allow setting arbitrary size for importc types

compiler/pragmas.nim

@@ -947,15 +947,19 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
       of wSize:
         if sym.typ == nil: invalidPragma(c, it)
         var size = expectIntLit(c, it)
-        case size
-        of 1, 2, 4:
-          sym.typ.size = size
-          sym.typ.align = int16 size
-        of 8:
-          sym.typ.size = 8
-          sym.typ.align = floatInt64Align(c.config)
+        if sfImportc in sym.flags:
+          # no restrictions on size for imported types
+          setImportedTypeSize(c.config, sym.typ, size)
         else:
-          localError(c.config, it.info, "size may only be 1, 2, 4 or 8")
+          case size
+          of 1, 2, 4:
+            sym.typ.size = size
+            sym.typ.align = int16 size
+          of 8:
+            sym.typ.size = 8
+            sym.typ.align = floatInt64Align(c.config)
+          else:
+            localError(c.config, it.info, "size may only be 1, 2, 4 or 8")
       of wAlign:
         let alignment = expectIntLit(c, it)
         if isPowerOfTwo(alignment) and alignment > 0:

compiler/types.nim

@@ -1515,6 +1515,17 @@ proc getSize*(conf: ConfigRef; typ: PType): BiggestInt =
   computeSizeAlign(conf, typ)
   result = typ.size
 
+proc setImportedTypeSize*(conf: ConfigRef, t: PType, size: int) =
+  t.size = size
+  if tfPacked in t.flags or size <= 1:
+    t.align = 1
+  elif size <= 2:
+    t.align = 2
+  elif size <= 4:
+    t.align = 4
+  else:
+    t.align = floatInt64Align(conf)
+
 proc isConcept*(t: PType): bool=
   case t.kind
   of tyConcept: true

doc/manual.md

@@ -7815,6 +7815,8 @@ The `size pragma` allows specifying the size of the enum type.
   doAssert sizeof(EventType) == sizeof(uint32)
   ```
 
+When used for enum types, the `size pragma` accepts only the values 1, 2, 4 or 8.
+
 The `size pragma` can also specify the size of an `importc` incomplete object type
 so that one can get the size of it at compile time even if it was declared without fields.
 
@@ -7827,8 +7829,6 @@ so that one can get the size of it at compile time even if it was declared witho
       echo sizeof(AtomicFlag)
   ```
 
-The `size pragma` accepts only the values 1, 2, 4 or 8.
-
 
 Align pragma
 ------------

tests/c/timportedsize.nim

@@ -0,0 +1,10 @@
+{.emit: """
+typedef struct Foo {
+  NI64 a;
+  NI64 b;
+} Foo;
+""".}
+
+type Foo {.importc: "Foo", size: 16.} = object
+
+var x: Foo