Improve error message

[gui1117]

@pkhry I tried to improve the error message, I just print all variant and there index, if there is a formula it will just be the formula expression without the evaluation but I still think it can be enough information for debugging, WDYT?

[pkhry]

Lgtm

[bkchr]

And then we can generate an even better error message here :P

[gui1117]

I thought it was impossible, but it is. Done in 876a14a

Also are we ok adding a dependency to const_format, if not I can always write the concatenation of strings inlined.
(convertion to array, then calculate the new length in const, then instantiate the new array and copy content + plus special code to format u8).

[bkchr]

Suggested change

	let msg = #crate_path::__private::concatcp!(
	"Found variants that have duplicate indexes. Both `",
	indices[DUP_INFO.1].1,
	"` and `",
	indices[DUP_INFO.2].1,
	"` have the index `",
	indices[DUP_INFO.1].0,
	"`. Use different indexes for each variant."
	);
	::core::panic!("{}", msg);
	::core::panic!("Found variants that have duplicate indexes. Both `{}` and `{}` have index `{}`. Each variant is required to have an unique index.",
	indices[DUP_INFO.1].1,
	indices[DUP_INFO.2].1,
	indices[DUP_INFO.1].0,
	);

DOesn't that work? :D

[gui1117]

I wish constant evaluation gets a bit more love from rustc developers.

But formatting macro doesn't actually work at compile-time:

		const _: () = {
		    const FOO: &str = "a";
		    const BAR: &str = "b";
			panic!("{}{}", FOO, BAR);
		};

error[E0015]: cannot call non-const formatting macro in constants
 --> src/lib.rs:4:12
  |
4 |             panic!("{}{}", FOO, BAR);
  |                     ^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants
  = note: this error originates in the macro `$crate::const_format_args` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)