Remove cat_discr

it seems to be some kind of GC-related mess
auto merge of #17978 : arielb1/rust/remaining-garbage, r=nikomatsakis

it seems to be some kind of leftover GC-related detritus
rollup merge of #18221 : jkleint/guide-boxes
rollup merge of #18229 : bjz/ttdelim
rollup merge of #18231 : cakebaker/fix_greater_than_forty_two_closure
rollup merge of #18235 : nikomatsakis/issue-18209
rollup merge of #18239 : msiemens/fix-ice-rename-failed
rollup merge of #18244 : areski/pr-fix-string-doc
rollup merge of #18250 : dotdash/fix_aliasing
rollup merge of #18251 : steveklabnik/build_module_guide
rollup merge of #18256 : SimonSapin/view_item_to_string
rollup merge of #18257 : globin/master
rollup merge of #18265 : jakub-/mailmap
rollup merge of #18266 : nick29581/vtable2.rs
rollup merge of #18303 : cgaebel/make-vec-match-slice
rollup merge of #18307 : Zoxc/stmesg-android
rollup merge of #18309 : cakebaker/fix_off_by_one
rollup merge of #18315 : arielb1/constexpr-addr
rollup merge of #18316 : thestinger/raw
rollup merge of #18320 : chastell/guide_simplify_formatting