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Improve error message for `printf`-style format strings Fixes #89173. The following is actually supported today: ```rust fn main() { let num = 5; let width = 20; print!("%*2$x", num, width); } ``` ``` error: multiple unused formatting arguments --> src/main.rs:4:21 | 4 | print!("%*2$x", num, width); | ------- ^^^ ^^^^^ argument never used | || | | || argument never used | |help: format specifiers use curly braces: `{:1$x}` | multiple missing formatting specifiers | = note: printf formatting not supported; see the documentation for `std::fmt` ``` However, as noted in #89173, something like ```rust print!("%0*x", width, num); ``` does not give a helpful suggestion. I think this is partly intended, because there actually _is_ no Rust equivalent to this; you always have to use a positional or named argument to specify the width (instead of just using the "next" argument, as `printf` or even `.*` as a precision specifier in Rust would). Therefore, I have added a note: ``` [...] note: format specifiers use curly braces, and you have to use a positional or named parameter for the width --> t2.rs:4:13 | 4 | print!("%0*x", width, num); | ^^^^ = note: printf formatting not supported; see the documentation for `std::fmt` ``` This is not perfect, but it should at least point the user in the right direction, instead of issuing no explanation at all. cc ```@lcnr```