Add dummy definition for referenced but never-defined structs

[lucic71]

This handles opaque pointers to user defined forward declared structs. It's allowed to use such pointers, even though the struct is not defined in any translation unit.

To implement this, I modified record_decls_ to also keep an extra bool with the following meaning:

    // record name -> true if a definition has been emitted, false if only
    // referenced.
    std::unordered_map<std::string, bool> entries_;

At the end of visiting all TU's, for the record_decls_ that are only referenced (not defined), a dummy struct definition is generated:

#[repr(C)]
pub struct opaque {
    _opaque: [u8; 0],
}

[lucic71]

pub struct opaque is enough, it doesn't have to be

#[repr(C)]
pub struct opaque {
    _opaque: [u8; 0],
}

[nunoplopes]

But we take the whole program as input. Why is something undefined?

[lucic71]

But we take the whole program as input. Why is something undefined?

The following case is a valid C/C++ program:

struct S; // forward declaration

#ifdef FLAG_THAT_CAN_BE_DISABLED_AT_BUILD_TIME
struct S {
  ... definition of struct ...
}
#endif 

void foo(S* s) {}

This becomes:

fn foo(s: *mut S) {}

And errors because there is no definition of S (we don't generate code on struct S; // forward declaration because rust does not support forward declarations.