[WIP] Relative VTables for Rust

[PiJoules]

This is a WIP patch for implementing rust-lang/compiler-team#903. It adds a new unstable flag -Zexperimental-relative-rust-abi-vtables that makes vtables PIC-friendly. This is only supported for LLVM codegen and not supported for other backends.

Early feedback on this is welcome. I'm not sure if how I implemented it is the best way of doing so since much of the actual vtable emission is heavily done during LLVM codegen. That is, the vtable to MIR looks like a normal table of pointers and byte arrays and I really only make the vtables relative on the codegen level.

Locally, I can build the stage 1 compiler and runtimes with relative vtables, but I couldn't figure out how to tell the build system to only build stage 1 binaries with this flag, so I work around this by unconditionally enabling relative vtables in rustc. The end goal I think we'd like is either something akin to multilibs in clang where the compiler chooses which runtimes to use based off compilation flags, or binding this ABI to the target and have it be part of the default ABI for that target (just like how relative vtables are the default for Fuchsia in C++ with Clang). I think the later is what target modifiers do (#136966).

Action Items:

• I'm still experimenting with building Fuchsia with this to assert it works e2e and I still need to do some measurements to see if this is still worth pursuing.
• More work will still be needed to ensure the correct relative intrinsics are emitted with CFI and LTO. Rn I'm experimenting on a normal build.

[bjorn3]

I wonder how hard it would be to store true 32bit pointers in the const eval allocation for the vtable. That would avoid all hacks elsewhere around the size mismatch between const eval and runtime.

[rust-log-analyzer]

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checking python file formatting
27 files already formatted
checking C++ file formatting
/checkout/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp:620:64: error: code should be clang-formatted [-Wclang-format-violations]
extern "C" LLVMValueRef LLVMBuildLoadRelative(LLVMBuilderRef B, LLVMValueRef Ptr,
                                                               ^
/checkout/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp:623:44: error: code should be clang-formatted [-Wclang-format-violations]
  Value *call = unwrap(B)->CreateIntrinsic(
                                           ^
/checkout/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp:624:43: error: code should be clang-formatted [-Wclang-format-violations]
      Intrinsic::load_relative, {Int32Ty}, {unwrap(Ptr), unwrap(ByteOffset)});
                                          ^

clang-format linting failed! Printing diff suggestions:
--- /checkout/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp (actual)
+++ /checkout/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp (formatted)
@@ -616,13 +616,14 @@
   LI->setAtomic(fromRust(Order));
   return wrap(LI);
 }
 
-extern "C" LLVMValueRef LLVMBuildLoadRelative(LLVMBuilderRef B, LLVMValueRef Ptr,
+extern "C" LLVMValueRef LLVMBuildLoadRelative(LLVMBuilderRef B,
+                                              LLVMValueRef Ptr,
                                               LLVMValueRef ByteOffset) {
   Type *Int32Ty = Type::getInt32Ty(unwrap(B)->getContext());
-  Value *call = unwrap(B)->CreateIntrinsic(
-      Intrinsic::load_relative, {Int32Ty}, {unwrap(Ptr), unwrap(ByteOffset)});
+  Value *call = unwrap(B)->CreateIntrinsic(Intrinsic::load_relative, {Int32Ty},
+                                           {unwrap(Ptr), unwrap(ByteOffset)});
   return wrap(call);
 }
 
 extern "C" LLVMValueRef LLVMRustBuildAtomicStore(LLVMBuilderRef B,

tidy error: checks with external tool 'clang-format' failed
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Build completed unsuccessfully in 0:01:16
  local time: Thu Aug  7 21:52:56 UTC 2025
  network time: Thu, 07 Aug 2025 21:52:56 GMT
##[error]Process completed with exit code 1.
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[bors]

☔ The latest upstream changes (presumably #147475) made this pull request unmergeable. Please resolve the merge conflicts.