Warn when match variables start with an uppercase letter

[kmcallister]

I had code like

match e.kind {
    EndOfFile => return Ok(()),
    _         => return Err(()),
}

and got error: unreachable pattern on the second pattern. I hadn't imported std::rt::io::EndOfFile so rustc interpreted EndOfFile as a variable rather than an enum constructor.

Most Rust code follows the stylistic rule of capitalizing enum constructors and not capitalizing variables. So I think rustc should warn when binding a capitalized variable in a match — at least when it makes another pattern unreachable, but probably always.

[nikomatsakis]

This has been proposed numerous times. I think it's a great idea. I couldn't find another issue though I'm sure one or two exists -- or maybe not. Here is a generalization: generate a warning if you create a variable that has the same name as a variant of the enum being matched (note that this variant name may be lower case). I actually think it'd possibly make sense to have both warnings, but checking for variables with the same name as an (unimported) variant seems more targeted.

[klutzy]

I like this suggestion (I've got similar problem several times), but I feel uppercase check is lint job. We already have unused variable lint so it should emit an warning for EndOfFile variable, but unreachable pattern error came first and stopped.

[huonw]

Couldn't we just go full-haskell and enforce* locals are snake_case (or mixedCase ... anything not starting with a capital) and statics and enum variants are UPPERCASE and/or CamelCase (well anything with an initial capital)?

This would solve #7526 too.

(* Possibly as lints on deny by default, but lints are only checked after the other errors happen.)

[pnkfelix]

cc @chris-morgan who was discussing related issues in the chat room yesterday. (I.e. the problem where users get surprised that a catch-all _ pattern is unreachable because it is preceded by a Var pattern that the user thought was already bound as a static.)

[pnkfelix]

Some of the previously filed issues for this are #4639 and #3070

[steveklabnik]

Triage: still no lint, still same error message.

[wesleywiser]

Is this still an issue? I tried the repro from #13995 and I got this

test.rs:9:9: 9:11 warning: pattern binding `V1` is named the same as one of the variants of the type `A::E` [E0170]
test.rs:9         V1 => 1,
                  ^~
test.rs:9:9: 9:11 help: run `rustc --explain E0170` to see a detailed explanation
test.rs:9:9: 9:11 help: if you meant to match on a variant, consider making the path in the pattern qualified: `A::E::V1`
test.rs:10:9: 10:11 warning: pattern binding `V2` is named the same as one of the variants of the type `A::E` [E0170]
test.rs:10         V2 => 2,
                   ^~
test.rs:10:9: 10:11 help: run `rustc --explain E0170` to see a detailed explanation
test.rs:10:9: 10:11 help: if you meant to match on a variant, consider making the path in the pattern qualified: `A::E::V2`
test.rs:10:9: 10:11 error: unreachable pattern [E0001]
test.rs:10         V2 => 2,
                   ^~
test.rs:10:9: 10:11 help: run `rustc --explain E0001` to see a detailed explanation
error: aborting due to previous error

[alexcrichton]

I also believe this is fixed, thanks @wesleywiser!