Named format arguments introduce implicit positional arguments

[Veetaha]

I tried this code:

fn main() {
    let a = "a";
    let b = "b";
    
    println!("{a} {b} {} {} {c} {}", c = "c");
}

I expected to see a compile error that says that there are missing positional arguments or that it is prohibited to mix positional and named parameters.

Instead, this compiles and prints the following:

a b c a c b

This code is available on the playground.

Meta

rustc --version --verbose:

rustc 1.58.1 (db9d1b20b 2022-01-20)
binary: rustc
commit-hash: db9d1b20bba1968c1ec1fc49616d4742c1725b4b
commit-date: 2022-01-20
host: x86_64-unknown-linux-gnu
release: 1.58.1
LLVM version: 13.0.0

[matthiaskrgr]

@rustbot prioritize

[m-ou-se]

Oh dear. We should fix this quickly, before any code starts depending on this.

[LunaBorowska]

println!("{c} {}", c = "c") appears to be valid since Rust 1.12.

[m-ou-se]

println!("{c} {}", c = "c") appears to be valid since Rust 1.12.

Yes, but that's not as bad, and arguably the right thing. The problem we should fix is the implicit captures getting used for {}.

[m-ou-se]

This should fix it: #93394