importc fails to generate stub type

[arnetheduck]

Description

type MyType {.importc, incompleteStruct.} = object

var v {.exportc.}: ptr MyType

My expectation, since there's no nodecl, would be a stub type generated (typedef struct MyType MyType; or similar.

Nim Version

devel, 2.0

Current Output

nim c testit.nim
/tmp/xxx/@mtest.nim.c:23:15: error: unknown type name ‘MyType’
   23 | N_LIB_PRIVATE MyType* v;
      |               ^~~~~~

Expected Output

No response

Known Workarounds

No response

Additional Information

No response

[litlighilit]

but why importc instead of exportc

[arnetheduck]

because the type is being imported from C (and defined in a C library)

[Araq]

Yet you want it to occur in the produced C file so as far as the compiler is concerned you export it...

[arnetheduck]

Yet you want it to occur in the produced C file so as far as the compiler is concerned you export it...

well, that doesn't seem logical - that would place the responsibility of defining the type on nim, which is not the intent. The type is defined in a C library - what nim is supposed to to is create an opaque struct to be used for pointers but whose definition remains unknown to the compilation unit - nothing else. In fact, this is also what happens, exportc tries to define the type which is a worse outcome:

typedef struct MyType MyType;
struct MyType {
	char dummy;
};

[Araq]

Hmm ok, I get what you're saying.

[ringabout]

It seems that lots of codebases relies on importc implying nodecl. e.g. status-im/nim-stint#166

[arnetheduck]

hm, that's an unfortunate mess - the manual seems pretty clear on the intent of nodecl / header here, in favor of the interpretation of this issue, so technically such code is buggy (as buggy as the compiler :) ).

stint at least is easy to fix since nodecl works with all nim versions as documented.

[Araq]

Well the issue is subtle, "this type comes from C but also generate it in the C code" ... it's no wonder why we never encountered it before.

[arnetheduck]

it's no wonder why we never encountered it before.

I think it's mostly because most of the time, people tend to use header which implies nodecl which is why it doesn't come up..

In this particular case, there's actually no header involved, or rather the nim file is the header because the relevant c code is also being generated and it doesn't need a header file. I could generate a c header file just to please nim, but that seems wrong too.