9752: feature: Declare proc-macro dependent crates in `rust-project.json` r=matklad a=tobywf
This adds the `is_proc_macro` flag in `rust-project.json`. By default, this is `false` and not required, so existing projects won't break/have the same behavior as before this change. If the flag is true, a dependency to the `proc_macro` sysroot crate is added (if it exists), so that rust-analyzer can resolve those imports.
This fixes#9726 .
I've also added some tests in the second commit. The first is a smoke test for a basic, minimal `rust-project.json` file. The second is a more targeted test for the flag. Both tests depend on the fake sysroot (a bunch of directories in the correct layout with empty `lib.rs` files), and also on `env!("CARGO_MANIFEST_DIR")` being an absolute path. I'm not sure if the later assumption is valid on all platforms. I wanted to at least try and add tests, but I'm happy to rework them or remove them if you don't think that's the way to go.
(You can license/relicense my contribution in any way you wish without contacting me.)
Co-authored-by: Toby Fleming <sourcecode@tobywf.com>
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