Auto merge of #43416 - tshepang:extra-layer, r=alexcrichton
Auto merge of #43416 - tshepang:extra-layer, r=alexcrichton

doc: provide an actual equivalent to filter_map

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