Allow Downcast projections unconditionally in const-checking
#71688
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Rollup of 8 pull requests Successful merges: - rust-lang#71148 (Vec drop and truncate: drop using raw slice *mut [T]) - rust-lang#71465 (Add a convenience method on `TyCtxt` for checking for thread locals) - rust-lang#71567 (Handle build completion message from Cargo) - rust-lang#71590 (MIR dump: print pointers consistently with Miri output) - rust-lang#71682 (Bump pulldown-cmark) - rust-lang#71688 (Allow `Downcast` projections unconditionally in const-checking) - rust-lang#71691 (Allow `Unreachable` terminators unconditionally in const-checking) - rust-lang#71719 (Update backtrace-sys) Failed merges: r? @ghost
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ProjectionElem::Downcastsounds scary, but it's really just the projection we use to access a particular enum variant. They usually appear in the lowering of amatchstatement, so they have been associated with control flow in const-checking, but they don't do any control flow by themselves. We already have a HIR pass that looks forifandmatch(even ones that have 1 or fewer reachable branches). That pass is double-checked by a MIR pass that looks forSwitchInts andFakeReads for match scrutinees. In my opinion, there's no need to look forDowncastas well.r? @oli-obk