Lint using blocks in compound expression operators

[Aaron1011]

Consider the following code:

#[derive(Copy, Clone)]
struct MyAdd;

impl std::ops::AddAssign<MyAdd> for MyAdd {
    fn add_assign(&mut self, other: MyAdd) {}
}

fn main() {
    let lhs = &mut MyAdd;
    let rhs = MyAdd;
    *{println!("LHS"); lhs} += {println!("RHS"); rhs};
}

This code uses a block express as both the LHS and RHS of a compound assignment operator (+=). This code is not only difficult to read - its execution order depends on the type of the expression produced by the block (see rust-lang/rust#28160 and rust-lang/rust#61572).

As written, this code prints:

LHS
RHS

However, this nearly identical code:

fn main() {
    let lhs = &mut 25;
    let rhs = 30;
    *{println!("LHS"); lhs} += {println!("RHS"); rhs};
}

prints:

RHS
LHS

That is, the evaluation order of the LHS and RHS is dependent on whether or not the type used is a primitive or not.

Using a block expression with a compound assignment operator is a bad idea in and of itself - it's very difficult to read, and makes it unclear what is actually happening. However, the inconsistent evaluation order makes this even worse - a seemly unrelated change (e.g. changing the type of a variable earlier up in a function) can result in the execution order of other expressions changing.

It would be useful if Clippy were to lint the use of anything other than a variable, literal, or (chain of) method calls with a compound assignment operator.

[camsteffen]

I think the block on the LHS is more egregious than the one on the RHS.