`clippy::indexing_slicing`

Use get or get_mut and handle the absent element instead of indexing, which panics out of bounds.

What it reads

Clippy, with the compiler behind it. The only pass that compiles.

Lowest tier: real, and rarely urgent.

The tier bounds how severe a rule may be; its category bounds the tier.

Turn it off, or make it blocking, without touching the code it flags:

rust-doctor --rule clippy::indexing_slicing=off .
rust-doctor --rule clippy::indexing_slicing=error .