0180b8fff0
Make `Ty::boxed_ty` return an `Option`
Looks like a good place to use Rust's type system.
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Most of 4ac7bcbaad/compiler/rustc_middle/src/ty/sty.rs (L971-L1963) looks like it could be moved to `TyKind` (then I guess `Ty` should be made to deref to `TyKind`).