match_single_binding is false positive on temporaries

[MikailBag]

Lint name: match_single_binding

I tried this code:

fn main() {
    match format_args!("hello {}", "world") {
        f => dbg!(f),
    };
}

(playground link)

Clippy prints a warning:

warning: this match could be written as a `let` statement

With a proposal:

let f = format_args!("hello {}", "world");
dbg!(f);

But suggested code does not compile, because f is bound to a temporary value and its lifetime ends before dbg!.

Meta

• cargo clippy -V: 0.1.51 (2021-01-20 a4cbb44)
• rustc -Vv:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-unknown-linux-gnu
release: 1.49.0

[camsteffen]

I tried to fix this with HIR and ScopeTree to no avail. I think this will require MIR analysis. Of course we could special-case format_args! but I think that is too rare of a use-case for that.

[Fishrock123]

Just ran into this with todo!(), which seems very silly:

warning: this match could be replaced by its body itself
  --> src/thing.rs:19:9
   |
19 | /         match event.action() {
20 | |             _ => todo!("make thing do"),
21 | |         }
   | |_________^ help: consider using the match body instead: `todo!("make thing do")`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding

[camsteffen]

I don't think it would be worth it to special-case todo! since that indicates incomplete code by definition.

[Fishrock123]

Oh, on re-reading this maybe this is a separate issue. I think it should not warn in the case of todo!. I think it would be worth it, that should be highlighted in different ways.

[bgeron]

I just ran into the same issue.

To be honest — matching with a single match arm is a bit silly, let is normally meant for that. And if you put a block in the match, now you have two indentation levels!

match format_args!("hello {}", "world") {
    f => {
        dbg!(f),
        dbg!(f),
    }
}

It seems nice to have different syntax for this use case. For instance:

let f = format_args!("hello {}", "world") in {
    dbg!(f),
    dbg!(f),
};

But the lint needs a fix irrespective of any new syntax.