fixes #1360; importc makes fields exportc by default (#1532)

fixes #1360

ref #1516

As in Nim, the fields of `importc` should be `exportc` by default, which
could be convenient without excess `exportc` pragmas on each fields

Author: ringabout

src/hexer/nifcgen.nim

@@ -91,6 +91,7 @@ type
     IsNodecl
     IsInheritable
     IsUnion
+    IsImportExternal
 
 proc trType(c: var EContext; n: var Cursor; flags: set[TypeFlag] = {})
 
@@ -124,6 +125,11 @@ proc trField(c: var EContext; n: var Cursor; flags: set[TypeFlag] = {}) =
 
   c.dest.addDotToken() # adds pragmas
 
+  if IsImportExternal in flags and prag.externName.len == 0:
+    var baseName = pool.syms[s]
+    extractBasename baseName
+    prag.externName = baseName
+
   if prag.externName.len > 0:
     c.registerMangle(s, c.toExtern(s, prag.externName))
 
@@ -1046,6 +1052,8 @@ proc trTypeDecl(c: var EContext; n: var Cursor; mode: TraverseMode) =
       flags.incl IsInheritable
     if UnionP in prag.flags:
       flags.incl IsUnion
+    if {ImportcP, ImportcppP} * prag.flags != {}:
+      flags.incl IsImportExternal
     trType c, n, flags
   takeParRi c, n
   swap dst, c.dest

tests/nimony/ffi/bar.h

@@ -1 +1,8 @@
 typedef struct {} Bar;
+
+typedef struct {
+    unsigned char r;
+    unsigned char g;
+    unsigned char b;
+    unsigned char a;
+} Color;

tests/nimony/ffi/theader.nim

@@ -3,3 +3,15 @@ type Bar1 {.importc: "Bar1", header: "${path}/bar1.h".} = object
 
 const header = "bar.h"
 type Bar2 {.importc: "Bar", header: header.} = object
+
+import std/assertions
+
+type
+  Color* {.bycopy, importc, header: header.} = object
+    r*: uint8
+    g*: uint8
+    b*: uint8
+    a*: uint8
+
+var color = Color(r: 8, g: 8, b: 8, a: 8)
+assert color.r == 8