Function rustc_trait_selection::traits::project::normalize_projection_type

pub fn normalize_projection_type<'a, 'b, 'tcx>(
    selcx: &'a mut SelectionContext<'b, 'tcx>,
    param_env: ParamEnv<'tcx>,
    projection_ty: AliasTy<'tcx>,
    cause: ObligationCause<'tcx>,
    depth: usize,
    obligations: &mut Vec<PredicateObligation<'tcx>>
) -> Term<'tcx>

The guts of normalize: normalize a specific projection like <T as Trait>::Item. The result is always a type (and possibly additional obligations). If ambiguity arises, which implies that there are unresolved type variables in the projection, we will instantiate it with a fresh type variable $X and generate a new obligation <T as Trait>::Item == $X for later.