Consider warning by default for `foo == bar;` statements

[crumblingstatue]

One can make a mistake where they mistype = to ==.

Consider the following code:

fn main() {
    let mut a = 10;
    println!("A is {}", a);
    if 1 == 2 {
        a = 20;
    } else {
        a == 30; // Oops, meant assignment
    }
    println!("A is now {}", a); // Still 10
}

No warning is produced for this code.

I'd argue that this is a common enough mistake to justify warning by default for it.

If the user explicitly wants to do this operation for side effects or whatever, they can do let _ = a == b;, similarly to the explicit version for #[must_use] types.

[mbrubeck]

I think this is a good candidate for an on-by-default builtin lint:

• It catches a typo that is easy to make and difficult to spot, and that won't be caught by type checking.
• It can be very narrowly targeted, only matching statements of the form EXPR == EXPR;.
• False positives are unlikely, because == should rarely if ever have side effects, so it almost never makes sense to discard its result.

[crumblingstatue]

False positives are unlikely, because == should rarely if ever have side effects, so it almost never makes sense to discard its result.

We could also show a note/hint to use the more explicit let _ = a == b version in case the user wants to only evaluate side effects.

[nagisa]

I think this is a good candidate for an on-by-default builtin lint

Needs RFC.

[crumblingstatue]

Closing in favor of rust-lang/rfcs#1812.