add section about string mixins

hands-on_dlang.md

@@ -84,6 +84,7 @@
             - [Example](#example)
             - [Executing unittests](#executing-unittests)
             - [Code coverage](#code-coverage)
+        - [String mixins](#string-mixins)
 
 ## Setup
 
@@ -1240,7 +1241,6 @@ composed to express complex operations in a readable way. They are based on
 ranges and will work on your datatypes, if they implement the range interface.
 
 ```D
-// Hey come on, just get the whole army!
 import std.algorithm : canFind, map,
   filter, sort, uniq, joiner, chunkBy, splitter;
 import std.array : array, empty;
@@ -1248,12 +1248,11 @@ import std.range : zip;
 import std.stdio : writeln;
 import std.string : format;
 
-void main()
-{
-    string text = q{This tour will give you an
+void main() {
+    string text = `This tour will give you an
 overview of this powerful and expressive systems
 programming language which compiles directly
-to efficient, *native* machine code.};
+to efficient, *native* machine code.`;
 
     // splitting predicate
     alias pred = c => canFind(" ,.\n", c);
@@ -1297,8 +1296,7 @@ D has built-in unittests via a very simple syntax, which can appear anywhere in
 a D module:
 
 ```D
-unittest
-{
+unittest {
     assert(myAbs(-1) == 1);
     assert(myAbs(1)  == 1);
 }
@@ -1334,8 +1332,7 @@ struct Vector3 {
     }
 }
 
-void main()
-{
+void main() {
     Vector3 vec = Vector3(0,1,0);
     writeln(`This vector has been tested: `, vec);
 }
@@ -1343,8 +1340,6 @@ void main()
 // Or just somewhere else
 unittest {
     Vector3 vec;
-    // .init a special built-in property that
-    // returns the initial value of type.
     assert(vec.x == double.init);
 }
 ```
@@ -1359,3 +1354,20 @@ using DUB, just run `dub test`.
 
 D also supports generating coverage reports out of the box, just compile with
 the `-cov` flag. When using DUB, just add the flag `--coverage`.
+
+### String mixins
+
+The mixin expression takes an arbitrary string and compiles it and generates instructions accordingly. This is a compile-time feature, so ths string needs
+to be known at compile-time.
+
+```D
+mixin("int b = 5"); // compiles just fine
+assert(b == 5);
+
+static immutable compileTimeKnownString = q{int c = 5;};
+mixin(compileTimeKnownString); // so does this
+assert(c == 5);
+
+immutable runTimeString = q{int c =5;};
+mixin(runTimeString); // compile error, runTimeString is not known at compile-time
+```