auto merge of #13976 : pnkfelix/rust/fsk-fix-13965, r=alexcrichton
auto merge of #13976 : pnkfelix/rust/fsk-fix-13965, r=alexcrichton

Fix #13965.

This commit adopts the second strategy I outlined in #13965, where the bulk of the code is still "smoke tested" (in the sense that rustdoc attempts to run it, sending all of the generated output into a locally allocated `MemWriter`).  The part of the code that is ignored (but included in the presentation) is isolated to a three-line `main` function that invokes the core rendering routine.

In the generated rustdoc output, this leads to a small break between the two code blocks, but I do not think this is a large issue.

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