Rollup merge of #64893 - SimonSapin:vec-of-option-box, r=sfackler
Rollup merge of #64893 - SimonSapin:vec-of-option-box, r=sfackler

Zero-initialize `vec![None; n]` for `Option<&T>`, `Option<&mut T>` and `Option<Box<T>>`

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