src/librustc_mir/monomorphize/mod.rs

use rustc::traits;
use rustc::ty::adjustment::CustomCoerceUnsized;
use rustc::ty::{self, Ty, TyCtxt};

pub mod collector;
pub mod partitioning;

pub fn custom_coerce_unsize_info<'tcx>(
    tcx: TyCtxt<'tcx>,
    source_ty: Ty<'tcx>,
    target_ty: Ty<'tcx>,
) -> CustomCoerceUnsized {
    let def_id = tcx.lang_items().coerce_unsized_trait().unwrap();

    let trait_ref = ty::Binder::bind(ty::TraitRef {
        def_id,
        substs: tcx.mk_substs_trait(source_ty, &[target_ty.into()]),
    });

    match tcx.codegen_fulfill_obligation((ty::ParamEnv::reveal_all(), trait_ref)) {
        Some(traits::VtableImpl(traits::VtableImplData { impl_def_id, .. })) => {
            tcx.coerce_unsized_info(impl_def_id).custom_kind.unwrap()
        }
        vtable => {
            bug!("invalid `CoerceUnsized` vtable: {:?}", vtable);
        }
    }
}