Improve box

[nsvke]

This patch introduces DST support for box, allowing it to correctly
handle unsized types.

It also implements the auto-deref mechanism for box. This ensures that
method dispatch, indexing, and tuple field access work correctly.

gcc/rust/ChangeLog:

* backend/rust-compile-expr.cc (compile_box): Remove DST guard.
(build_box_inner_ptr): Add fat pointer control.
(CompileExpr::visit): Add auto-deref for box.
(HIRCompileBase::resolve_deref_adjustment): Return fat or thin
pointer.
* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit):
Add auto-deref for box.
* typecheck/rust-tyty.cc (ADTType::is_box): New function.
* typecheck/rust-tyty.h (class BaseType): New declaration.

gcc/testsuite/ChangeLog:

* rust/execute/box-dispatch-1.rs: New test.
* rust/execute/box-dispatch-2.rs: New test.
* rust/execute/unsized-adt-size.rs: Format test.

---

This patch implements the 'dispatch_from_dyn' lang item specifically for
'Box'. While there are other smart pointers and types that require this
lang item for dynamic dispatch, they are omitted in this patch as they
are not yet fully supported by the compiler.

gcc/rust/ChangeLog:

* backend/rust-compile-expr.cc (CompileExpr::visit): Handle Box
receivers correctly during dynamic dispatch.
* typecheck/rust-hir-dot-operator.cc (MethodResolver::try_hook):
Extract the inner type of a Box to resolve trait methods.
* util/rust-lang-item.cc (Rust::LangItem::lang_items): Register
dispatch_from_dyn to the BiMap.
* util/rust-lang-item.h (class LangItem): Add DISPATCH_FROM_DYN
to the Kind enum.

gcc/testsuite/ChangeLog:

* rust/execute/box-dispatch-from-dyn.rs: New test.

[philberty]

LGTM nice work

[philberty]

oh wait i see there is a base mr to this