Auto merge of #25698 - mdinger:flat_map, r=steveklabnik
Auto merge of #25698 - mdinger:flat_map, r=steveklabnik

I'm not sure why `core` is on but it's blocking the playpen. Doesn't seem to be needed but I'm not sure. It's not on the playpen template and playpen works on release and nightly.

Seems easier to understand without `take()`.

nwin/rust,mvdnes/rust,jashank/rust,mihneadb/rust,pelmers/rust,omasanori/rust,GBGamer/rust,pelmers/rust,jroesch/rust,dwillmer/rust,jashank/rust,pshc/rust,XMPPwocky/rust,vhbit/rust,krzysz00/rust,robertg/rust,zubron/rust,KokaKiwi/rust,nwin/rust,reem/rust,nwin/rust,aepsil0n/rust,jroesch/rust,philyoon/rust,l0kod/rust,zachwick/rust,ejjeong/rust,pshc/rust,nwin/rust,XMPPwocky/rust,krzysz00/rust,aepsil0n/rust,TheNeikos/rust,aepsil0n/rust,aidancully/rust,KokaKiwi/rust,aneeshusa/rust,mvdnes/rust,dwillmer/rust,zubron/rust,KokaKiwi/rust,gifnksm/rust,mihneadb/rust,aidancully/rust,reem/rust,vhbit/rust,zachwick/rust,gifnksm/rust,jashank/rust,jroesch/rust,krzysz00/rust,omasanori/rust,ejjeong/rust,nwin/rust,seanrivera/rust,TheNeikos/rust,omasanori/rust,andars/rust,kwantam/rust,aepsil0n/rust,graydon/rust,KokaKiwi/rust,ejjeong/rust,vhbit/rust,dwillmer/rust,GBGamer/rust,zubron/rust,aidancully/rust,graydon/rust,pelmers/rust,mihneadb/rust,pelmers/rust,TheNeikos/rust,l0kod/rust,ejjeong/rust,XMPPwocky/rust,kwantam/rust,KokaKiwi/rust,jroesch/rust,zubron/rust,pshc/rust,omasanori/rust,aneeshusa/rust,TheNeikos/rust,seanrivera/rust,sae-bom/rust,aidancully/rust,mvdnes/rust,sae-bom/rust,sae-bom/rust,XMPPwocky/rust,zachwick/rust,jroesch/rust,ejjeong/rust,vhbit/rust,omasanori/rust,jashank/rust,GBGamer/rust,sae-bom/rust,philyoon/rust,jashank/rust,graydon/rust,GBGamer/rust,vhbit/rust,jroesch/rust,robertg/rust,andars/rust,aneeshusa/rust,reem/rust,aepsil0n/rust,vhbit/rust,l0kod/rust,aidancully/rust,robertg/rust,robertg/rust,TheNeikos/rust,kwantam/rust,XMPPwocky/rust,zubron/rust,jroesch/rust,reem/rust,mvdnes/rust,l0kod/rust,pshc/rust,krzysz00/rust,philyoon/rust,reem/rust,andars/rust,aneeshusa/rust,nwin/rust,mvdnes/rust,pelmers/rust,l0kod/rust,graydon/rust,pshc/rust,jashank/rust,graydon/rust,gifnksm/rust,seanrivera/rust,gifnksm/rust,dwillmer/rust,dwillmer/rust,nwin/rust,zubron/rust,seanrivera/rust,pshc/rust,zachwick/rust,dwillmer/rust,sae-bom/rust,aidancully/rust,robertg/rust,jroesch/rust,andars/rust,vhbit/rust,vhbit/rust,GBGamer/rust,GBGamer/rust,robertg/rust,l0kod/rust,XMPPwocky/rust,aneeshusa/rust,philyoon/rust,aepsil0n/rust,andars/rust,pshc/rust,pshc/rust,dwillmer/rust,zubron/rust,philyoon/rust,l0kod/rust,mihneadb/rust,graydon/rust,andars/rust,zachwick/rust,kwantam/rust,krzysz00/rust,GBGamer/rust,l0kod/rust,jashank/rust,philyoon/rust,seanrivera/rust,pelmers/rust,dwillmer/rust,reem/rust,gifnksm/rust,mihneadb/rust,TheNeikos/rust,zubron/rust,zachwick/rust,mihneadb/rust,ejjeong/rust,jashank/rust,mvdnes/rust,nwin/rust,krzysz00/rust,sae-bom/rust,kwantam/rust,seanrivera/rust,KokaKiwi/rust,aneeshusa/rust,kwantam/rust,gifnksm/rust,omasanori/rust,GBGamer/rust