[rust] bindgen path supports env!() macro

[kvcache]

This makes it possible to refer to a wit path by a path that includes
things like OUT_DIR or CARGO_MANIFEST_DIR. Relative paths that navigate
up, like ../wit/, don't work well with cargo publish, and symlinks
have difficulties between git and cargo.

Using macro-string, this replaces the path option's syn::LitStr with MacroString.

See the example crates/guest-rust/src/examples/_5_import_directory.rs for
how this can be used. I need this capability so I can write separate crates for
a partially inter-related set of WIT's without having to copy/paste the whole
WIT tree for each package.

Before: error: expected string literal
After: Example 5 compiles.

[kvcache]

For context, this is the expression I'm able to compile locally with this change:

wit_bindgen::generate!({
    world: "momento:bytes/imports",
    path: [ concat!(env!("OUT_DIR"), "/wit/host") ],
});

[alexcrichton]

Thanks! Would it be ok to place this behind a Cargo feature as well since it's an optional dependency. Perhaps an on-by-default optional feature?

[kvcache]

Thanks! Would it be ok to place this behind a Cargo feature as well since it's an optional dependency. Perhaps an on-by-default optional feature?

Thanks @alexcrichton, I added this as a default-on feature. I'm not sure why someone would disable it but the affordance is there now!

[alexcrichton]

Thanks! Can you also edit this dependency to disable default features, and then also configure the default features on the wit-bindgen crate as well? Most users don't depend on the macro crate directly but instead only indirectly though the wit-bindgen crate.

[kvcache]

@alexcrichton I think I got the feature lifted to wit-bindgen now, but I'm not the most experienced with cargo features. I also merged main, because github couldn't automatically figure out the line diff in cargo.toml with the 0.54.0 update.

[alexcrichton]

Thanks!