[llvm-dev] [RFC] clang support for API information generation in JSON

[Zixu Wang via llvm-dev]

Hi All!

I’m writing to propose clang-extract-api, a tool to collect and serialize API information from header files, for example function signatures, Objective-C interfaces and protocols, and inline documentation comments. We hope it could help future tools to understand clang-based language APIs without needing to dig into the AST themselves.

Background

Motivation

Library and SDK providers may find it useful to be able to create and inspect a “snapshot” of APIs they expose, for example, to check for API/ABI-breaking changes between two versions, or to automate generating documentation for the APIs. Here is a list of examples of information we want to extract from APIs:
	• the name (spelling/mangled) of the symbol;
	• the unique identifier of the symbol, for example the Unified Symbol Resolution (USR);
	• the source location of the API declaration (file, line, column);
	• access control of the API (public/private/protected);
	• availability (available/unavailable/deprecated);
	• function signatures (return/parameters);
	• documentation comments attached to a symbol;
	• relations with other symbols (class methods, typedef relations, struct data fields, enum constants, etc.)
Since these API information is available in the header files, which declare and distribute the APIs, we can implement a tool to extract them without invoking a compilation of the whole project to enable easy access to the information for tooling.

Existing solutions

While there are some existing solutions in clang to dump symbols or AST information, they either expose unnecessary low-level details or fail to provide enough information of APIs. For example, clang -ast-dump dumps low-level details for all declarations for debug purposes and the output is not machine-parsable. Doxygen also extracts documentation comments and other information from API declarations, but its output is rendered documentation in web formats which is not flexible for other uses and tools.

Proposal

We propose to implement this tool as a new frontend action invoked by `clang -extract-api` as show in the example below.

clang -extract-api \
  header.h [more_header.h ...] or a filelist\
  -isysroot <SDK> \
  -target <TARGET> \
  -I <INCLUDE PATH> \
  -isystem <SYS INCLUDE PATH> \
  ...
  -o output.json

It takes in the header file(s) or a filelist file containing paths to the header file(s) as the input. The header files will be parsed by clang and the extract-api action will visit the AST to extract needed information and serialize to a JSON output. Please find an example input and output attached.

The example output is based on the symbol graph format that's already used by Swift for serializing symbol information and their relations. This format can represent the required API information and is flexible and extendable as demonstrated in the example so we think it's a good starting point.

We are excited about this idea and its potential uses, and we’d love to hear feedback and suggestions!



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