Auto merge of #38713 - clarcharr:trusted_len, r=brson
Auto merge of #38713 - clarcharr:trusted_len, r=brson

TrustedLen for Empty and Once.

These implementations were missing, so, I went ahead and added them.

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