Auto merge of #135031 - RalfJung:intrinsics-without-body, r=oli-obk

rustc_intrinsic: support functions without body

We synthesize a HIR body `loop {}` but such bodyless intrinsics.

Most of the diff is due to turning `ItemKind::Fn` into a brace (named-field) enum variant, because it carries a `bool`-typed field now. This is to remember whether the function has a body. MIR building panics to avoid ever translating the fake `loop {}` body, and the intrinsic logic uses the lack of a body to implicitly mark that intrinsic as must-be-overridden.

I first tried actually having no body rather than generating the fake body, but there's a *lot* of code that assumes that all function items have HIR and MIR, so this didn't work very well. Then I noticed that even `rustc_intrinsic_must_be_overridden` intrinsics have MIR generated (they are filled with an `Unreachable` terminator) so I guess I am not the first to discover this. ;)

r? `@oli-obk`

Author: bors

src/bin/miri.rs

@@ -242,7 +242,7 @@ impl rustc_driver::Callbacks for MiriBeRustCompilerCalls {
                             let is_reachable_non_generic = matches!(
                                 tcx.hir_node_by_def_id(local_def_id),
                                 Node::Item(&hir::Item {
-                                    kind: hir::ItemKind::Static(..) | hir::ItemKind::Fn(..),
+                                    kind: hir::ItemKind::Static(..) | hir::ItemKind::Fn{ .. },
                                     ..
                                 }) | Node::ImplItem(&hir::ImplItem {
                                     kind: hir::ImplItemKind::Fn(..),