Auto merge of #101212 - eholk:dyn-star, r=compiler-errors

Initial implementation of dyn*

This PR adds extremely basic and incomplete support for [dyn*](https://smallcultfollowing.com/babysteps//blog/2022/03/29/dyn-can-we-make-dyn-sized/). The goal is to get something in tree behind a flag to make collaboration easier, and also to make sure the implementation so far is not unreasonable. This PR does quite a few things:

* Introduce `dyn_star` feature flag
* Adds parsing for `dyn* Trait` types
* Defines `dyn* Trait` as a sized type
* Adds support for explicit casts, like `42usize as dyn* Debug`
  * Including const evaluation of such casts
* Adds codegen for drop glue so things are cleaned up properly when a `dyn* Trait` object goes out of scope
* Adds codegen for method calls, at least for methods that take `&self`

Quite a bit is still missing, but this gives us a starting point. Note that this is never intended to become stable surface syntax for Rust, but rather `dyn*` is planned to be used as an implementation detail for async functions in dyn traits.

Joint work with `@nikomatsakis` and `@compiler-errors.`

r? `@bjorn3`

compiler/rustc_middle/src/ty/mod.rs

@@ -58,6 +58,7 @@ use std::ops::ControlFlow;
 use std::{fmt, str};
 
 pub use crate::ty::diagnostics::*;
+pub use rustc_type_ir::DynKind::*;
 pub use rustc_type_ir::InferTy::*;
 pub use rustc_type_ir::RegionKind::*;
 pub use rustc_type_ir::TyKind::*;
@@ -1113,6 +1114,12 @@ pub trait ToPredicate<'tcx> {
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx>;
 }
 
+impl<'tcx> ToPredicate<'tcx> for Predicate<'tcx> {
+    fn to_predicate(self, _tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {
+        self
+    }
+}
+
 impl<'tcx> ToPredicate<'tcx> for Binder<'tcx, PredicateKind<'tcx>> {
     #[inline(always)]
     fn to_predicate(self, tcx: TyCtxt<'tcx>) -> Predicate<'tcx> {