Rollup merge of #76823 - RalfJung:black-box-warn, r=joshtriplett
Rollup merge of #76823 - RalfJung:black-box-warn, r=joshtriplett

black_box: silence unused_mut warning when building with cfg(miri)

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