Auto merge of #120675 - oli-obk:intrinsics3.0, r=pnkfelix

Add a scheme for moving away from `extern "rust-intrinsic"` entirely

All `rust-intrinsic`s can become free functions now, either with a fallback body, or with a dummy body and an attribute, requiring backends to actually implement the intrinsic.

This PR demonstrates the dummy-body scheme with the `vtable_size` intrinsic.

cc https://github.com/rust-lang/rust/issues/63585

follow-up to #120500

MCP at https://github.com/rust-lang/compiler-team/issues/720

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1749,7 +1749,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.root.tables.attr_flags.get(self, index)
     }
 
-    fn get_intrinsic(self, index: DefIndex) -> Option<Symbol> {
+    fn get_intrinsic(self, index: DefIndex) -> Option<ty::IntrinsicDef> {
         self.root.tables.intrinsic.get(self, index).map(|d| d.decode(self))
     }
 

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1053,11 +1053,14 @@ fn should_encode_mir(
         // Full-fledged functions + closures
         DefKind::AssocFn | DefKind::Fn | DefKind::Closure => {
             let generics = tcx.generics_of(def_id);
-            let opt = tcx.sess.opts.unstable_opts.always_encode_mir
+            let mut opt = tcx.sess.opts.unstable_opts.always_encode_mir
                 || (tcx.sess.opts.output_types.should_codegen()
                     && reachable_set.contains(&def_id)
                     && (generics.requires_monomorphization(tcx)
                         || tcx.cross_crate_inlinable(def_id)));
+            if let Some(intrinsic) = tcx.intrinsic(def_id) {
+                opt &= !intrinsic.must_be_overridden;
+            }
             // The function has a `const` modifier or is in a `#[const_trait]`.
             let is_const_fn = tcx.is_const_fn_raw(def_id.to_def_id())
                 || tcx.is_const_default_method(def_id.to_def_id());
@@ -1409,9 +1412,9 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             if let DefKind::Fn | DefKind::AssocFn = def_kind {
                 self.tables.asyncness.set_some(def_id.index, tcx.asyncness(def_id));
                 record_array!(self.tables.fn_arg_names[def_id] <- tcx.fn_arg_names(def_id));
-                if let Some(name) = tcx.intrinsic(def_id) {
-                    record!(self.tables.intrinsic[def_id] <- name);
-                }
+            }
+            if let Some(name) = tcx.intrinsic(def_id) {
+                record!(self.tables.intrinsic[def_id] <- name);
             }
             if let DefKind::TyParam = def_kind {
                 let default = self.tcx.object_lifetime_default(def_id);

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -375,7 +375,7 @@ macro_rules! define_tables {
 
 define_tables! {
 - defaulted:
-    intrinsic: Table<DefIndex, Option<LazyValue<Symbol>>>,
+    intrinsic: Table<DefIndex, Option<LazyValue<ty::IntrinsicDef>>>,
     is_macro_rules: Table<DefIndex, bool>,
     is_type_alias_impl_trait: Table<DefIndex, bool>,
     type_alias_is_lazy: Table<DefIndex, bool>,