Add support for environment variable RDMD_DMD into rdmd

changelog/rdmd-environment-args.dd

@@ -0,0 +1,3 @@
+rdmd now supports specifying the D compiler using the `RDMD_DMD` environment variable
+
+rdmd now uses the `RDMD_DMD` environment variable, if it is present in the environment, to choose the D compiler to use. As with the `--compiler` option, the variable's value must specify the name or path of a compiler with a DMD-like command line syntax, such as `gdmd` or `ldmd2`. The variable overrides the default (which is decided at the time `rdmd` was built), but can still be overridden by the `--compiler` option.

man/man1/rdmd.1

@@ -89,6 +89,12 @@ of the options.
 Specify directory to store cached program and other
 temporaries [default = as per \fIhttp://dlang.org/phobos/std_file.html#.tempDir\fR]
 
+.SH ENVIRONMENT
+.TP
+.B RDMD_DMD
+Specifies the D compiler to use, in the same way as \fB--compiler\fR, when \fB--compiler\fR is not specified.
+.PP
+
 .SH NOTES
 .B dmd
 or

rdmd.d

@@ -64,18 +64,11 @@ else version (LDC)
     private enum defaultCompiler = "ldmd2";
 else
     static assert(false, "Unknown compiler");
-
-private string compiler = defaultCompiler;
+private string compiler = null;
 
 version(unittest) {} else
 int main(string[] args)
 {
-    // Look for the D compiler rdmd invokes automatically in the same directory as rdmd
-    // and fall back to using the one in your path otherwise.
-    string compilerPath = buildPath(dirName(thisExePath()), defaultCompiler ~ binExt);
-    if (Filesystem.existsAsFile(compilerPath))
-        compiler = compilerPath;
-
     if (args.length > 1 && args[1].startsWith("--shebang ", "--shebang="))
     {
         // multiple options wrapped in one
@@ -142,6 +135,7 @@ int main(string[] args)
     string[] eval;     // set by --eval
     bool makeDepend;
     string makeDepFile;
+
     try
     {
         getopt(argsBeforeProgram,
@@ -173,6 +167,24 @@ int main(string[] args)
     if (bailout) return 0;
     if (dryRun) chatty = true; // dry-run implies chatty
 
+    // If we don't have a known compiler specified by the user,
+    // then we need to look to see if it was specified via an environmental argument.
+    // We need to do this due to rdmd being used as the execution engine for shebang files.
+    // This was originally tested with both $DMD, and $DC variable support.
+    // It was removed due to the current test suites being fragile enough that it broke them.
+    if (!compiler)
+        compiler = environment.get("RDMD_DMD", null);
+    if (!compiler)
+    {
+        compiler = defaultCompiler;
+
+        // Look for the D compiler rdmd invokes automatically in the same directory as rdmd
+        // and fall back to using the one in your path otherwise.
+        string compilerPath = buildPath(dirName(thisExePath()), compiler ~ binExt);
+        if (Filesystem.existsAsFile(compilerPath))
+            compiler = compilerPath;
+    }
+
     /* Only -of is supported because Make is very susceptible to file names, and
      * it doesn't do a good job resolving them. One option would be to use
      * std.path.buildNormalizedPath(), but some corner cases will break, so it