Change behavior when invoking LDC/LDMD without source files

Only display the appropriate usage help (and then fail) if invoked
without any explicit cmdline options. Otherwise emit an error about
missing source files and fail immediately, without displaying the usage
help.

Besides making LDC and LDMD behave identically in this regard, it makes
just more sense IMO (when forgetting to specify a file, LDC previously
just printed the cmdline help without any error message).

It also makes `ldmd2 -transition=?` and `ldmd2 -preview=help` etc. print
the expected help without LDMD special cases.

dmd/mars.d

@@ -66,7 +66,6 @@ version (IN_LLVM)
 
     // in driver/main.cpp
     void registerPredefinedVersions();
-    void printLDCUsage();
     void codegenModules(ref Modules modules);
     // in driver/archiver.cpp
     int createStaticLibrary();
@@ -379,7 +378,7 @@ version (IN_LLVM) {} else
         }
 version (IN_LLVM)
 {
-        printLDCUsage();
+        error(Loc.initial, "No source files");
 }
 else
 {

driver/ldmd.cpp

@@ -371,7 +371,6 @@ void translateArgs(size_t originalArgc, char **originalArgv,
   ldcArgs.push_back("-ldmd");
 
   bool vdmd = false;
-  bool noFiles = true;
   bool pic = false; // -fPIC already encountered?
 
   for (size_t i = 1; i < args.size(); i++) {
@@ -529,13 +528,9 @@ void translateArgs(size_t originalArgc, char **originalArgv,
         }
       }
       /* -extern-std
-       */
-      else if (strcmp(p + 1, "transition=?") == 0) {
-        const char *transitionargs[] = {ldcPath.c_str(), p, nullptr};
-        execute(ldcPath, transitionargs);
-        exit(EXIT_SUCCESS);
-      }
-      /* -transition=<id>
+       * -transition
+       * -preview
+       * -revert
        * -w
        * -wi
        * -O
@@ -656,11 +651,9 @@ void translateArgs(size_t originalArgc, char **originalArgv,
         exit(EXIT_SUCCESS);
       } else if (strcmp(p + 1, "run") == 0) {
         ldcArgs.insert(ldcArgs.end(), args.begin() + i, args.end());
-        noFiles = (i == args.size() - 1);
         break;
       } else if (p[1] == '\0') {
         ldcArgs.push_back("-");
-        noFiles = false;
       } else if (p[1] == 'C') {
         ldcArgs.push_back(concat("-", p + 2));
       } else {
@@ -686,21 +679,9 @@ void translateArgs(size_t originalArgc, char **originalArgv,
       }
 #endif
       ldcArgs.push_back(p);
-      noFiles = false;
     }
   }
 
-  // at least one file is mandatory, except when `-Xi=…` is used
-  if (noFiles && std::find_if(args.begin(), args.end(), [](const char *arg) {
-                   return startsWith(arg, "-Xi=");
-                 }) == args.end()) {
-    printUsage(originalArgv[0], ldcPath);
-    if (originalArgc == 1)
-      exit(EXIT_FAILURE); // compatible with DMD
-    else
-      error("No source file specified.");
-  }
-
   if (vdmd) {
     printf(" -- Invoking:");
     for (const auto &arg : ldcArgs) {
@@ -762,6 +743,11 @@ int cppmain(int argc, char **argv) {
     error("Could not locate " LDC_EXE_NAME " executable.");
   }
 
+  if (argc == 1) {
+    printUsage(argv[0], ldcPath);
+    exit(EXIT_FAILURE);
+  }
+
   // We need to manually set up argv[0] and the terminating NULL.
   std::vector<const char *> args;
   args.push_back(ldcPath.c_str());

driver/main.cpp

@@ -896,6 +896,11 @@ int cppmain(int argc, char **argv) {
   Strings files;
   parseCommandLine(argc, argv, files);
 
+  if (argc == 1) {
+    cl::PrintHelpMessage(/*Hidden=*/false, /*Categorized=*/true);
+    exit(EXIT_FAILURE);
+  }
+
   if (global.errors) {
     fatal();
   }
@@ -977,10 +982,6 @@ int cppmain(int argc, char **argv) {
   return mars_mainBody(global.params, files, libmodules);
 }
 
-void printLDCUsage() {
-  cl::PrintHelpMessage(/*Hidden=*/false, /*Categorized=*/true);
-}
-
 void codegenModules(Modules &modules) {
   // Generate one or more object/IR/bitcode files/dcompute kernels.
   if (global.params.obj && !modules.empty()) {