auto merge of #8992 : chris-morgan/rust/unreachable-macro, r=brson
auto merge of #8992 : chris-morgan/rust/unreachable-macro, r=brson

Rationale: having a function which fails means that the location of
failure which is output is that of the unreachable() function, rather
than the caller.

This is part of #8991 but is not all of it; current usage of
``std::util::unreachable()`` must remain so for the moment, until a new
snapshot is made; then I will remove that function entirely in favour of
using this macro.

mdinger/rust,aneeshusa/rust,aidancully/rust,seanrivera/rust,aepsil0n/rust,KokaKiwi/rust,AerialX/rust-rt-minimal,KokaKiwi/rust,andars/rust,erickt/rust,gifnksm/rust,l0kod/rust,mdinger/rust,vhbit/rust,cllns/rust,mahkoh/rust,dwillmer/rust,barosl/rust,avdi/rust,ebfull/rand,aepsil0n/rust,LeoTestard/rust,nham/rust,rohitjoshi/rust,GBGamer/rust,cllns/rust,nham/rust,zubron/rust,pshc/rust,jbclements/rust,P1start/rust,bluss/rand,P1start/rust,seanrivera/rust,gifnksm/rust,KokaKiwi/rust,TheNeikos/rust,aidancully/rust,jroesch/rust,aneeshusa/rust,stepancheg/rust-ide-rust,dwillmer/rust,reem/rust,emk/rust,stepancheg/rust-ide-rust,reem/rust,richo/rust,philyoon/rust,pczarn/rust,jashank/rust,retep998/rand,jroesch/rust,dwillmer/rust,zachwick/rust,mihneadb/rust,zachwick/rust,andars/rust,nwin/rust,seanrivera/rust,michaelballantyne/rust-gpu,bhickey/rand,GBGamer/rust,quornian/rust,barosl/rust,vhbit/rust,andars/rust,ejjeong/rust,mvdnes/rust,ruud-v-a/rust,aidancully/rust,pshc/rust,pythonesque/rust,jashank/rust,pshc/rust,0x73/rust,untitaker/rust,krzysz00/rust,pczarn/rust,rohitjoshi/rust,l0kod/rust,philyoon/rust,jroesch/rust,mitsuhiko/rust,quornian/rust,pczarn/rust,0x73/rust,victorvde/rust,quornian/rust,sae-bom/rust,jashank/rust,sae-bom/rust,nwin/rust,Ryman/rust,mahkoh/rust,pshc/rust,l0kod/rust,zubron/rust,andars/rust,Ryman/rust,jashank/rust,zubron/rust,XMPPwocky/rust,gifnksm/rust,carols10cents/rust,aturon/rust,fabricedesre/rust,quornian/rust,XMPPwocky/rust,mihneadb/rust,defuz/rust,miniupnp/rust,robertg/rust,stepancheg/rust-ide-rust,erickt/rust,Ryman/rust,jroesch/rust,fabricedesre/rust,emk/rust,mitsuhiko/rust,AerialX/rust,ebfull/rust,carols10cents/rust,zubron/rust,richo/rust,cllns/rust,miniupnp/rust,aidancully/rust,P1start/rust,defuz/rust,mvdnes/rust,miniupnp/rust,P1start/rust,pczarn/rust,zachwick/rust,sae-bom/rust,miniupnp/rust,SiegeLord/rust,aepsil0n/rust,dwillmer/rust,erickt/rust,mdinger/rust,hauleth/rust,0x73/rust,mdinger/rust,pshc/rust,servo/rust,bombless/rust,XMPPwocky/rust,0x73/rust,richo/rust,l0kod/rust,stepancheg/rust-ide-rust,philyoon/rust,aepsil0n/rust,aneeshusa/rust,miniupnp/rust,aidancully/rust,rprichard/rust,omasanori/rust,AerialX/rust,miniupnp/rust,SiegeLord/rust,mahkoh/rust,nwin/rust,GBGamer/rust,achanda/rand,sae-bom/rust,zaeleus/rust,pelmers/