src/librustc_typeck/variance/solve.rs

//! Constraint solving
//!
//! The final phase iterates over the constraints, refining the variance
//! for each inferred until a fixed point is reached. This will be the
//! optimal solution to the constraints. The final variance for each
//! inferred is then written into the `variance_map` in the tcx.

use rustc::hir::def_id::DefId;
use rustc::ty;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::Lrc;

use super::constraints::*;
use super::terms::*;
use super::terms::VarianceTerm::*;
use super::xform::*;

struct SolveContext<'a, 'tcx: 'a> {
    terms_cx: TermsContext<'a, 'tcx>,
    constraints: Vec<Constraint<'a>>,

    // Maps from an InferredIndex to the inferred value for that variable.
    solutions: Vec<ty::Variance>,
}

pub fn solve_constraints(constraints_cx: ConstraintContext<'_, '_>) -> ty::CrateVariancesMap {
    let ConstraintContext { terms_cx, constraints, .. } = constraints_cx;

    let mut solutions = vec![ty::Bivariant; terms_cx.inferred_terms.len()];
    for &(id, ref variances) in &terms_cx.lang_items {
        let InferredIndex(start) = terms_cx.inferred_starts[&id];
        for (i, &variance) in variances.iter().enumerate() {
            solutions[start + i] = variance;
        }
    }

    let mut solutions_cx = SolveContext {
        terms_cx,
        constraints,
        solutions,
    };
    solutions_cx.solve();
    let variances = solutions_cx.create_map();
    let empty_variance = Lrc::new(Vec::new());

    ty::CrateVariancesMap { variances, empty_variance }
}

impl<'a, 'tcx> SolveContext<'a, 'tcx> {
    fn solve(&mut self) {
        // Propagate constraints until a fixed point is reached.  Note
        // that the maximum number of iterations is 2C where C is the
        // number of constraints (each variable can change values at most
        // twice). Since number of constraints is linear in size of the
        // input, so is the inference process.
        let mut changed = true;
        while changed {
            changed = false;

            for constraint in &self.constraints {
                let Constraint { inferred, variance: term } = *constraint;
                let InferredIndex(inferred) = inferred;
                let variance = self.evaluate(term);
                let old_value = self.solutions[inferred];
                let new_value = glb(variance, old_value);
                if old_value != new_value {
                    debug!("Updating inferred {} \
                            from {:?} to {:?} due to {:?}",
                           inferred,
                           old_value,
                           new_value,
                           term);

                    self.solutions[inferred] = new_value;
                    changed = true;
                }
            }
        }
    }

    fn create_map(&self) -> FxHashMap<DefId, Lrc<Vec<ty::Variance>>> {
        let tcx = self.terms_cx.tcx;

        let solutions = &self.solutions;
        self.terms_cx.inferred_starts.iter().map(|(&id, &InferredIndex(start))| {
            let def_id = tcx.hir().local_def_id_from_hir_id(id);
            let generics = tcx.generics_of(def_id);

            let mut variances = solutions[start..start+generics.count()].to_vec();

            debug!("id={} variances={:?}", id, variances);

            // Functions can have unused type parameters: make those invariant.
            if let ty::FnDef(..) = tcx.type_of(def_id).sty {
                for variance in &mut variances {
                    if *variance == ty::Bivariant {
                        *variance = ty::Invariant;
                    }
                }
            }

            (def_id, Lrc::new(variances))
        }).collect()
    }

    fn evaluate(&self, term: VarianceTermPtr<'a>) -> ty::Variance {
        match *term {
            ConstantTerm(v) => v,

            TransformTerm(t1, t2) => {
                let v1 = self.evaluate(t1);
                let v2 = self.evaluate(t2);
                v1.xform(v2)
            }

            InferredTerm(InferredIndex(index)) => self.solutions[index],
        }
    }
}