Hi,
This patch attempts to add returns_arg attribute for c-family
languages. For C++ methods, first arg is assumed to be this pointer,
similar to alloc_size.
I have a couple of doubts:

(a) I am not sure why DECL_ARGUMENTS (decl) in
handle_returns_arg_attribute returns NULL ? My intent was to check
that the return-type and argument-types are compatible.

(b) AFAIU the bottom two bits of call return flags are used for storing
args from 0 - 3 and the arg number can be obtained by
masking with ERF_RETURN_ARG_MASK.
So, does that mean we can only allow first 4 arguments to be annotated
with returns_arg attribute ?
In the patch, I use fn_spec ('argnum'), which gimple_call_return_flags
uses to mark the corresponding arg with ERF_RETURN_ARG:

(c) How to write dejaGNU test to check if fn_spec ('argnum') has been correctly
applied to the corresponding parameter ? In the patch, I am just
testing for validation.

Thanks,
Prathamesh