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```
``````//! 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::DefIdMap;
use rustc_middle::ty;

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

struct SolveContext<'a, 'tcx> {
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<'tcx>(
constraints_cx: ConstraintContext<'_, 'tcx>,
) -> ty::CrateVariancesMap<'tcx> {
let ConstraintContext { terms_cx, constraints, .. } = constraints_cx;

let mut solutions = vec![ty::Bivariant; terms_cx.inferred_terms.len()];
for (id, 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();

ty::CrateVariancesMap { variances }
}

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 enforce_const_invariance(&self, generics: &ty::Generics, variances: &mut [ty::Variance]) {
let tcx = self.terms_cx.tcx;

// Make all const parameters invariant.
for param in generics.own_params.iter() {
if let ty::GenericParamDefKind::Const { .. } = param.kind {
variances[param.index as usize] = ty::Invariant;
}
}

// Make all the const parameters in the parent invariant (recursively).
if let Some(def_id) = generics.parent {
self.enforce_const_invariance(tcx.generics_of(def_id), variances);
}
}

fn create_map(&self) -> DefIdMap<&'tcx [ty::Variance]> {
let tcx = self.terms_cx.tcx;

let solutions = &self.solutions;
DefIdMap::from(self.terms_cx.inferred_starts.items().map(
|(&def_id, &InferredIndex(start))| {
let generics = tcx.generics_of(def_id);
let count = generics.count();

let variances = tcx.arena.alloc_slice(&solutions[start..(start + count)]);

// Const parameters are always invariant.
self.enforce_const_invariance(generics, variances);

// Functions are permitted to have unused generic parameters: make those invariant.
if let ty::FnDef(..) = tcx.type_of(def_id).instantiate_identity().kind() {
for variance in variances.iter_mut() {
if *variance == ty::Bivariant {
*variance = ty::Invariant;
}
}
}

(def_id.to_def_id(), &*variances)
},
))
}

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],
}
}
}
``````