sets up a path that performs all required cleanup for dropping a coroutine, starting from the given block that ends in TerminatorKind::Yield.

This path terminates in CoroutineDrop.

pub(crate) fn build_drop_and_replace( &mut self, block: BasicBlock, span: Span, place: Place<'tcx>, value: Rvalue<'tcx>, ) -> BlockAnd<()>

Utility function for non-scope code to build their own drops Force a drop at this point in the MIR by creating a new block.

pub(crate) fn assert( &mut self, block: BasicBlock, cond: Operand<'tcx>, expected: bool, msg: AssertMessage<'tcx>, span: Span, ) -> BasicBlock

Creates an Assert terminator and return the success block. If the boolean condition operand is not the expected value, a runtime panic will be caused with the given message.

pub(crate) fn clear_top_scope(&mut self, region_scope: Scope)

Unschedules any drops in the top scope.

This is only needed for match arm scopes, because they have one entrance per pattern, but only one exit.

impl<'a, 'tcx: 'a> Builder<'a, 'tcx>

fn build_exit_tree( &mut self, drops: DropTree, else_scope: Scope, span: Span, continue_block: Option<BasicBlock>, ) -> Option<BlockAnd<()>>

Build a drop tree for a breakable scope.

If continue_block is Some, then the tree is for continue inside a loop. Otherwise this is for break or return.

pub(crate) fn build_drop_trees(&mut self)

Build the unwind and coroutine drop trees.

fn build_coroutine_drop_trees(&mut self)

fn build_unwind_tree( cfg: &mut CFG<'tcx>, drops: &mut DropTree, fn_span: Span, resume_block: &mut Option<BasicBlock>, )

impl<'a, 'tcx> Builder<'a, 'tcx>

fn is_bound_var_in_guard(&self, id: LocalVarId) -> bool

fn var_local_id(&self, id: LocalVarId, for_guard: ForGuard) -> Local

impl<'a, 'tcx> Builder<'a, 'tcx>

fn new( thir: &'a Thir<'tcx>, infcx: InferCtxt<'tcx>, def: LocalDefId, hir_id: HirId, span: Span, arg_count: usize, return_ty: Ty<'tcx>, return_span: Span, coroutine: Option<Box<CoroutineInfo<'tcx>>>, ) -> Builder<'a, 'tcx>

fn finish(self) -> Body<'tcx>

fn insert_upvar_arg(&mut self)

fn args_and_body( &mut self, block: BasicBlock, arguments: &IndexSlice<ParamId, Param<'tcx>>, argument_scope: Scope, expr_id: ExprId, ) -> BlockAnd<()>

fn set_correct_source_scope_for_arg( &mut self, arg_hir_id: HirId, original_source_scope: SourceScope, pattern_span: Span, )

fn get_unit_temp(&mut self) -> Place<'tcx>

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 1600 bytes