Port to use the new Unify code, which has no UnifyValue trait

but is otherwise mostly the same.

6 files changed, 363 insertions, 125 deletions

diff --git a/src/librustc/middle/infer/freshen.rs b/src/librustc/middle/infer/freshen.rs
index 29f74d12ea3..d93d13beec8 100644
--- a/src/librustc/middle/infer/freshen.rs
+++ b/src/librustc/middle/infer/freshen.rs
@@ -37,7 +37,7 @@ use middle::ty_fold::TypeFolder;
 use std::collections::hash_map::{self, Entry};
 
 use super::InferCtxt;
-use super::unify::ToType;
+use super::unify_key::ToType;
 
 pub struct TypeFreshener<'a, 'tcx:'a> {
     infcx: &'a InferCtxt<'a, 'tcx>,
diff --git a/src/librustc/middle/infer/mod.rs b/src/librustc/middle/infer/mod.rs
index 0f62b440bf3..b0921a266f3 100644
--- a/src/librustc/middle/infer/mod.rs
+++ b/src/librustc/middle/infer/mod.rs
@@ -29,6 +29,7 @@ use middle::ty::replace_late_bound_regions;
 use middle::ty::{self, Ty};
 use middle::ty_fold::{TypeFolder, TypeFoldable};
 use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use rustc_data_structures::unify::{self, UnificationTable};
 use std::cell::{RefCell};
 use std::fmt;
 use std::rc::Rc;
@@ -41,8 +42,8 @@ use util::ppaux::{Repr, UserString};
 
 use self::combine::CombineFields;
 use self::region_inference::{RegionVarBindings, RegionSnapshot};
-use self::unify::{ToType, UnificationTable};
 use self::error_reporting::ErrorReporting;
+use self::unify_key::ToType;
 
 pub mod bivariate;
 pub mod combine;
@@ -57,7 +58,7 @@ pub mod resolve;
 mod freshen;
 pub mod sub;
 pub mod type_variable;
-pub mod unify;
+pub mod unify_key;
 
 pub type Bound<T> = Option<T>;
 pub type UnitResult<'tcx> = RelateResult<'tcx, ()>; // "unify result"
diff --git a/src/librustc/middle/infer/unify_key.rs b/src/librustc/middle/infer/unify_key.rs
new file mode 100644
index 00000000000..6b23e2c5029
--- /dev/null
+++ b/src/librustc/middle/infer/unify_key.rs
@@ -0,0 +1,48 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use middle::ty::{self, IntVarValue, Ty};
+use rustc_data_structures::unify::UnifyKey;
+use syntax::ast;
+
+pub trait ToType<'tcx> {
+    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx>;
+}
+
+impl UnifyKey for ty::IntVid {
+    type Value = Option<IntVarValue>;
+    fn index(&self) -> u32 { self.index }
+    fn from_index(i: u32) -> ty::IntVid { ty::IntVid { index: i } }
+    fn tag(_: Option<ty::IntVid>) -> &'static str { "IntVid" }
+}
+
+impl<'tcx> ToType<'tcx> for IntVarValue {
+    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            ty::IntType(i) => ty::mk_mach_int(tcx, i),
+            ty::UintType(i) => ty::mk_mach_uint(tcx, i),
+        }
+    }
+}
+
+// Floating point type keys
+
+impl UnifyKey for ty::FloatVid {
+    type Value = Option<ast::FloatTy>;
+    fn index(&self) -> u32 { self.index }
+    fn from_index(i: u32) -> ty::FloatVid { ty::FloatVid { index: i } }
+    fn tag(_: Option<ty::FloatVid>) -> &'static str { "FloatVid" }
+}
+
+impl<'tcx> ToType<'tcx> for ast::FloatTy {
+    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
+        ty::mk_mach_float(tcx, *self)
+    }
+}
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index d90a40941cb..6562a748898 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -35,3 +35,4 @@ extern crate serialize as rustc_serialize; // used by deriving
 pub mod snapshot_vec;
 pub mod graph;
 pub mod bitvec;
+pub mod unify;
diff --git a/src/librustc/middle/infer/unify.rs b/src/librustc_data_structures/unify/mod.rs
index 5aec4227136..aff79e25956 100644
--- a/src/librustc/middle/infer/unify.rs
+++ b/src/librustc_data_structures/unify/mod.rs
@@ -8,16 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-pub use self::VarValue::*;
-
 use std::marker;
-
-use middle::ty::{IntVarValue};
-use middle::ty::{self, Ty};
 use std::fmt::Debug;
 use std::marker::PhantomData;
-use syntax::ast;
-use rustc_data_structures::snapshot_vec as sv;
+use snapshot_vec as sv;
+
+#[cfg(test)]
+mod test;
 
