Improve error message for printf-style format strings

[FabianWolff]

Fixes #89173. The following is actually supported today:

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

    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

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[petrochenkov]

@bors r+

[bors]

📌 Commit 6490ed3 has been approved by petrochenkov