src/librustc_mir/monomorphize/mod.rs

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

use rustc_hir::lang_items::CoerceUnsizedTraitLangItem;

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.require_lang_item(CoerceUnsizedTraitLangItem, None);

    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)) {
        Ok(traits::ImplSourceUserDefined(traits::ImplSourceUserDefinedData {
            impl_def_id,
            ..
        })) => tcx.coerce_unsized_info(impl_def_id).custom_kind.unwrap(),
        impl_source => {
            bug!("invalid `CoerceUnsized` impl_source: {:?}", impl_source);
        }
    }
}