fixes #8405: -d:useNimRtl now works even when {.rtl.} procs are used at compile time; CTFFI now works with {dynlib} (#11635)

(cherry picked from commit 64168d4aea)

compiler/semstmts.nim

@@ -1863,7 +1863,7 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
         " operator has to be enabled with {.experimental: \"callOperator\".}")
 
   if n.sons[bodyPos].kind != nkEmpty and sfError notin s.flags:
-    # for DLL generation it is annoying to check for sfImportc!
+    # for DLL generation we allow sfImportc to have a body, for use in VM
     if sfBorrow in s.flags:
       localError(c.config, n.sons[bodyPos].info, errImplOfXNotAllowed % s.name.s)
     let usePseudoGenerics = kind in {skMacro, skTemplate}
@@ -1881,12 +1881,11 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
 
         c.p.wasForwarded = proto != nil
         maybeAddResult(c, s, n)
-        if lfDynamicLib notin s.loc.flags:
-          # no semantic checking for importc:
-          s.ast[bodyPos] = hloBody(c, semProcBody(c, n.sons[bodyPos]))
-          # unfortunately we cannot skip this step when in 'system.compiles'
-          # context as it may even be evaluated in 'system.compiles':
-          trackProc(c, s, s.ast[bodyPos])
+        # semantic checking also needed with importc in case used in VM
+        s.ast[bodyPos] = hloBody(c, semProcBody(c, n.sons[bodyPos]))
+        # unfortunately we cannot skip this step when in 'system.compiles'
+        # context as it may even be evaluated in 'system.compiles':
+        trackProc(c, s, s.ast[bodyPos])
         if s.kind == skMethod: semMethodPrototype(c, s, n)
       else:
         if (s.typ.sons[0] != nil and kind != skIterator) or kind == skMacro:
@@ -1899,8 +1898,9 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
           fixupInstantiatedSymbols(c, s)
         if s.kind == skMethod: semMethodPrototype(c, s, n)
       if sfImportc in s.flags:
-        # so we just ignore the body after semantic checking for importc:
-        n.sons[bodyPos] = c.graph.emptyNode
+        # don't ignore the body in case used in VM
+        # n.sons[bodyPos] = c.graph.emptyNode
+        discard
       popProcCon(c)
   else:
     if s.kind == skMethod: semMethodPrototype(c, s, n)

compiler/vm.nim

@@ -1082,12 +1082,15 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
           VmArgs(ra: ra, rb: rb, rc: rc, slots: cast[pointer](regs),
                  currentException: c.currentExceptionA,
                  currentLineInfo: c.debug[pc]))
-      elif sfImportc in prc.flags:
+      elif importcCond(prc):
         if compiletimeFFI notin c.config.features:
           globalError(c.config, c.debug[pc], "VM not allowed to do FFI, see `compiletimeFFI`")
         # we pass 'tos.slots' instead of 'regs' so that the compiler can keep
         # 'regs' in a register:
         when hasFFI:
+          if prc.position - 1 < 0:
+            globalError(c.config, c.debug[pc],
+              "VM call invalid: prc.position: " & $prc.position)
           let prcValue = c.globals.sons[prc.position-1]
           if prcValue.kind == nkEmpty:
             globalError(c.config, c.debug[pc], "cannot run " & prc.name.s)

compiler/vmgen.nim

@@ -1560,8 +1560,11 @@ proc genTypeLit(c: PCtx; t: PType; dest: var TDest) =
   n.typ = t
   genLit(c, n, dest)
 
-proc importcCond(s: PSym): bool {.inline.} =
-  sfImportc in s.flags and (lfDynamicLib notin s.loc.flags or s.ast == nil)
+proc importcCond*(s: PSym): bool {.inline.} =
+  ## return true to importc `s`, false to execute its body instead (refs #8405)
+  if sfImportc in s.flags:
+    if s.kind in routineKinds:
+      return s.ast.sons[bodyPos].kind == nkEmpty
 
 proc importcSym(c: PCtx; info: TLineInfo; s: PSym) =
   when hasFFI:
@@ -1569,7 +1572,8 @@ proc importcSym(c: PCtx; info: TLineInfo; s: PSym) =
       c.globals.add(importcSymbol(c.config, s))
       s.position = c.globals.len
     else:
-      localError(c.config, info, "VM is not allowed to 'importc'")
+      localError(c.config, info,
+        "VM is not allowed to 'importc' without --experimental:compiletimeFFI")
   else:
     localError(c.config, info,
                "cannot 'importc' variable at compile time; " & s.name.s)