warning for assignment to temporary

[no111u3]

For simple useless code:

    fn main() {
        (0, ).0 = 1;
    }

Compiler not generate any warnings/errors, but clippy generate one warning:

warning: assignment to temporary
 --> src/main.rs:4:5
  |
4 |     (0, ).0 = 1;
  |     ^^^^^^^^^^^
  |

I think this warning need to implement in compiler too.

[sanxiyn]

FYI, this is temporary_assignment lint. rust-lang/rust#53224 is the general issue for upstreaming Clippy lints. (temporary_assignment is not included in that issue, but it probably should be?)

[flip1995]

The rust issue only looks at correctness lints. temporary_assignment is a complexity lint. Doing something like this cannot result in wrong code, since you can't use the structure you temporary assigned a value to (The assignment operator = always returns () in Rust, so a = b = 1, will result in a = () and b = 1*).

So this is valid rust code, that does not have any influence on the correctness of your program. So this doesn't qualify for a rustc lint, because for such cases Clippy exists.

---

* This seems that it may cause errors, but if you would try to use a = () as an integer somewhere in your program, this would be caught by the rust type system, so we're good here.

[sanxiyn]

I very much disagree in that Rust's standard unused_variables lint is not a correctness issue either. It does make sense to prioritize correctness lints though.

[flip1995]

I agree, that rust has lints, that aren't really correctness concerns (most of them are allow-by-default though). But on the other hand, the rust compiler still has a pretty strict "no new lints" rule, except, there are really good reasons to add a new lint. That's why Clippy exists.

---

As a site note: unused_variables can lead to errors:

let similar_name1 = 10;
let similar_nameI = 11; // unused_variable

assert_eq!(similar_name1, 11); // panic

That is a very constructed example, but it does happen in actual code. (BTW, Clippy has a lint for this: https://rust-lang.github.io/rust-clippy/master/index.html#similar_names)