Except the language can choose from suitable templates (eg instead of a generic matrix multiply template for floats, it can use a library like LAPACK) and does so in a systematic way.

It also has a feature (I can’t recall the name) which is a bit like fexprs (let’s say macros who’s inputs are the types of the arguments of a function) that can generate customised code (eg an FFT depending on the input size) on the fly.