Rollup merge of #70194 - kornelski:must_split, r=joshtriplett
Rollup merge of #70194 - kornelski:must_split, r=joshtriplett

#[must_use] on split_off()

I've noticed this function used for truncation in the wild. `must_use` will clear that up.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust