Use `if let` instead of `if matches!`

[GideonBear]

What it does

Warn against use of if matches! where it could be written as if let. I don't know in which cases this is or isn't safe.

This should probably be a pedantic lint (see drawbacks).

Advantage

• More readable
• Shorter
• More idiomatic

Drawbacks

• The pattern and the value are switched around, which might've been done intentionally for readability.

Example

enum E {
    A,
    B,
}

fn main() {
    let x = E::A;
    if matches!(x, E::A) {
        println!("It's an A");
    }
}

Could be written as:

enum E {
    A,
    B,
}

fn main() {
    let x = E::A;
    if let E::A = x {
        println!("It's an A");
    }
}

And:

enum E {
    A(u32),
    B,
}

fn main() {
    let x = E::A(1);
    if matches!(x, E::A(n) if n == 1) {
        println!("It's an A(1)");
    }
}

Could be written as:

enum E {
    A(u32),
    B,
}

fn main() {
    let x = E::A(1);
    if let E::A(n) = x && n == 1 {
        println!("It's an A(1)");
    }
}

(As long as let chains are available in the current Rust version)

Comparison with existing lints

No response

Additional Context

No response

[samueltardieu]

This would probably be a restriction lint, as the goal is to restrict the use of if matches!(…). And some people prefer not to use if let if there no binding being created and used after the if let expression, so the reverse lint could also be created.

[Taym95]

@rustbot claim

[sagudev]

We need to be sure there is no variable use in the block:

let n = 5;
if matches!(x, E::A(n) if n == 1) {
        println!("It's an A(1)");
        dbg!(n); // prints 5
}

but

let n = 5;
if let E::A(n) = x && n == 1 {
        println!("It's an A(1)");
        dbg!(n); // prints 1
}