Add support for custom attributes

[mkroening]

This adds support for custom attributes:

• ParseCallbacks::add_attributes(&self, info: &AttributeInfo<'_>) -> Vec<TokenStream>
• --with-attribute-custom <CUSTOM>
• --with-attribute-custom-struct <CUSTOM>
• --with-attribute-custom-enum <CUSTOM>
• --with-attribute-custom-union <CUSTOM>

The implementation is very similar to the custom derives functionality. One thing I am not sure about is returning Vec<TokenStream> instead of Vec<String>, but that seemed natural to me.

Fixes #2520

[pvdrz]

Hi @mkroening sorry for the delay with the review.

I only have two requests for you because otherwise this looks great:

• It is better to use String instead of TokenStream as the latter leaks proc_macro2 into our public API, meaning that we would have to do releases from time to time just to keep up to date with proc_macro2.
• There should be a test for the CLI args and also some documentation explaining the syntax of the attributes as right now.

Edit: I also have an additional question for you that is non-blocking and can be addressed later (even by someone else). Would it be possible to re-implement the custom derives functionality using the custom attributes machinery instead?

[mkroening]

Thanks for the review!

• It is better to use String instead of TokenStream as the latter leaks proc_macro2 into our public API, meaning that we would have to do releases from time to time just to keep up to date with proc_macro2.

✅

• There should be a test for the CLI args

✅

and also some documentation explaining the syntax of the attributes as right now.

Where would I put that? I did not find anything similar for --with-derive-custom.

Would it be possible to re-implement the custom derives functionality using the custom attributes machinery instead?

That should be possible! Would you like me to tackle this in this PR or in a separate one?

I have also implemented support for annotations, but error reporting is difficult there. It currently panics, if the attribute is not parsable as TokenStream.

[pvdrz]

and also some documentation explaining the syntax of the attributes as right now.

Where would I put that? I did not find anything similar for --with-derive-custom.

This would go in bindgen-cli as part of the docstring for the respective fields so clap can generate the --help message with that documentation. This is how it looks for the --with-derive-custom.* flags:

$ bindgen --help
Generates Rust bindings from C/C++ headers.

Usage: bindgen <FLAGS> <OPTIONS> <HEADER> -- <CLANG_ARGS>...

Arguments:
  [HEADER]         C or C++ header file
  [CLANG_ARGS]...  Arguments to be passed straight through to clang

Options:
      ...
      --with-derive-custom <CUSTOM>
          Derive custom traits on any kind of type. The CUSTOM value must be of the shape REGEX=DERIVE where DERIVE is a coma-separated list of derive macros
      --with-derive-custom-struct <CUSTOM>
          Derive custom traits on a `struct`. The CUSTOM value must be of the shape REGEX=DERIVE where DERIVE is a coma-separated list of derive macros
      --with-derive-custom-enum <CUSTOM>
          Derive custom traits on an `enum. The CUSTOM value must be of the shape REGEX=DERIVE where DERIVE is a coma-separated list of derive macros
      --with-derive-custom-union <CUSTOM>
          Derive custom traits on a `union`. The CUSTOM value must be of the shape REGEX=DERIVE where DERIVE is a coma-separated list of derive macros

Would it be possible to re-implement the custom derives functionality using the custom attributes machinery instead?

That should be possible! Would you like me to tackle this in this PR or in a separate one?

I'd do it on a separate one so it is easier to review and track.

Thanks!

[mkroening]

This would go in bindgen-cli as part of the docstring for the respective fields so clap can generate the --help message with that documentation. This is how it looks for the --with-derive-custom.* flags:

Ah, that's what you mean. I wondered, since I had already done that when opening this PR. Perhaps you overlooked it?

I have also fixed the formatting for CI.

[pvdrz]

Oh my bad, it seems my head just skipped that section of code 🤣. It seems there are some compilation errors due to the change from TokenStream to String, could you fix those so we can roll this up?. Thanks!

[mkroening]

The CI is green now. :)