[cfe-dev] [analyzer] Retrieving macro expansions in the plist output

[Kristóf Umann via cfe-dev]

Hi!

I'm currently trying to implement a new node in the plist output that would
show the expansion of a macro, like this:

void setToNull(int **vptr) {
  *vptr = nullptr;
}

void print(void*);

#define TO_NULL(x) \
  setToNull(x)

#define DOES_NOTHING(x) \
  {                     \
    int b;              \
    b = 5;              \
  }                     \
  print(x)

#define DEREF(x)   \
  DOES_NOTHING(x); \
  *x

void f() {
  int *a = new int(5);
  TO_NULL(&a);
  DEREF(a) = 5;
}

For this code, two PathDiagnosticMacroPieces should be generated, and a
message like this should be displayed:
  Expanding macro 'TO_NULL' to 'print(&a); setToNull(&a)'
  Expanding macro 'DEREF' to '{ int sajt; sajt = 5; } print(a); *a'
I've made some progress on this issue, but here are the problems I faced.

Currently, HTML file generation supports macro expansions fairly well,
however, the code that achieves this seems to be held together by sheer
luck:
https://clang.llvm.org/doxygen/namespaceclang_1_1html.html#a3283736376c3e436ff65975acdb1d399
As I understand it, the entire file is re-lexed and preprocessed, during
which macro expansions are added to the HTML file. I attempted to reuse the
code here without having to re-lex everything, but so far my efforts lead
to little success. I also found that any modification of this code quickly
leads to instabilities -- although this part of Clang is somewhat of a
mystery to me just yet, so I'd concede to this being my fault.

I also fear that since HTML output is not as used as other outputs, and
could lack vigorous testing, it could be crash-prone.

Do you know any way of obtaining the expansion of a macro expression that
doesn't involve such abusement of the preprocessor? Something like this
would be ideal:

SourceLocation Loc = /* PathDiagnosticMacroPiece location */;
Preproc.getMacroExpansionForExpression(Loc));

Cheers,
Kristóf
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