Conditionally enabling a feature

I am currently working on a ranged unsigned integer type,and I am using typenum as a dependency.
I am looking for a way to conditionally enable the i128 feature,because I want to support Rust versions going back as much as possible,and enabling it uncoditionally would mean I could not support Rust 1.25.

The only way is to have a feature yourself:

i128 = ["typenum/i128"]

This is why I ended up adding auto-detect build scripts for this in the num crates, so i128 is always enabled when the compiler supports it.

This is what i'll use,thanks.