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`

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 35699acb318..339acbad6b9 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -190,7 +190,8 @@ impl<'tcx> Inliner<'tcx> {
 
         // Intrinsic fallback bodies are automatically made cross-crate inlineable,
         // but at this stage we don't know whether codegen knows the intrinsic,
-        // so just conservatively don't inline it.
+        // so just conservatively don't inline it. This also ensures that we do not
+        // accidentally inline the body of an intrinsic that *must* be overridden.
         if self.tcx.has_attr(callsite.callee.def_id(), sym::rustc_intrinsic) {
             return Err("Callee is an intrinsic, do not inline fallback bodies");
         }