Support C modules via ImportC

[vladimmi]

At this point both DMD and LDC support compilation of C modules, automatically invoking external preprocessor for them if needed (DMD for some time, LDC since 1.33-beta2 version), dub also can treat C modules as part of project for building. Is it theoretically possible to have IDE support for this? I mean resolving C modules, having C functions in suggestions/goto, etc.

[jnms-me]

Starting off, code completion and other features use the dparse library very heavily (imported from 20+ modules). That library can't parse c files.

As I believe the D compiler frontend emits a D AST for C source code, I have some ideas:

• Let the D compiler convert the C file to a D file, probably losing line:col positions of tokens.
• Prefer using the dmd compiler as a library over dparse?
• Impement lexing and parsing of C code into a D AST in libdparse, regarding C as a subset of D (not too hard).

Take them with a grain of salt, I'm very new to this project.

[jnms-me]

dmd -o- -Hf/dev/stdout cfile.c prints an equivalent .di file.

[jnms-me]

As for code completion, no changes need be made here. This feature is the responsibility of DCD.

[jnms-me]

See dlang-community/DCD#789.
Seems like some things are still missing for complete signatures (this side) and documentation (dmd side) to show properly. But even __attribute__ ((deprecated)) shows in my editor as strikethrough, which is more than I expected.