Improve and investigate error messages about cxx-qt

[ahayzen-kdab]

https://blog.jetbrains.com/rust/2022/12/05/what-every-rust-developer-should-know-about-macro-support-in-ides/ is relevant here too with the RUST_IDE_PROC_MACRO_COMPLETION_DUMMY_IDENTIFIER env var.

[LeonMatthesKDAB]

Also see my comment on #455

[ahayzen-kdab]

Interestingly internally cxx_gen has two options gen_header and gen_implementation https://github.com/dtolnay/cxx/blob/5ebaee1a1dd668ab48a231890a26bae5fa81888a/gen/src/mod.rs#L58 these then avoid the generation phase https://github.com/dtolnay/cxx/blob/5ebaee1a1dd668ab48a231890a26bae5fa81888a/gen/src/mod.rs#L178

Say that these were public or we could modify them somehow, this would allow us to pass the tokens through CXX and find out if it was the parsing/checks phase that has failed or the write phase. Which then means if this first phase failed we could stop the build script and emit a warning, then we know that the macro expansion will fail ?

But as the later write phases don't fail there is actually no advantage here? So if the generate_header_and_cc fails this means the same and we should try to warn.

[ahayzen-kdab]

Think i've figured it out 😅

[ahayzen-kdab]

#547 has massively helped with this. I think this issue is done now ?

[ahayzen-kdab]

We have separate issues for any remaining tasks.