Basic macro support

[Manishearth]

For the purposes of cleanly writing code, it would be nice if the AST parser supported using macros. These could expand to types or methods, and would not support nested interpolation.

We'd only support:

• macros with a single arm
• the single arm must be a sequence of comma separated matchers, no fancy parsing
• the macro can be found in item or impl item position, nowhere else (not type position).
• the macro does not use interpolation matchers

This type of thing is relatively straightforward to parse and interpolate.

These macro definitions would be tagged with diplomat::macro and used in the source code.

The main challenge is that the Diplomat macro would only be able to see macros defined in the same bridge module they are used in. In theory we could apply a #[diplomat::macro_load = src/macros.rs] attribute, and have a global cache of macro_load paths to named Diplomat macros to avoid reparsing each time. It's not pretty, but it would work. We'd have to (at runtime) read CARGO_MANIFEST_DIR.

[Manishearth]

I'm not planning on doing this, but it would be cool. It would be pure Diplomat AST code, and would not affect the HIR or backends.

[Manishearth]

I found myself wanting something like this when I was redoing ICU4X's constructors over FFI, which follow a pattern. That would be cleaner with a single macro invocation on every formatter type that generates the compiled and buffer ctors.

It would also probably be useful in unicode-org/icu4x#5940 in case we decide to make a ton of formatter types.

[ambiguousname]

Seconding that this would be cool. I love Rust's macro system and I think it'd be great to bring any small part of that to Diplomat