 /// This trait is implemented by any type that can serve as a type
 /// variable. We call such variables *unification keys*. For example,
@@ -28,9 +25,10 @@ use rustc_data_structures::snapshot_vec as sv;
 /// `IntVid`, this is `Option<IntVarValue>`, representing some
 /// (possibly not yet known) sort of integer.
 ///
-/// Implementations of this trait are at the end of this file.
-pub trait UnifyKey : Clone + Debug + PartialEq {
-    type Value : UnifyValue;
+/// Clients are expected to provide implementations of this trait; you
+/// can see some examples in the `test` module.
+pub trait UnifyKey : Copy + Clone + Debug + PartialEq {
+    type Value: Clone + PartialEq + Debug;
 
     fn index(&self) -> u32;
 
@@ -39,15 +37,6 @@ pub trait UnifyKey : Clone + Debug + PartialEq {
     fn tag(k: Option<Self>) -> &'static str;
 }
 
-/// Trait for valid types that a type variable can be set to. Note that
-/// this is typically not the end type that the value will take on, but
-/// rather an `Option` wrapper (where `None` represents a variable
-/// whose value is not yet set).
-///
-/// Implementations of this trait are at the end of this file.
-pub trait UnifyValue : Clone + PartialEq + Debug {
-}
-
 /// Value of a unification key. We implement Tarjan's union-find
 /// algorithm: when two keys are unified, one of them is converted
 /// into a "redirect" pointing at the other. These redirects form a
@@ -57,9 +46,10 @@ pub trait UnifyValue : Clone + PartialEq + Debug {
 /// time of the algorithm under control. For more information, see
 /// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
 #[derive(PartialEq,Clone,Debug)]
-pub enum VarValue<K:UnifyKey> {
-    Redirect(K),
-    Root(K::Value, usize),
+pub struct VarValue<K:UnifyKey> {
+    parent: K,       // if equal to self, this is a root
+    value: K::Value, // value assigned (only relevant to root)
+    rank: u32,       // max depth (only relevant to root)
 }
 
 /// Table of unification keys and their values.
@@ -76,16 +66,46 @@ pub struct Snapshot<K:UnifyKey> {
     snapshot: sv::Snapshot,
 }
 
-/// Internal type used to represent the result of a `get()` operation.
-/// Conveys the current root and value of the key.
-pub struct Node<K:UnifyKey> {
-    pub key: K,
-    pub value: K::Value,
-    pub rank: usize,
-}
-
 #[derive(Copy, Clone)]
-pub struct Delegate<K>(PhantomData<K>);
+struct Delegate<K>(PhantomData<K>);
+
+impl<K:UnifyKey> VarValue<K> {
+    fn new_var(key: K, value: K::Value) -> VarValue<K> {
+        VarValue::new(key, value, 0)
+    }
+
+    fn new(parent: K, value: K::Value, rank: u32) -> VarValue<K> {
+        VarValue { parent: parent, // this is a root
+                   value: value,
+                   rank: rank }
+    }
+
+    fn redirect(self, to: K) -> VarValue<K> {
+        VarValue { parent: to, ..self }
+    }
+
+    fn root(self, rank: u32, value: K::Value) -> VarValue<K> {
+        VarValue { rank: rank, value: value, ..self }
+    }
+
+    /// Returns the key of this node. Only valid if this is a root
+    /// node, which you yourself must ensure.
+    fn key(&self) -> K {
+        self.parent
+    }
+
+    fn parent(&self, self_key: K) -> Option<K> {
+        self.if_not_self(self.parent, self_key)
+    }
+
+    fn if_not_self(&self, key: K, self_key: K) -> Option<K> {
+        if key == self_key {
+            None
+        } else {
+            Some(key)
+        }
+    }
+}
 
