Mixing the two types of positional specifiers does not always produce an error

[bhaible]

The "Limitations" section at https://crates.io/crates/formatx says: "Intermingling the two types of positional specifiers isn't supported."
and "Examples given below will always panic."

But mixing unnumbered and numbered positional specifiers does not produce an error.

How to reproduce:

use formatx::formatx;

fn identity (s: &str) -> &str
{
  return s;
}

fn main ()
{
  println!("{}", format!("{} {2} {} {1} {4} {} {3} {5} {6}", "zero", "one", "two", "three", "four", "five", "six"));
  println!("{}", formatx!(identity("{} {2} {} {1} {4} {} {3} {5} {6}"), "zero", "one", "two", "three", "four", "five", "six", "seven", "eight").unwrap());
}

Running this program produces:

zero two one one four two three five six
zero two seven one four eight three five six

Which means:

• For the built-in format!, the first {} accesses argument 0, the second {} accesses argument 1, and so on. (Like it's documented in https://doc.rust-lang.org/std/fmt/ .)
• For formatx!, the first {} accesses the lowest-numbered arguments that is not consumed by the numbered directives, the second {} accesses the second-to-lowest-numbered such argument, and so on.

It would be good if formatx! would either produce an error in such cases (so that everyone knows it's really unsupported) or would use the same numbering scheme as the built-in format!.

[clitic]

Thanks for mentioning this issue. formatx does have a very basic approach to handle positional arguments and I do not want to complicate things. Since formatx is unable to see upcoming arguments in advance an error could not be also raised as it does in format macro. So, I just mention this as a limitation of formatx.