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use crate::build::ForGuard::OutsideGuard;
use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
use rustc_middle::middle::region::Scope;
use rustc_middle::thir::*;
use rustc_middle::{mir::*, ty};
use rustc_span::Span;
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<()> {
let Block { region_scope, span, ref stmts, expr, targeted_by_break, safety_mode: _ } =
self.thir[ast_block];
self.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
if targeted_by_break {
this.in_breakable_scope(None, destination, span, |this| {
Some(this.ast_block_stmts(destination, block, span, stmts, expr, region_scope))
})
} else {
this.ast_block_stmts(destination, block, span, stmts, expr, region_scope)
}
})
}
fn ast_block_stmts(
&mut self,
destination: Place<'tcx>,
mut block: BasicBlock,
span: Span,
stmts: &[StmtId],
expr: Option<ExprId>,
region_scope: Scope,
) -> BlockAnd<()> {
let this = self;
// This convoluted structure is to avoid using recursion as we walk down a list
// of statements. Basically, the structure we get back is something like:
//
// let x = <init> in {
// expr1;
// let y = <init> in {
// expr2;
// expr3;
// ...
// }
// }
//
// The let bindings are valid till the end of block so all we have to do is to pop all
// the let-scopes at the end.
//
// First we build all the statements in the block.
let mut let_scope_stack = Vec::with_capacity(8);
let outer_source_scope = this.source_scope;
// This scope information is kept for breaking out of the parent remainder scope in case
// one let-else pattern matching fails.
// By doing so, we can be sure that even temporaries that receive extended lifetime
// assignments are dropped, too.
let mut last_remainder_scope = region_scope;
let source_info = this.source_info(span);
for stmt in stmts {
let Stmt { ref kind } = this.thir[*stmt];
match kind {
StmtKind::Expr { scope, expr } => {
this.block_context.push(BlockFrame::Statement { ignores_expr_result: true });
let si = (*scope, source_info);
unpack!(
block = this.in_scope(si, LintLevel::Inherited, |this| {
this.stmt_expr(block, *expr, Some(*scope))
})
);
}
StmtKind::Let {
remainder_scope,
init_scope,
pattern,
initializer: Some(initializer),
lint_level,
else_block: Some(else_block),
span: _,
} => {
// When lowering the statement `let <pat> = <expr> else { <else> };`,
// the `<else>` block is nested in the parent scope enclosing this statement.
// That scope is usually either the enclosing block scope,
// or the remainder scope of the last statement.
// This is to make sure that temporaries instantiated in `<expr>` are dropped
// as well.
// In addition, even though bindings in `<pat>` only come into scope if
// the pattern matching passes, in the MIR building the storages for them
// are declared as live any way.
// This is similar to `let x;` statements without an initializer expression,
// where the value of `x` in this example may or may be assigned,
// because the storage for their values may not be live after all due to
// failure in pattern matching.
// For this reason, we declare those storages as live but we do not schedule
// any drop yet- they are scheduled later after the pattern matching.
// The generated MIR will have `StorageDead` whenever the control flow breaks out
// of the parent scope, regardless of the result of the pattern matching.
// However, the drops are inserted in MIR only when the control flow breaks out of
// the scope of the remainder scope associated with this `let .. else` statement.
// Pictorial explanation of the scope structure:
// ┌─────────────────────────────────┐
// │ Scope of the enclosing block, │
// │ or the last remainder scope │
// │ ┌───────────────────────────┐ │
// │ │ Scope for <else> block │ │
// │ └───────────────────────────┘ │
// │ ┌───────────────────────────┐ │
// │ │ Remainder scope of │ │
// │ │ this let-else statement │ │
// │ │ ┌─────────────────────┐ │ │
// │ │ │ <expr> scope │ │ │
// │ │ └─────────────────────┘ │ │
// │ │ extended temporaries in │ │
// │ │ <expr> lives in this │ │
// │ │ scope │ │
// │ │ ┌─────────────────────┐ │ │
// │ │ │ Scopes for the rest │ │ │
// │ │ └─────────────────────┘ │ │
// │ └───────────────────────────┘ │
// └─────────────────────────────────┘
// Generated control flow:
// │ let Some(x) = y() else { return; }
// │
// ┌────────▼───────┐
// │ evaluate y() │
// └────────┬───────┘
// │ ┌────────────────┐
// ┌────────▼───────┐ │Drop temporaries│
// │Test the pattern├──────►in y() │
// └────────┬───────┘ │because breaking│
// │ │out of <expr> │
// ┌────────▼───────┐ │scope │
// │Move value into │ └───────┬────────┘
// │binding x │ │
// └────────┬───────┘ ┌───────▼────────┐
// │ │Drop extended │
// ┌────────▼───────┐ │temporaries in │
// │Drop temporaries│ │<expr> because │
// │in y() │ │breaking out of │
// │because breaking│ │remainder scope │
// │out of <expr> │ └───────┬────────┘
// │scope │ │
// └────────┬───────┘ ┌───────▼────────┐
// │ │Enter <else> ├────────►
// ┌────────▼───────┐ │block │ return;
// │Continue... │ └────────────────┘
// └────────────────┘
let ignores_expr_result = matches!(pattern.kind, PatKind::Wild);
this.block_context.push(BlockFrame::Statement { ignores_expr_result });
// Lower the `else` block first because its parent scope is actually
// enclosing the rest of the `let .. else ..` parts.
