--gc:arc is freely using incomplete structs

[kasperk81]

Type FILE is imported as incomplete structs in two places: https://github.com/nim-lang/Nim/blob/9d3a813fb96950631c69858e8e26b6503abd02fe/lib/system/io.nim#L18-L20

https://github.com/nim-lang/Nim/blob/fb86271556b4ea4485195febaa02b972149ca088/lib/system/ansi_c.nim#L117-L119

However the compiler/cgen does not make distinction when choosing the ctypes to emit. This assumption that FILE "can be" complete struct (regardless of what C standard says and how nim itself imports it; abstract / incomplete struct) naturally does NOT hold in non-GNU environments which abide by the meaning of "incomplete struct".

Consequently, a test fails on Alpine Linux, musl-libc:

FAIL: tests/arc/trepr.nim C
Test "tests/arc/trepr.nim" in category "arc"
Failure: reNimcCrash
$ bin/nim c --gc:arc tests/arc/trepr.nim
Hint: used config file '/builds/alpine/aports/community/nim/src/nim-1.4.8/config/nim.cfg' [Conf]
Hint: used config file '/builds/alpine/aports/community/nim/src/nim-1.4.8/config/config.nims' [Conf]
Hint: used config file '/builds/alpine/aports/community/nim/src/nim-1.4.8/tests/config.nims' [Conf]
Hint: used config file '/builds/alpine/aports/community/nim/src/nim-1.4.8/tests/arc/nim.cfg' [Conf]
..........
(a: true, n: doAssert)
Table[system.string, trepr.MyType](data: @[], counter: 0)
nil
..........
../../lib/system/repr_v2.nim(96, 7) Hint: 'firstElement' is declared but not used [XDeclaredButNotUsed]
trepr.nim(43, 8) Warning: imported and not used: 'os' [UnusedImport]
CC: stdlib_assertions.nim
CC: stdlib_dollars.nim
CC: stdlib_repr_v2.nim
CC: stdlib_io.nim
CC: stdlib_system.nim
CC: stdlib_times.nim
CC: stdlib_os.nim
CC: trepr.nim
/home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c:237:92: error: parameter 2 ('x') has incomplete type
  237 | N_LIB_PRIVATE N_NIMCALL(void, reprObject__oUb9a0hqZE8qgrd3cWM89cvg)(NimStringV2* res, FILE x) {
      |                                                                                       ~~~~~^
/home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c: In function 'repr__jIXMkVs9aUgCuk4CQ63oMmw':
/home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c:270:51: error: invalid use of incomplete typedef 'FILE'
  270 |  reprObject__oUb9a0hqZE8qgrd3cWM89cvg((&result), (*x));
      |                                                  ~^~~
/home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c:270:50: error: type of formal parameter 2 is incomplete
  270 |  reprObject__oUb9a0hqZE8qgrd3cWM89cvg((&result), (*x));
      |                                                  ^~~~
Error: execution of an external compiler program 'gcc -c  -w -fmax-errors=3   -I/builds/alpine/aports/community/nim/src/nim-1.4.8/lib -I/builds/alpine/aports/community/nim/src/nim-1.4.8/tests/arc -o /home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c.o /home/buildozer/.cache/nim/trepr_d/stdlib_repr_v2.nim.c' failed with exit code: 1

[kasperk81]

@timotheecour do you have any ideas where the fix of this one should go?

[richfelker]

Note: I looked at this a bit yesterday but, not being familiar with Nim, I was unable to figure out whether the problem was coming just from the tests, or from part of the Nim standard library that could also affect applications.