[cfe-dev] AST for multiple files and copying the clang diagnostic system design

[Kenneth Camann]

Hello everyone,

How can I build an AST that represents code in multiple translation units?
I am working on a code generation tool whose DSL is full C++, plus a few
extensions related to what the tool does (it adds new AST nodes). Some of
the declarations are namespace-like: redeclarable (can be "re-opened"
several times) and acting as DeclContext's. I would prefer that each the
time they are reopened (in any file), the same Decl object gets populated.

One thing I could do is paste all the translation units into a single
buffer to satisfy the need for a single "main file," and keep a mapping of
SourceLocations in the merged file back to their origin file.

It occurs to me that clang already has a lot of machinery for doing this,
used by the include stack. I am wondering if anyone has an examples or
advice on how I could reuse that machinery; I'm guessing I'm not the first
one to want to do this.

I'll also explain the use case, which you may find interesting: it is an
attempt to copy the design of clang's diagnostic system, but as a "resuable
library" for my other projects, and without using tablegen:

I like the idea of logging/error reporting by defining "diagnostics" in a
simple declarative language, along with a tool that generates boilerplate
C++ diagnostic classes. Part of my "library" is the actual library (which
has the DiagnosticConsumer-like stuff in it), and part is the code
generation tool. I cannot use tablegen -- unlike the clang diagnostic
system, diagnostics can be parameterized by any C++ type, and are printable
as long as the type has an ostream operator<< overload. Because of how it
works, it makes sense for the diagnostic declaration language to be a
superset of C++, since parts of the code generator need to understand C++.

I created this tool once already, using some macro weirdness. Diagnostic
files were normal C++ headers, and the "diagnostic declarations," which
were just macros (DIAG, DIAG_LIBRARY, DIAG_CATEGORY). The macros would make
"dummy" placeholder decls that would encode the info I needed, and a
libtooling-based clang tool would find them up and spit out the generated
code, while leaving all "normal" declarations alone.

I would like to reimplement this tool as actual fork of clang that adds a
few custom Decl nodes (e.g., DiagDecl, DiagCategoryDecl, DiagLibraryDecl).
Although it's probably not worth the complexity over the earlier approach,
I want to take my understanding of the clang AST to the next level anyway,
and I think this will be a fun project for doing that. It will also make it
possible to write a "diagnostic declaration" on more than one line without
ending every line with a "\" the way you have to do with macros, which
drives me crazy.

Thanks!
Ken
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