New lint: prefer bool::then

[magurotuna]

What it does

Checks for if-else that could be rewritten to bool::then

Categories (optional)

• Kind: style restriction

Reduces redundancy and lines of code by leveraging a new stable API bool::then

Drawbacks

None.

Example

let a = if foo {
    println!("hello foo!");
    Some(bar())
} else {
    None
};

Could be written as:

let a = foo.then(|| {
    println!("hello foo!");
    bar()
});

Note that the following cannot be rewritten using bool::then because it has some action in else block:

let a = if foo {
    println!("hello foo!");
    Some(bar())
} else {
    println!("else!");
    None
};

[magurotuna]

I would like to implement it myself. If there's any concern or negative opinion on this lint, please let me know.
@rustbot claim

[camsteffen]

We definitely should have this. But I have a feeling that bool::then is "a little too functional" for some people and so it should not warn-by-default. So either pedantic or restriction. I would lean towards restriction.

[magurotuna]

Sounds reasonable. I'm in favor of choosing "restriction" too. I'll go with it.