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)

compiler/rustc_middle/src/query/erase.rs

@@ -241,6 +241,7 @@ trivial! {
     Option<rustc_target::abi::FieldIdx>,
     Option<rustc_target::spec::PanicStrategy>,
     Option<usize>,
+    Option<rustc_span::Symbol>,
     Result<(), rustc_errors::ErrorGuaranteed>,
     Result<(), rustc_middle::traits::query::NoSolution>,
     Result<rustc_middle::traits::EvaluationResult, rustc_middle::traits::OverflowError>,

compiler/rustc_middle/src/query/mod.rs

@@ -1760,8 +1760,8 @@ rustc_queries! {
         separate_provide_extern
     }
     /// Whether the function is an intrinsic
-    query is_intrinsic(def_id: DefId) -> bool {
-        desc { |tcx| "checking whether `{}` is an intrinsic", tcx.def_path_str(def_id) }
+    query intrinsic(def_id: DefId) -> Option<Symbol> {
+        desc { |tcx| "fetch intrinsic name if `{}` is an intrinsic", tcx.def_path_str(def_id) }
         separate_provide_extern
     }
     /// Returns the lang items defined in another crate by loading it from metadata.

compiler/rustc_middle/src/ty/util.rs

@@ -18,7 +18,7 @@ use rustc_hir::def_id::{CrateNum, DefId, LocalDefId};
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_macros::HashStable;
 use rustc_session::Limit;
-use rustc_span::sym;
+use rustc_span::{sym, Symbol};
 use rustc_target::abi::{Integer, IntegerType, Primitive, Size};
 use rustc_target::spec::abi::Abi;
 use smallvec::SmallVec;
@@ -1552,9 +1552,15 @@ pub fn is_doc_notable_trait(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
         .any(|items| items.iter().any(|item| item.has_name(sym::notable_trait)))
 }
 
-/// Determines whether an item is an intrinsic by Abi.
-pub fn is_intrinsic(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
-    matches!(tcx.fn_sig(def_id).skip_binder().abi(), Abi::RustIntrinsic | Abi::PlatformIntrinsic)
+/// Determines whether an item is an intrinsic by Abi. or by whether it has a `rustc_intrinsic` attribute
+pub fn intrinsic(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<Symbol> {
+    if matches!(tcx.fn_sig(def_id).skip_binder().abi(), Abi::RustIntrinsic | Abi::PlatformIntrinsic)
+        || tcx.has_attr(def_id, sym::rustc_intrinsic)
+    {
+        Some(tcx.item_name(def_id.into()))
+    } else {
+        None
+    }
 }
 
 pub fn provide(providers: &mut Providers) {
@@ -1562,7 +1568,7 @@ pub fn provide(providers: &mut Providers) {
         reveal_opaque_types_in_bounds,
         is_doc_hidden,
         is_doc_notable_trait,
-        is_intrinsic,
+        intrinsic,
         ..*providers
     }
 }