mixin expression takes an arbitrary string and
compiles it and generates instructions accordingly. It
is purely a compile-time mechanism and can only work
on strings available during compilation - a comparison
eval would be highly unfair.
mixin("int b = 5;");
assert(b == 5); // compiles just fine
mixin also works with strings that are constructed
dynamically as long as the available information doesn't
depend on runtime values.
mixin together with CTFE from the next section allows
writing impressive libraries like Pegged
a grammar parser from a grammar defined as a string
in the source code.