error out on unknown environment variables

source/dub/project.d

@@ -1519,11 +1519,23 @@ unittest
 	assert(expandVars!expandVar("$${DUB_EXE:-dub}") == "${DUB_EXE:-dub}");
 }
 
-string expandEnvironmentVariables(string s)
+/// Expands the variables in the input string with the same rules as command
+/// variables inside custom dub commands.
+///
+/// Params:
+///     s = the input string where environment variables in form `$VAR` should be replaced
+///     throwIfMissing = if true, throw an exception if the given variable is not found,
+///                      otherwise replace unknown variables with the empty string.
+string expandEnvironmentVariables(string s, bool throwIfMissing = true)
 {
 	import std.process : environment;
 
-	return expandVars!(v => environment.get(v))(s);
+	return expandVars!((v) {
+		auto ret = environment.get(v);
+		if (ret is null && throwIfMissing)
+			throw new Exception("Specified environment variable `$" ~ v ~ "` is not set");
+		return ret;
+	})(s);
 }
 
 // Keep the following list up-to-date if adding more build settings variables.