Auto merge of #120500 - oli-obk:intrinsics2.0, r=WaffleLapkin

Implement intrinsics with fallback bodies

fixes #93145 (though we can port many more intrinsics)
cc #63585

The way this works is that the backend logic for generating custom code for intrinsics has been made fallible. The only failure path is "this intrinsic is unknown". The `Instance` (that was `InstanceDef::Intrinsic`) then gets converted to `InstanceDef::Item`, which represents the fallback body. A regular function call to that body is then codegenned. This is currently implemented for

* codegen_ssa (so llvm and gcc)
* codegen_cranelift

other backends will need to adjust, but they can just keep doing what they were doing if they prefer (though adding new intrinsics to the compiler will then require them to implement them, instead of getting the fallback body).

cc `@scottmcm` `@WaffleLapkin`

### todo

* [ ] miri support
* [x] default intrinsic name to name of function instead of requiring it to be specified in attribute
* [x] make sure that the bodies are always available (must be collected for metadata)

4 files changed, 7 insertions, 5 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 13d2af7a78e..2cfbaff35ef 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1749,8 +1749,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.root.tables.attr_flags.get(self, index)
     }
 
-    fn get_is_intrinsic(self, index: DefIndex) -> bool {
-        self.root.tables.is_intrinsic.get(self, index)
+    fn get_intrinsic(self, index: DefIndex) -> Option<Symbol> {
+        self.root.tables.intrinsic.get(self, index).map(|d| d.decode(self))
     }
 
     fn get_doc_link_resolutions(self, index: DefIndex) -> DocLinkResMap {
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index 178bfc3a380..9df28799f4f 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -356,7 +356,7 @@ provide! { tcx, def_id, other, cdata,
         cdata.get_stability_implications(tcx).iter().copied().collect()
     }
     stripped_cfg_items => { cdata.get_stripped_cfg_items(cdata.cnum, tcx) }
-    is_intrinsic => { cdata.get_is_intrinsic(def_id.index) }
+    intrinsic => { cdata.get_intrinsic(def_id.index) }
     defined_lang_items => { cdata.get_lang_items(tcx) }
     diagnostic_items => { cdata.get_diagnostic_items() }
     missing_lang_items => { cdata.get_missing_lang_items(tcx) }
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 74750250ab8..fdb2b4f2024 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1409,7 +1409,9 @@ 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));
-                self.tables.is_intrinsic.set(def_id.index, tcx.is_intrinsic(def_id));
+                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);
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 5d67295d407..9c0e8029571 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -375,7 +375,7 @@ macro_rules! define_tables {
 
 define_tables! {
 - defaulted:
-    is_intrinsic: Table<DefIndex, bool>,
+    intrinsic: Table<DefIndex, Option<LazyValue<Symbol>>>,
     is_macro_rules: Table<DefIndex, bool>,
     is_type_alias_impl_trait: Table<DefIndex, bool>,
     type_alias_is_lazy: Table<DefIndex, bool>,