match with default branch that would better be let-else.

[ltratt]

What it does

I'm working on a moderately old codebase that has a number of instances of this idiom:

let z = match x {
  Y(z) => z,
  _ => panic!()
};

In other words: a match with exactly two arms, the first of which extracts "the only expected variant at this point in execution" and the other a default branch which does something (e.g. panics) if that expected variant isn't found.

This seems exactly what let-else is meant for i.e. the above would be better as:

let Y(z) = x else { panic!() };

Advantage

Advantages:

• if-let is terser than the match equivalent (sometimes collapsing 4 lines down to 1).
• I believe let-else, is now the expected idiom for "extract the only expected variant and if it's not found something has gone wrong".

Drawbacks

No response

Example

enum E {
  Y(z),
  A(b),
  C(d)
}

fn f() {
  let z = match x {
    Y(z) => z,
    _ => p
  };
  ...
}

Could be written as:

enum E {
  Y(z),
  A(b),
  C(d)
}

fn f() {
  let Y(z) = x else { p };
  ...
}

[y21]

This lint already exists: https://rust-lang.github.io/rust-clippy/master/index.html#/manual_let_else

warning: this could be rewritten as `let...else`
  --> src/main.rs:10:3
   |
10 | /   let z = match x {
11 | |     Y(z) => z,
12 | |     _ => todo!()
13 | |   };
   | |____^ help: consider writing: `let Y(z) = x else { todo!() };`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_let_else

Since it's in the pedantic category, it's allowed by default and needs to be manually enabled.

[ltratt]

You're quite right! I've checked and this does fire on most (though not all, for reasons I haven't yet understood) instances of the idiom in our codebase. FWIW this lint doesn't feel pedantic to me, but maybe I'm biased.

[GuillaumeGomez]

Then please open another issue to change the category. I think it should be in the style passes. Anyway, better to have an issue with a title related to what you suggest so you can attract more commenters. ;)