questionable write_literal warning: write!(f, "{}", "{integer}")

[matthiaskrgr]

Lint name: write_literal

I tried this code:

use std::fmt;

struct S {}

impl fmt::Display for S {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", "{integer}")
    }
}

fn main() {
    println!("{}", S {});
}

warning: literal with an empty format string
 --> src/main.rs:7:25
  |
7 |         write!(f, "{}", "{integer}")
  |                         ^^^^^^^^^^^
  |
  = note: `#[warn(clippy::write_literal)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#write_literal

In this code we use dummy format string {} to insert the string literal "{integer}" because if we had
write!(f, "{ìnteger}"), the {integer} would not be parsed as a raw literal(?):

error: there is no argument named `integer`
 --> src/main.rs:7:20
  |
7 |         write!(f, "{integer}")
  |                    ^^^^^^^^^

This is quite an interesting edge-case :)

I'm not sure what the best solution is here, if we can somehow provide a better suggestion or should just ignore literals that enclose anything in curly braces.

Meta

• cargo clippy -V: clippy 0.1.52 (9b471a3 2021-02-19)
• rustc -Vv:

rustc 1.52.0-nightly (9b471a3 2021-02-19)
binary: rustc
commit-hash: 9b471a3
commit-date: 2021-02-19
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

[ghost]

Unless I misunderstood, I can (and prefer to) escape the curly braces instead (playground):

use std::fmt;

struct S {}

impl fmt::Display for S {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{{integer}}")
    }
}

fn main() {
    println!("{}", S {});
}

So... please do not "ignore literals that enclose anything in curly braces", at least provide a way to opt-in to the warning.

[matthiaskrgr]

Ah, I forgot about the escaping with double curly braces!
Shall I close this again? :)

[camsteffen]

No, this is still a false positive. I think this is related to pre_expansion_pass.