not all c style formatting specifiers are detected

[lcnr]

i've tried

fn main() {
    let num = 0x0abcde;
    let width = 6;
    print!("%0*x", width, num);
}

i expected:

error: multiple unused formatting arguments
 --> src/main.rs:4:19
  |
4 |     print!("%0*x", width, num);
  |            -----  ^^^^^  ^^^ argument never used
  |            ||     |
  |            ||     argument never used
  |            |help: format specifiers use curly braces: `{1:00$x}`
  |            multiple missing formatting specifiers
  |
  = note: printf formatting not supported; see the documentation for `std::fmt`

the actual error message does not contain such a help message:

error: multiple unused formatting arguments
 --> src/main.rs:4:20
  |
4 |     print!("%0*x", width, num);
  |            ------  ^^^^^  ^^^ argument never used
  |            |       |
  |            |       argument never used
  |            multiple missing formatting specifiers

[lcnr]

note that the following does suggest {:06x}

fn main() {
    let num = 0x0abcde;
    print!("%06x", num);
}

[jamesmunns]

Is there a typo in your snippet?

print!("%0*x", width, num);

The * looks out of place to me, even for printf formatting. It's also not matched in your "expected" output (the * is not there).

[lcnr]

no, * is used to get the width specifier from the arguments, which is why the equivalent rust specifier would be {1:00$x} here. see https://rust.godbolt.org/z/MvdsPTWTa for working c code

fixed my expected output though, copied a similar example where it does work

[jamesmunns]

Wow! Thanks for explaining, TIL (in both Rust and C!)