Add a macro to track a file

[GuillaumeGomez]

I recently released doc-comment 0.4 which converted declarative macros to proc-macros. I encountered a "funny" issue though: I needed the compiler to track files that were included using my proc-macros. In order to do so, I generated anonymous constants which looked like this:

const _: &'static str = include_str!("file.md");

However, I have no use for those constants and they make the source code heavier for just keeping track of a file. Therefore, I propose to add a track!("file") macro which would allow to track files like include_str! without requiring the usage of anonymous constants.

I can make the implementation if the rust teams agree to add this feature. :)

[Mark-Simulacrum]

IMO, the proper solution is to add proc_macro APIs that indicate "interest" in a file, rather than further expanding the set of macros.

[estebank]

Isn't this already supported using build.rs?

[ehuss]

The user of the proc-macro would need to add it to their build.rs, which is a bit tedious (and I suspect most people won't even know they should do that). It would also cause the entire build script to rerun, which could be quite expensive if it is doing other unrelated things. Build scripts can also be tricky for inexperienced users (such as requiring "rerun-if-changed=build.rs" to avoid full rebuilds).

I've seen it mentioned in several places where there is desire to register change detection for various things (including environment variables mentioned here).

Grepping through crates.io, I found about 10 proc-macros that use the include_str/include_bytes trick to force a dependency check.

[ehuss]

cc #74690, where the ability to register the usage of an environment variable was added. I would suspect adding support for tracking files could use a similar mechanism. I'm not sure if this specifically needs to use a macro, or if a function call would be sufficient (tracked_env::file(...)? tracked_env::path(...)?).

[petrochenkov]

The bare minimum would be ability to add file paths to dependencies.

However, it would be significantly more useful to also have ability to load the file into a token stream and create a source map for it, so all the spans are assigned properly.

[luser]

However, it would be significantly more useful to also have ability to load the file into a token stream and create a source map for it, so all the spans are assigned properly.

This is #55904