Source transformations in frontend plugin before compilation

Hi all,

I want to annotate malloc(sizeof(X)) calls with the type in the sizeof so that I can use the information in the IR later on (in a transformation plugin). My current approach is to do a somewhat hacky source transformation, replacing the call with:
malloc(({ ty * volatile ptr = NULL; (size_t)(ptr + 1); }))
so that I can take the pointer type and replace the size back with a constant in the IR.

If someone can think of a better way to do this inside a clang frontend plugin, I’m open to that, but currently I’m having issues just getting this to work. I have a working plugin that finds the sizeof expressions and a Rewriter instance that replaces them. However, I don’t know how to apply the changes to the input file so that the input with be parsed again before being passed on to the codegen. There is a Reparse function in ASTUnit, but I cannot find (in the docs or online) how this is supposed to be used.
Does anyone know how to correctly use this?