use crate::build::matches::ArmHasGuard;
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;
&mut self,
destination: Place<'tcx>,
block: BasicBlock,
- ast_block: &Block,
+ ast_block: BlockId,
source_info: SourceInfo,
) -> BlockAnd<()> {
let Block {
expr,
targeted_by_break,
safety_mode,
- } = *ast_block;
+ } = self.thir[ast_block];
let expr = expr.map(|expr| &self.thir[expr]);
self.in_opt_scope(opt_destruction_scope.map(|de| (de, source_info)), move |this| {
this.in_scope((region_scope, source_info), LintLevel::Inherited, move |this| {
&stmts,
expr,
safety_mode,
+ region_scope,
))
})
} else {
- this.ast_block_stmts(destination, block, span, &stmts, expr, safety_mode)
+ this.ast_block_stmts(
+ destination,
+ block,
+ span,
+ &stmts,
+ expr,
+ safety_mode,
+ region_scope,
+ )
}
})
})
stmts: &[StmtId],
expr: Option<&Expr<'tcx>>,
safety_mode: BlockSafety,
+ region_scope: Scope,
) -> BlockAnd<()> {
let this = self;
let mut let_scope_stack = Vec::with_capacity(8);
let outer_source_scope = this.source_scope;
let outer_in_scope_unsafe = this.in_scope_unsafe;
+ // 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;
this.update_source_scope_for_safety_mode(span, safety_mode);
let source_info = this.source_info(span);
)
);
}
+ StmtKind::Let {
+ remainder_scope,
+ init_scope,
+ pattern,
+ initializer: Some(initializer),
+ lint_level,
+ else_block: Some(else_block),
+ } => {
+ // When lowering the statement `let <pat> = <expr> else { <else> };`,
+ // the `<else>` block is nested in the parent scope enclosing this statment.
+ // 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, None));
+
+ let init = &this.thir[*initializer];
+ let initializer_span = init.span;
+ this.declare_bindings(
+ visibility_scope,
+ remainder_span,
+ pattern,
+ ArmHasGuard(false),
+ Some((None, initializer_span)),
+ );
+ this.visit_primary_bindings(
+ pattern,
+ UserTypeProjections::none(),
+ &mut |this, _, _, _, node, span, _, _| {
+ this.storage_live_binding(block, node, span, OutsideGuard, true);
+ },
+ );
+ let failure = unpack!(
+ block = this.in_opt_scope(
+ opt_destruction_scope.map(|de| (de, source_info)),
+ |this| {
+ let scope = (*init_scope, source_info);
+ this.in_scope(scope, *lint_level, |this| {
+ this.ast_let_else(
+ block,
+ init,
+ 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,
+ else_block: None,
} => {
- let ignores_expr_result = matches!(*pattern.kind, PatKind::Wild);
+ 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| {
let scope = (*init_scope, source_info);
this.in_scope(scope, *lint_level, |this| {
- if let Some(else_block) = else_block {
- this.ast_let_else(
- block,
- init,
- initializer_span,
- else_block,
- visibility_scope,
- *remainder_scope,
- remainder_span,
- pattern,
- )
- } else {
- this.declare_bindings(
- visibility_scope,
- remainder_span,
- pattern,
- ArmHasGuard(false),
- Some((None, initializer_span)),
- );
- this.expr_into_pattern(block, pattern.clone(), init) // irrefutable pattern
- }
+ this.declare_bindings(
+ visibility_scope,
+ remainder_span,
+ pattern,
+ ArmHasGuard(false),
+ Some((None, initializer_span)),
+ );
+ this.expr_into_pattern(block, &pattern, init) // irrefutable pattern
})
},
)
if let Some(source_scope) = visibility_scope {
this.source_scope = source_scope;
}
+ last_remainder_scope = *remainder_scope;
}
}
projects / rustc.git / blobdiff