fixes #25885; incompleteStruct ignored without importc (#25898)

fixes #25885

compiler/sizealignoffsetimpl.nim

@@ -394,9 +394,10 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
           accum.offset = 1
         computeObjectOffsetsFoldFunction(conf, typ.n, false, accum)
       let paddingAtEnd = int16(accum.finish())
-      if typ.sym != nil and
-         typ.sym.flags * {sfCompilerProc, sfImportc} == {sfImportc} and
-         tfCompleteStruct notin typ.flags:
+      if (typ.sym != nil and
+          typ.sym.flags * {sfCompilerProc, sfImportc} == {sfImportc} and
+          tfCompleteStruct notin typ.flags) or
+         tfIncompleteStruct in typ.flags:
         typ.size = szUnknownSize
         typ.align = szUnknownSize
         typ.paddingAtEnd = szUnknownSize

compiler/vmgen.nim

@@ -786,8 +786,12 @@ proc genBinaryABCD(c: PCtx; n: PNode; dest: var TDest; opc: TOpcode) =
   c.freeTemp(tmp2)
   c.freeTemp(tmp3)
 
-template sizeOfLikeMsg(name): string =
-  "'$1' requires '.importc' types to be '.completeStruct'" % [name]
+template sizeOfLikeMsg(name, incompleteStruct): string =
+  block:
+    if incompleteStruct:
+      "'$1' cannot be used with '.incompleteStruct' types" % [name]
+    else:
+      "'$1' requires '.importc' types to be '.completeStruct'" % [name]
 
 proc genNarrow(c: PCtx; n: PNode; dest: TDest) =
   let t = skipTypes(n.typ, abstractVar-{tyTypeDesc})
@@ -1476,11 +1480,14 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; flags: TGenFlags = {}, m: TMag
     else:
       globalError(c.config, n.info, "expandToAst requires a call expression")
   of mSizeOf:
-    globalError(c.config, n.info, sizeOfLikeMsg("sizeof"))
+    let arg = n[1].typ.skipTypes({tyTypeDesc})
+    globalError(c.config, n.info, sizeOfLikeMsg("sizeof", tfIncompleteStruct in arg.flags))
   of mAlignOf:
-    globalError(c.config, n.info, sizeOfLikeMsg("alignof"))
+    let arg = n[1].typ.skipTypes({tyTypeDesc})
+    globalError(c.config, n.info, sizeOfLikeMsg("alignof", tfIncompleteStruct in arg.flags))
   of mOffsetOf:
-    globalError(c.config, n.info, sizeOfLikeMsg("offsetof"))
+    let arg = n[1].typ.skipTypes({tyTypeDesc})
+    globalError(c.config, n.info, sizeOfLikeMsg("offsetof", tfIncompleteStruct in arg.flags))
   of mRunnableExamples:
     discard "just ignore any call to runnableExamples"
   of mDestroy, mTrace: discard "ignore calls to the default destructor"

doc/manual.md

@@ -7996,6 +7996,9 @@ underlying C `struct`:c: in a `sizeof` expression:
            pure, incompleteStruct.} = object
   ```
 
+Attempting to use `sizeof` on an `incompleteStruct` type at compile-time
+will error with "'sizeof' cannot be used with '.incompleteStruct' types".
+
 
 CompleteStruct pragma
 ---------------------

tests/misc/tsizeof_incompleteStruct.nim

@@ -0,0 +1,12 @@
+discard """
+errormsg: "'sizeof' cannot be used with '.incompleteStruct' types"
+line: 10
+"""
+
+type
+  MyStruct {.incompleteStruct.} = object
+    field: int
+
+const i = sizeof(MyStruct)
+
+echo i