Auto merge of #29458 - tshepang:better, r=alexcrichton
Auto merge of #29458 - tshepang:better, r=alexcrichton

I see that `extend()` is not called if new_len > len()

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