[WIP] 5 - Transmutes#159082
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This patch implements Rust's equivalent of Clang's function pointer type discriminator computation used in pointer authentication. Compatibility with Clang is a primary goal. The discriminator produced for a given external "C" function type must match the value computed by Clang so that function pointers can be exchanged safely between Rust and C code while preserving pointer authentication semantics. The implementation mirrors Clang's behavior in `ASTContext::encodeTypeForFunctionPointerAuth`, ensuring that identical C-compatible function types produce identical discriminators. See: <https://clang.llvm.org/doxygen/ASTContext_8cpp.html#abb1375e068e807917527842d05cadea3>.
This patch introduces the following: * Extends `FnAbi` (`callconv`) with a `ptrauth_type_discriminator` field. This field is only used when emitting pointer authentication call bundles. It is stored in `FnAbi` because the call site is not guaranteed to have access to an `Instance`, so the discriminator cannot always be computed on demand. * Adds support for `llvm.ptrauth.resign`. This intrinsic will be used when support for semantic transmute is added. * Performs a minor API redesign as groundwork for allowing call sites to modify schemas in place.
Also remove error messages/tests that used to guarded it.
The codegen now walks the layout of static initializer types to find extern "C" function pointer fields, computes their type discriminators, and applies those discriminators when emitting authenticated function pointer relocations. Also make sure that type discrimination is never applied to init/fini entries.
Implement pointer authentication domain handling for function pointer transmutes. When function pointer type discrimination is enabled, transmuting between function pointer types with different authentication domains now re-signs the pointer using the appropriate discriminator.
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Implement pointer authentication domain handling for function pointer transmutes. When function pointer type discrimination is enabled, transmuting between function pointer types with different authentication domains now re-signs the pointer using the appropriate discriminator.
This is part 5 of a sequence of 8 PRs, that together aim to bring function pointer type discrimination support:
Useful links:
pauthtestintroduction: Introduce aarch64-unknown-linux-pauthtest target #155722