Rollup merge of #68441 - Centril:pprust-as_deref, r=Mark-Simulacrum
Rollup merge of #68441 - Centril:pprust-as_deref, r=Mark-Simulacrum

pprust: use as_deref

Some drive-by cleanup.

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