+= desugars wrong

[nikomatsakis]

If you run

class Point(var x, var y)

async fn main() {
    p = Point(22, 44)
    test(p).x += 1
    print(p).await
}

async fn test(p) {
    print("Hi").await
    p
}

you get a panic right now:

thread 'main' panicked at 'cannot force local variable origin to be synthesized', components/dada-validate/src/validate/validator.rs:771:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

In general, we need to improve this to desugar to

tmp = test(p).lease
tmp.x = tmp.x + 1

or something like that.

I bellieve that form should also work for e.g. foo.x += 1 (where you would get tmp = foo.lease)

[nikomatsakis]

General area of the code:

dada/components/dada-validate/src/validate/validator.rs

Lines 379 to 402 in 0089325

	syntax::ExprData::OpEq(lhs_expr, op, rhs_expr) => {
	let result = try {
	let (validated_opt_temp_expr, validated_lhs_place) =
	self.validate_expr_as_place(*lhs_expr)?;
	let validated_lhs_expr =
	self.add(validated::ExprData::Give(validated_lhs_place), expr);
	let validated_rhs_expr = self.validate_expr(*rhs_expr);
	let validated_op = self.validated_op(*op);
	let validated_op_expr = self.add(
	validated::ExprData::Op(
	validated_lhs_expr,
	validated_op,
	validated_rhs_expr,
	),
	expr,
	);
	let assign_expr = self.add(
	validated::ExprData::Assign(validated_lhs_place, validated_op_expr),
	expr,
	);
	self.maybe_seq(validated_opt_temp_expr, assign_expr, expr)
	};
	self.or_error(result, expr)
	}

[nikomatsakis]

I'm fixing this "en passane" in the modal harmony branch