0000-eq-statement-warning.md

• Feature Name: eq_statement_warning
• Start Date: 2016-12-07
• RFC PR: (leave this empty)
• Rust Issue: (leave this empty)

Summary

Warn by default when encountering a statement which only consists of an equality comparison.

Motivation

It is easy to accidentally write == when one actually meant =.

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
}

At the time of this RFC, no warning is produced for this code, making it easy not to notice this mistake. This can result in wasted time trying to debug a simple mistake.

We can rectify this by giving a warning to the user.

Detailed design

Add a new lint called eq_statement, which checks for statements that are of the form lhs == rhs;. This lint should warn by default.

The message should tell the user that the result of the equality comparison is not used. It should also hint that the user probably intended lhs = rhs. Optionally, it can also tell the user that they if they only want the side effects, they can explicitly express that with let _ = lhs == rhs;.

Drawbacks

This adds an additional lint to maintain.

False positives shouldn't be an issue, as let _ = rhs == rhs; expresses the same thing more explicitly.

Alternatives

Clippy already has a lint that warns about this, called no_effect.

It looks like this:

warning: statement with no effect, #[warn(no_effect)] on by default
 --> src/main.rs:7:9
  |
7 |         a == 30; // Oops, meant assignment
  |         ^^^^^^^^

However, not everyone uses clippy, and I believe this is a common enough mistake to justify including a lint for it in rustc itself.

Unresolved questions

None.