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Make the lowering of thir::ExprKind::If easier to follow #122063

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merged 3 commits into from Mar 7, 2024
Merged

Make the lowering of thir::ExprKind::If easier to follow #122063

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7396fd1
Clarify how lowering `if` produces then/else blocks
Zalathar Mar 6, 2024
3402f39
Clarify lowering the `else` arm into the else block
Zalathar Mar 6, 2024
250e697
Additional comments for lowering `if`
Zalathar Mar 6, 2024
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81 changes: 45 additions & 36 deletions compiler/rustc_mir_build/src/build/expr/into.rs
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Expand Up @@ -58,52 +58,61 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
this.thir[scrutinee].span,
),
ExprKind::If { cond, then, else_opt, if_then_scope } => {
let then_blk;
let then_span = this.thir[then].span;
let then_source_info = this.source_info(then_span);
let condition_scope = this.local_scope();

let mut else_blk = unpack!(
then_blk = this.in_scope(
(if_then_scope, then_source_info),
LintLevel::Inherited,
|this| {
let source_info = if this.is_let(cond) {
let variable_scope =
this.new_source_scope(then_span, LintLevel::Inherited, None);
this.source_scope = variable_scope;
SourceInfo { span: then_span, scope: variable_scope }
} else {
this.source_info(then_span)
};
let (then_block, else_block) =
this.in_if_then_scope(condition_scope, then_span, |this| {
let then_blk = unpack!(this.then_else_break(
block,
cond,
Some(condition_scope), // Temp scope
condition_scope,
source_info,
true, // Declare `let` bindings normally
));

this.expr_into_dest(destination, then_blk, then)
});
then_block.and(else_block)
},
)
let then_and_else_blocks = this.in_scope(
(if_then_scope, then_source_info),
LintLevel::Inherited,
|this| {
// FIXME: Does this need extra logic to handle let-chains?
let source_info = if this.is_let(cond) {
let variable_scope =
this.new_source_scope(then_span, LintLevel::Inherited, None);
this.source_scope = variable_scope;
SourceInfo { span: then_span, scope: variable_scope }
} else {
this.source_info(then_span)
};

// Lower the condition, and have it branch into `then` and `else` blocks.
let (then_block, else_block) =
this.in_if_then_scope(condition_scope, then_span, |this| {
let then_blk = unpack!(this.then_else_break(
block,
cond,
Some(condition_scope), // Temp scope
condition_scope,
source_info,
true, // Declare `let` bindings normally
));

// Lower the `then` arm into its block.
this.expr_into_dest(destination, then_blk, then)
});

// Pack `(then_block, else_block)` into `BlockAnd<BasicBlock>`.
then_block.and(else_block)
},
);

else_blk = if let Some(else_opt) = else_opt {
unpack!(this.expr_into_dest(destination, else_blk, else_opt))
// Unpack `BlockAnd<BasicBlock>` into `(then_blk, else_blk)`.
let (then_blk, mut else_blk);
else_blk = unpack!(then_blk = then_and_else_blocks);

// If there is an `else` arm, lower it into `else_blk`.
if let Some(else_expr) = else_opt {
unpack!(else_blk = this.expr_into_dest(destination, else_blk, else_expr));
} else {
// Body of the `if` expression without an `else` clause must return `()`, thus
// we implicitly generate an `else {}` if it is not specified.
// There is no `else` arm, so we know both arms have type `()`.
// Generate the implicit `else {}` by assigning unit.
let correct_si = this.source_info(expr_span.shrink_to_hi());
this.cfg.push_assign_unit(else_blk, correct_si, destination, this.tcx);
else_blk
};
}

// The `then` and `else` arms have been lowered into their respective
// blocks, so make both of them meet up in a new block.
let join_block = this.cfg.start_new_block();
this.cfg.goto(then_blk, source_info, join_block);
this.cfg.goto(else_blk, source_info, join_block);
Expand Down