CFI: Restore typeid_for_instance default behavior

compiler/rustc_codegen_llvm/src/declare.rs

@@ -147,7 +147,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
                 for options in [
                     TypeIdOptions::GENERALIZE_POINTERS,
                     TypeIdOptions::NORMALIZE_INTEGERS,
-                    TypeIdOptions::ERASE_SELF_TYPE,
+                    TypeIdOptions::USE_CONCRETE_SELF,
                 ]
                 .into_iter()
                 .powerset()
@@ -173,9 +173,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
 
         if self.tcx.sess.is_sanitizer_kcfi_enabled() {
             // LLVM KCFI does not support multiple !kcfi_type attachments
-            // Default to erasing the self type. If we need the concrete type, there will be a
-            // hint in the instance.
-            let mut options = TypeIdOptions::ERASE_SELF_TYPE;
+            let mut options = TypeIdOptions::empty();
             if self.tcx.sess.is_sanitizer_cfi_generalize_pointers_enabled() {
                 options.insert(TypeIdOptions::GENERALIZE_POINTERS);
             }

compiler/rustc_symbol_mangling/src/typeid.rs

@@ -24,9 +24,14 @@ bitflags! {
         /// `-fsanitize-cfi-icall-experimental-normalize-integers` option for cross-language LLVM
         /// CFI and  KCFI support.
         const NORMALIZE_INTEGERS = 4;
-        /// Generalize the instance by erasing the concrete `Self` type where possible.
-        /// Only has an effect on `{kcfi_,}typeid_for_instance`.
-        const ERASE_SELF_TYPE = 8;
+        /// Do not perform self type erasure for attaching a secondary type id to methods with their
+        /// concrete self so they can be used as function pointers.
+        ///
+        /// (This applies to typeid_for_instance only and should be used to attach a secondary type
+        /// id to methods during their declaration/definition so they match the type ids returned by
+        /// either typeid_for_instance or typeid_for_fnabi at call sites during code generation for
+        /// type membership tests when methods are used as function pointers.)
+        const USE_CONCRETE_SELF = 8;
     }
 }
 
@@ -69,10 +74,23 @@ pub fn kcfi_typeid_for_instance<'tcx>(
     instance: Instance<'tcx>,
     mut options: TypeIdOptions,
 ) -> u32 {
-    // If we receive a `ReifyShim` intended to produce a function pointer, we need to remain
-    // concrete - abstraction is for vtables.
+    // KCFI support for Rust shares most of its implementation with the CFI support, with some key
+    // differences:
+    //
+    // 1. KCFI performs type tests differently and are implemented as different LLVM passes than CFI
+    //    to not require LTO.
+    // 2. KCFI has the limitation that a function or method may have one type id assigned only.
+    //
+    // Because of the limitation listed above (2), the current KCFI implementation (not CFI) does
+    // reifying of types (i.e., adds shims/trampolines for indirect calls in these cases) for:
+    //
+    // * Supporting casting between function items, closures, and Fn trait objects.
+    // * Supporting methods being cast as function pointers.
+    //
+    // This was implemented for KCFI support in #123106 and #123052 (which introduced the
+    // ReifyReason). The tracking issue for KCFI support for Rust is #123479.
     if matches!(instance.def, InstanceDef::ReifyShim(_, Some(ReifyReason::FnPtr))) {
-        options.remove(TypeIdOptions::ERASE_SELF_TYPE);
+        options.insert(TypeIdOptions::USE_CONCRETE_SELF);
     }
     // A KCFI type metadata identifier is a 32-bit constant produced by taking the lower half of the
     // xxHash64 of the type metadata identifier. (See llvm/llvm-project@cff5bef.)

compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs

@@ -1098,7 +1098,7 @@ pub fn typeid_for_instance<'tcx>(
         instance.args = tcx.mk_args_trait(invoke_ty, trait_ref.args.into_iter().skip(1));
     }
 
-    if options.contains(EncodeTyOptions::ERASE_SELF_TYPE) {
+    if !options.contains(EncodeTyOptions::USE_CONCRETE_SELF) {
         if let Some(impl_id) = tcx.impl_of_method(instance.def_id())
             && let Some(trait_ref) = tcx.impl_trait_ref(impl_id)
         {

tests/codegen/sanitizer/cfi/emit-type-metadata-id-itanium-cxx-abi-method-secondary-typeid.rs

@@ -18,5 +18,5 @@ impl Trait1 for Type1 {
 }
 
 
-// CHECK: ![[TYPE1]] = !{i64 0, !"_ZTSFvu3refIu{{[0-9]+}}NtC{{[[:print:]]+}}_{{[[:print:]]+}}5Type1EE"}
-// CHECK: ![[TYPE2]] = !{i64 0, !"_ZTSFvu3refIu3dynIu{{[0-9]+}}NtC{{[[:print:]]+}}_{{[[:print:]]+}}6Trait1u6regionEEE"}
+// CHECK: ![[TYPE1]] = !{i64 0, !"_ZTSFvu3refIu3dynIu{{[0-9]+}}NtC{{[[:print:]]+}}_{{[[:print:]]+}}6Trait1u6regionEEE"}
+// CHECK: ![[TYPE2]] = !{i64 0, !"_ZTSFvu3refIu{{[0-9]+}}NtC{{[[:print:]]+}}_{{[[:print:]]+}}5Type1EE"}