compiler/rustc_infer/src/infer/projection.rs

use rustc_middle::traits::ObligationCause;
use rustc_middle::ty::{self, Ty};

use crate::traits::{Obligation, PredicateObligation};

use super::InferCtxt;

impl<'tcx> InferCtxt<'tcx> {
    /// Instead of normalizing an associated type projection,
    /// this function generates an inference variable and registers
    /// an obligation that this inference variable must be the result
    /// of the given projection. This allows us to proceed with projections
    /// while they cannot be resolved yet due to missing information or
    /// simply due to the lack of access to the trait resolution machinery.
    pub fn infer_projection(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        projection_ty: ty::AliasTy<'tcx>,
        cause: ObligationCause<'tcx>,
        recursion_depth: usize,
        obligations: &mut Vec<PredicateObligation<'tcx>>,
    ) -> Ty<'tcx> {
        debug_assert!(!self.next_trait_solver());
        let def_id = projection_ty.def_id;
        let ty_var = self.next_ty_var(self.tcx.def_span(def_id));
        let projection = ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::Projection(
            ty::ProjectionPredicate { projection_ty, term: ty_var.into() },
        )));
        let obligation =
            Obligation::with_depth(self.tcx, cause, recursion_depth, param_env, projection);
        obligations.push(obligation);
        ty_var
    }
}