determining aggregate member from MEM_REF

[Martin Sebor <msebor at gmail dot com>]

There are APIs to determine the base object and an offset
into it from all sorts of expressions, including ARRAY_REF,
COMPONENT_REF, and MEM_REF, but none of those I know about
makes it also possible to discover the member being referred
to.

Is there an API that I'm missing or a combination of calls
to some that would let me determine the (approximate) member
and/or element of an aggregate from a MEM_REF expression,
plus the offset from its beginning?

Say, given

  struct A
  {
     void *p;
     char b[3][9];
  } a[2];

and an expression like

  a[1].b[2] + 3

represented as the expr

  MEM_REF (char[9], a, 69)

where offsetof (struct A, a[1].b[2]) == 66

I'd like to be able to determine that expr refers to the field
b of struct A, and more specifically, b[2], plus 3.  It's not
important what the index into the array a is, or any other
arrays on the way to b.

I realize the reference can be ambiguous in some cases (arrays
of structs with multiple array members) and so the result wouldn't
be guaranteed to be 100% reliable.  It would only be used in
diagnostics.  (I think with some effort the type of the MEM_REF
could be used to disambiguate the majority (though not all) of
these references in practice.)

Thanks
Martin