aa01891700
make vtable pointers entirely opaque This implements the scheme discussed in https://github.com/rust-lang/unsafe-code-guidelines/issues/338: vtable pointers should be considered entirely opaque and not even readable by Rust code, similar to function pointers. - We have a new kind of `GlobalAlloc` that symbolically refers to a vtable. - Miri uses that kind of allocation when generating a vtable. - The codegen backends, upon encountering such an allocation, call `vtable_allocation` to obtain an actually dataful allocation for this vtable. - We need new intrinsics to obtain the size and align from a vtable (for some `ptr::metadata` APIs), since direct accesses are UB now. I had to touch quite a bit of code that I am not very familiar with, so some of this might not make much sense... r? `@oli-obk`
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.