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Change destructuring assignment lowering to use match. #96588

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Change destructuring assignment lowering to use match. #96588

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27 changes: 11 additions & 16 deletions compiler/rustc_ast_lowering/src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -938,7 +938,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
}

/// Destructure the LHS of complex assignments.
/// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
/// For instance, lower `(a, b) = t` to `match t { (lhs1, lhs2) => { a = lhs1; b = lhs2; } }`.
fn lower_expr_assign(
&mut self,
lhs: &Expr,
Expand Down Expand Up @@ -979,22 +979,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
let rhs = self.lower_expr(rhs);

// Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
let destructure_let = self.stmt_let_pat(
None,
whole_span,
Some(rhs),
pat,
hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
);
// Block of statements for the assignments: `{ a = lhs1; b = lhs2; }`.
let stmts = self.arena.alloc_from_iter(assignments);
let block = self.block_all(whole_span, stmts, None);
let assignments_block = self.arena.alloc(self.expr_block(block, ThinVec::new()));

// `a = lhs1; b = lhs2;`.
let stmts = self
.arena
.alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));

// Wrap everything in a block.
hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
// Produce the `match` for destructuring: `match t { (lhs1, lhs2) => ... }
hir::ExprKind::Match(
rhs,
arena_vec![self; self.arm(pat, assignments_block)],
hir::MatchSource::Normal, // FIXME: is `Normal` fine here?
)
}

/// If the given expression is a path to a tuple struct, returns that path.
Expand Down