If you have &strs already (or things that Deref to that, such as Strings), as it looks like you do here, then I suggest this way:
let x = ["A", " ", "simple", " ", "demo", "."].concat();
That way it's short to write, nice to read, and fully efficient -- it does the with_capacity+push_str thing that kornel mentioned internally.
EDIT: A previous conversation, which ironically also had both me and kornel...