New lint: Repetitive branches

What it does

Identifies code where each branch performs the same transformation on some value.

Categories (optional)

• Kind: clippy::style?

What is the advantage of the recommended code over the original code

It is less repetitive and often creates opportunities for further simplification.

Drawbacks

In some cases, the benefit may be negligible or may differ from stylistic preference.

Example

match n {
    1 => Ok("one")
    2 => Ok("two")
    3 => {
        // suppose there is more code here
        Ok("three")
    }
    _ => Ok("other")
}

Could be written as:

let s = match n {
    1 => "one"
    2 => "two"
    3 => {
        // suppose there is more code here
        "three"
    }
    _ => "other"
};
Ok(s)

This lint can apply to several constructs: match, if/else if/else, if let

The transformation could be a wrapper struct or enum or a function call.

The variable value should be a single value. Not, for example, a list of function arguments.

I'm not sure if this should be pedantic. Personally, I would probably want to #[warn] it.

Start by reviewing the proposed match and conditional examples in the issue, then determine how the lint should recognize a single transformed value across match, if/else, and if let constructs. Define the accepted transformation shapes and whether the lint belongs in the default or pedantic category. Done means the behavior and diagnostics are specified well enough to implement and test consistently.