cValidAssemblers -> const set

Added assembler validity check

compiler/extccomp.nim

@@ -368,7 +368,9 @@ var
   cIncludes*: seq[string] = @[]   # directories to search for included files
   cLibs*: seq[string] = @[]       # directories to search for lib files
   cLinkedLibs*: seq[string] = @[] # libraries to link
-  cValidAssemblers* = @[asmFasm]
+
+const
+  cValidAssemblers* = {asmFasm}
 
 # implementation
 
@@ -567,6 +569,8 @@ proc getCompileCFileCmd*(cfilename: string, isExternal = false): string =
 
     if c == ccNone:
       rawMessage(errExternalAssemblerNotFound, "")
+    elif c notin cValidAssemblers:
+      rawMessage(errExternalAssemblerNotValid, customAssembler)
 
   var options = cFileSpecificOptions(cfilename)
   var exe = getConfigVar(c, ".exe")

compiler/msgs.nim

@@ -109,6 +109,7 @@ type
     errGenericLambdaNotAllowed,
     errCompilerDoesntSupportTarget,
     errExternalAssemblerNotFound,
+    errExternalAssemblerNotValid,
     errUser,
     warnCannotOpenFile,
     warnOctalEscape, warnXIsNeverRead, warnXmightNotBeenInit,
@@ -372,6 +373,7 @@ const
                                 "of the generic paramers can be inferred from the expected signature.",
     errCompilerDoesntSupportTarget: "The current compiler \'$1\' doesn't support the requested compilation target",
     errExternalAssemblerNotFound: "External assembler not found",
+    errExternalAssemblerNotValid: "External assembler '$1' is not a valid assembler",
     errUser: "$1",
     warnCannotOpenFile: "cannot open \'$1\'",
     warnOctalEscape: "octal escape sequences do not exist; leading zero is ignored",