Function is_call_from_compiler_builtins_to_upstream_monomorphization 

pub fn is_call_from_compiler_builtins_to_upstream_monomorphization<'tcx>(
    tcx: TyCtxt<'tcx>,
    instance: Instance<'tcx>,
) -> bool

Returns whether a call from the current crate to the Instance would produce a call from compiler_builtins to a symbol the linker must resolve.

Such calls from compiler_builtins are effectively impossible for the linker to handle. Some linkers will optimize such that dead calls to unresolved symbols are not an error, but this is not guaranteed. So we use this function in codegen backends to ensure we do not generate any unlinkable calls.

Note that calls to LLVM intrinsics are uniquely okay because they won’t make it to the linker. Note also that calls to foreign items that are actually exported by the local crate are also okay. This situation arises because compiler-builtins calls functions in core that are #[inline] wrappers for extern "C" declarations in core, which resolve to a symbol exported by compiler-builtins.