Add @thestinger comment explaining that shrink_to_fit might drop down as close as possible but not to the minimun
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
rollup merge of #18321 : chastell/guide_refresh_testing_output
rollup merge of #18324 : jakub-/eneedstest
rollup merge of #18326 : sfackler/cfg-final
rollup merge of #18327 : vadimcn/17982
rollup merge of #18329 : sfackler/memwriter-clear
rollup merge of #18332 : jbcrail/fix-comments
rollup merge of #18337 : bkoropoff/unboxed-imm-upvar-fixes
rollup merge of #18344 : aochagavia/show-arc
rollup merge of #18346 : aochagavia/closure-fields
rollup merge of #18347 : cakebaker/ffi
rollup merge of #18356 : jakub-/const-pattern-suptype
rollup merge of #18362 : kevinmehall/pprint-struct-pat-shorthand
rollup merge of #18364 : cakebaker/missing_a
rollup merge of #18366 : aochagavia/ascii
rollup merge of #18334 : csherratt/arc-encodable