Added environment variable expansion support to the server config file

- use a replaceAll of a compile time regex to expand environment variables of
the form ${ENV_VAR} in the import paths
- added a unit test for the regex
- updated docs

README.md

@@ -224,7 +224,11 @@ The server will attempt to read the file ```${XDG_CONFIG_HOME}/dcd/dcd.conf```
 ```dcd.conf``` on Windows in the current working directory on startup.
 If it exists, each line of the file is interpreted as a path that should be
 searched when looking for module imports. Lines that start with the "#" character
-are ignored.
+are ignored. Lines can contain environment variables which will be expanded
+during loading. The name of the environment variable needs to the enclosed in
+${VAR}. For example:
+
+	${HOME}/sysroot/usr/include/dmd/phobos
 
 Keep in mind that DCD treats import paths the same way that the compiler does.
 For example, a configuration file like this will not work as expected:

src/server/server.d

@@ -376,11 +376,29 @@ void warnAboutOldConfigLocation()
 	}
 }
 
+import std.regex : ctRegex;
+alias envVarRegex = ctRegex!(`\$\{([_a-zA-Z][_a-zA-Z 0-9]*)\}`);
+
+private unittest
+{
+	import std.regex : replaceAll;
+
+	enum input = `${HOME}/aaa/${_bb_b}/ccc`;
+
+	assert(replaceAll!(m => m[1])(input, envVarRegex) == `HOME/aaa/_bb_b/ccc`);
+}
+
 /**
  * Loads import directories from the configuration file
  */
 string[] loadConfiguredImportDirs()
 {
+	string expandEnvVars(string l)
+	{
+		import std.regex : replaceAll;
+		return replaceAll!(m => environment.get(m[1], ""))(l, envVarRegex);
+	}
+
 	warnAboutOldConfigLocation();
 	immutable string configLocation = getConfigurationLocation();
 	if (!configLocation.exists())
@@ -388,8 +406,10 @@ string[] loadConfiguredImportDirs()
 	info("Loading configuration from ", configLocation);
 	File f = File(configLocation, "rt");
 	return f.byLine(KeepTerminator.no)
-		.filter!(a => a.length > 0 && a[0] != '#' && existanceCheck(a))
+		.filter!(a => a.length > 0 && a[0] != '#')
 		.map!(a => a.idup)
+		.map!(expandEnvVars)
+		.filter!(a => existanceCheck(a))
 		.array();
 }