the mainmodule option has effect only when no explicit module is given

compiler/main.nim

@@ -382,15 +382,14 @@ proc CommandSuggest =
 
 proc wantMainModule =
   if gProjectFull.len == 0:
-    Fatal(gCmdLineInfo, errCommandExpectsFilename)
+    if optMainModule.len == 0:
+      Fatal(gCmdLineInfo, errCommandExpectsFilename)
+    else:
+      gProjectName = optMainModule
+      gProjectFull = gProjectPath / gProjectName
+
   gProjectMainIdx = addFileExt(gProjectFull, nimExt).fileInfoIdx
 
-var oss: PGenericSeq
-proc dbgseqimp(x: PGenericSeq) {.cdecl.} =
-  oss = x
-
-seqdbg = dbgseqimp
-
 proc resetMemory =
   resetCompilationLists()
   ccgutils.resetCaches()