rust,aepsil0n/rust,bombless/rust-docs-chinese,mitsuhiko/rust,barosl/rust,mihneadb/rust,XMPPwocky/rust,jbclements/rust,untitaker/rust,pelmers/rust,aturon/rust,philyoon/rust,cllns/rust,kwantam/rust,ktossell/rust,victorvde/rust,kwantam/rust,bombless/rust,zachwick/rust,erickt/rust,servo/rust,quornian/rust,TheNeikos/rust,ebfull/rust,GBGamer/rust,zachwick/rust,jbclements/rust,servo/rust,kimroen/rust,TheNeikos/rust,ruud-v-a/rust,bombless/rust,mahkoh/rust,rohitjoshi/rust,Ryman/rust,P1start/rust,ruud-v-a/rust,AerialX/rust,jbclements/rust,michaelballantyne/rust-gpu,rprichard/rust,defuz/rust,Ryman/rust,SiegeLord/rust,ktossell/rust,dwillmer/rust,robertg/rust,kimroen/rust,rohitjoshi/rust,graydon/rust,mvdnes/rust,emk/rust,pythonesque/rust,AerialX/rust,arthurprs/rand,jroesch/rust,krzysz00/rust,pczarn/rust,andars/rust,pczarn/rust,stepancheg/rust-ide-rust,robertg/rust,hauleth/rust,hauleth/rust,mahkoh/rust,jashank/rust,nwin/rust,ejjeong/rust,nham/rust,SiegeLord/rust,sae-bom/rust,untitaker/rust,kwantam/rust,ebfull/rust,kwantam/rust,aepsil0n/rust,vhbit/rust,richo/rust,emk/rust,mvdnes/rust,ktossell/rust,aneeshusa/rust,carols10cents/rust,barosl/rust,aturon/rust,mitsuhiko/rust,fabricedesre/rust,mdinger/rust,rprichard/rust,bombless/rust,barosl/rust,ruud-v-a/rust,krzysz00/rust,kwantam/rust,mihneadb/rust,kimroen/rust,aturon/rust,jashank/rust,zaeleus/rust,reem/rust,philyoon/rust,avdi/rust,vhbit/rust,avdi/rust,jbclements/rust,KokaKiwi/rust,aturon/rust,bombless/rust,rohitjoshi/rust,graydon/rust,kwantam/rust,P1start/rust,ruud-v-a/rust,kimroen/rust,michaelballantyne/rust-gpu,nham/rust,GrahamDennis/rand,jbclements/rust,jbclements/rust,aturon/rust,SiegeLord/rust,Ryman/rust,pshc/rust,ktossell/rust,victorvde/rust,graydon/rust,jbclements/rust,0x73/rust,hauleth/rust,michaelballantyne/rust-gpu,aneeshusa/rust,carols10cents/rust,ejjeong/rust,robertg/rust,erickt/rust,nwin/rust,fabricedesre/rust,nham/rust,nwin/rust,l0kod/rust,zaeleus/rust,AerialX/rust-rt-minimal,miniupnp/rust,P1start/rust,zubron/rust,ebfull/rust,erickt/rust,AerialX/rust-rt-minimal,seanrivera/rust,victorvde/rust,quornian/rust,LeoTestard/rust,defuz/rust,LeoTestard/rust,kimroen/rust,AerialX/rust-rt-minimal,servo/rust,pythonesque/rust,dwillmer/rust,0x73/rust,defuz/rust,aidancully/rust,zachwick/rust,untitaker/rust,richo/rust,LeoTestard/rust,avdi/rust,l0kod/rust,zubron/rust,LeoTestard/rust,nham/rust,victorvde/rust,andars/rust,pelmers/rust,michaelballantyne/rust-gpu,mihneadb/rust,jashank/rust,jroesch/rust,jroesch/rust,graydon/rust,cllns/rust,richo/rust,TheNeikos/rust,ruud-v-a/rust,zaeleus/rust,pelmers/rust,rprichard/rust,reem/rust,stepancheg/rust-ide-rust,ejjeong/rust,mihneadb/rust,Ryman/rust,stepancheg/rust-ide-rust,philyoon/rust,aneeshusa/rust,TheNeikos/rust,unti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