Horrible...

Everything I said about templates? Double for mixins.

A short mixin can be a wonderful thing. Like for unittests:

string test(string name)()
{
return `
   Stdout("` ~ name ~ `");
   scope (failure) Stdout("failed").newline;
   scope (success) Stdout("passed").newline;`
}
unittest
{
mixin(test!("test naming test"));
}

But mixins should be passive, delegating to concrete methods and classes whenever possible. Line numbers are that important.