let else_block_span = this.thir[*else_block].span;
// This place is not really used because this destination place
// should never be used to take values at the end of the failure
// block.
let dummy_place = this.temp(this.tcx.types.never, else_block_span);
let failure_entry = this.cfg.start_new_block();
let failure_block;
unpack!(
failure_block = this.ast_block(
dummy_place,
failure_entry,
*else_block,
this.source_info(else_block_span),
)
);
this.cfg.terminate(
failure_block,
this.source_info(else_block_span),
TerminatorKind::Unreachable,
);
// Declare the bindings, which may create a source scope.
let remainder_span = remainder_scope.span(this.tcx, this.region_scope_tree);
this.push_scope((*remainder_scope, source_info));
let_scope_stack.push(remainder_scope);
let visibility_scope =
Some(this.new_source_scope(remainder_span, LintLevel::Inherited));
let initializer_span = this.thir[*initializer].span;
let scope = (*init_scope, source_info);
let failure = unpack!(
block = this.in_scope(scope, *lint_level, |this| {
this.declare_bindings(
visibility_scope,
remainder_span,
pattern,
None,
Some((Some(&destination), initializer_span)),
);
this.visit_primary_bindings(
pattern,
UserTypeProjections::none(),
&mut |this, _, _, node, span, _, _| {
this.storage_live_binding(
block,
node,
span,
OutsideGuard,
true,
);
},
);
this.ast_let_else(
block,
*initializer,
initializer_span,
*else_block,
&last_remainder_scope,
pattern,
)
})
);
this.cfg.goto(failure, source_info, failure_entry);
if let Some(source_scope) = visibility_scope {
this.source_scope = source_scope;
}
last_remainder_scope = *remainder_scope;
}
StmtKind::Let { init_scope, initializer: None, else_block: Some(_), .. } => {
span_bug!(
init_scope.span(this.tcx, this.region_scope_tree),
"initializer is missing, but else block is present in this let binding",
)
}
StmtKind::Let {
remainder_scope,
init_scope,
ref pattern,
initializer,
lint_level,
else_block: None,
span: _,
} => {
let ignores_expr_result = matches!(pattern.kind, PatKind::Wild);
this.block_context.push(BlockFrame::Statement { ignores_expr_result });
// Enter the remainder scope, i.e., the bindings' destruction scope.
this.push_scope((*remainder_scope, source_info));
let_scope_stack.push(remainder_scope);
// Declare the bindings, which may create a source scope.
let remainder_span = remainder_scope.span(this.tcx, this.region_scope_tree);
let visibility_scope =
Some(this.new_source_scope(remainder_span, LintLevel::Inherited));
// Evaluate the initializer, if present.
if let Some(init) = *initializer {
let initializer_span = this.thir[init].span;
let scope = (*init_scope, source_info);
unpack!(
block = this.in_scope(scope, *lint_level, |this| {
this.declare_bindings(
visibility_scope,
remainder_span,
pattern,
None,
Some((None, initializer_span)),
);
this.expr_into_pattern(block, &pattern, init)
// irrefutable pattern
})
)
} else {
let scope = (*init_scope, source_info);
unpack!(this.in_scope(scope, *lint_level, |this| {
this.declare_bindings(
visibility_scope,
remainder_span,
pattern,
None,
None,
);
block.unit()
}));
debug!("ast_block_stmts: pattern={:?}", pattern);
this.visit_primary_bindings(
pattern,
UserTypeProjections::none(),
&mut |this, _, _, node, span, _, _| {
this.storage_live_binding(block, node, span, OutsideGuard, true);
this.schedule_drop_for_binding(node, span, OutsideGuard);
},
)
}
// Enter the visibility scope, after evaluating the initializer.
if let Some(source_scope) = visibility_scope {
this.source_scope = source_scope;
}
last_remainder_scope = *remainder_scope;
}
}
let popped = this.block_context.pop();
assert!(popped.is_some_and(|bf| bf.is_statement()));
}
// Then, the block may have an optional trailing expression which is a “return” value
// of the block, which is stored into `destination`.
let tcx = this.tcx;
let destination_ty = destination.ty(&this.local_decls, tcx).ty;
if let Some(expr_id) = expr {
let expr = &this.thir[expr_id];
let tail_result_is_ignored =
destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
this.block_context
.push(BlockFrame::TailExpr { tail_result_is_ignored, span: expr.span });
unpack!(block = this.expr_into_dest(destination, block, expr_id));
let popped = this.block_context.pop();
assert!(popped.is_some_and(|bf| bf.is_tail_expr()));
} else {
// If a block has no trailing expression, then it is given an implicit return type.
// This return type is usually `()`, unless the block is diverging, in which case the
// return type is `!`. For the unit type, we need to actually return the unit, but in
// the case of `!`, no return value is required, as the block will never return.
// Opaque types of empty bodies also need this unit assignment, in order to infer that their
// type is actually unit. Otherwise there will be no defining use found in the MIR.
if destination_ty.is_unit()
|| matches!(destination_ty.kind(), ty::Alias(ty::Opaque, ..))
{
// We only want to assign an implicit `()` as the return value of the block if the
// block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
this.cfg.push_assign_unit(block, source_info, destination, this.tcx);
}
}
// Finally, we pop all the let scopes before exiting out from the scope of block
// itself.
for scope in let_scope_stack.into_iter().rev() {
unpack!(block = this.pop_scope((*scope, source_info), block));
}
// Restore the original source scope.
this.source_scope = outer_source_scope;
block.unit()
}
}