 // We can't use V:LatticeValue, much as I would like to,
 // because frequently the pattern is that V=Option<U> for some
@@ -95,7 +115,7 @@ pub struct Delegate<K>(PhantomData<K>);
 impl<K:UnifyKey> UnificationTable<K> {
     pub fn new() -> UnificationTable<K> {
         UnificationTable {
-            values: sv::SnapshotVec::new(),
+            values: sv::SnapshotVec::new()
         }
     }
 
@@ -121,12 +141,13 @@ impl<K:UnifyKey> UnificationTable<K> {
     }
 
     pub fn new_key(&mut self, value: K::Value) -> K {
-        let index = self.values.push(Root(value, 0));
-        let k = UnifyKey::from_index(index as u32);
+        let len = self.values.len();
+        let key: K = UnifyKey::from_index(len as u32);
+        self.values.push(VarValue::new_var(key, value));
         debug!("{}: created new key: {:?}",
                UnifyKey::tag(None::<K>),
-               k);
-        k
+               key);
+        key
     }
 
     /// Find the root node for `vid`. This uses the standard
@@ -135,36 +156,34 @@ impl<K:UnifyKey> UnificationTable<K> {
     ///
     /// NB. This is a building-block operation and you would probably
     /// prefer to call `probe` below.
-    fn get(&mut self, vid: K) -> Node<K> {
+    fn get(&mut self, vid: K) -> VarValue<K> {
         let index = vid.index() as usize;
-        let value = (*self.values.get(index)).clone();
-        match value {
-            Redirect(redirect) => {
-                let node: Node<K> = self.get(redirect.clone());
-                if node.key != redirect {
+        let mut value: VarValue<K> = self.values.get(index).clone();
+        match value.parent(vid) {
+            Some(redirect) => {
+                let root: VarValue<K> = self.get(redirect);
+                if root.key() != redirect {
                     // Path compression
-                    self.values.set(index, Redirect(node.key.clone()));
+                    value.parent = root.key();
+                    self.values.set(index, value);
                 }
-                node
+                root
             }
-            Root(value, rank) => {
-                Node { key: vid, value: value, rank: rank }
+            None => {
+                value
             }
         }
     }
 
-    fn is_root(&self, key: &K) -> bool {
+    fn is_root(&self, key: K) -> bool {
         let index = key.index() as usize;
-        match *self.values.get(index) {
-            Redirect(..) => false,
-            Root(..) => true,
-        }
+        self.values.get(index).parent(key).is_none()
     }
 
     /// Sets the value for `vid` to `new_value`. `vid` MUST be a root
     /// node! This is an internal operation used to impl other things.
     fn set(&mut self, key: K, new_value: VarValue<K>) {
-        assert!(self.is_root(&key));
+        assert!(self.is_root(key));
 
         debug!("Updating variable {:?} to {:?}",
                key, new_value);
@@ -181,31 +200,36 @@ impl<K:UnifyKey> UnificationTable<K> {
     /// really more of a building block. If the values associated with
     /// your key are non-trivial, you would probably prefer to call
     /// `unify_var_var` below.
-    fn unify(&mut self, node_a: &Node<K>, node_b: &Node<K>, new_value: K::Value) {
-        debug!("unify(node_a(id={:?}, rank={:?}), node_b(id={:?}, rank={:?}))",
-               node_a.key,
-               node_a.rank,
-               node_b.key,
-               node_b.rank);
-
-        let (new_root, new_rank) = if node_a.rank > node_b.rank {
+    fn unify(&mut self, root_a: VarValue<K>, root_b: VarValue<K>, new_value: K::Value) {
+        debug!("unify(root_a(id={:?}, rank={:?}), root_b(id={:?}, rank={:?}))",
+               root_a.key(),
+               root_a.rank,
+               root_b.key(),
+               root_b.rank);
+
+        if root_a.rank > root_b.rank {
             // a has greater rank, so a should become b's parent,
             // i.e., b should redirect to a.
-            self.set(node_b.key.clone(), Redirect(node_a.key.clone()));
-            (node_a.key.clone(), node_a.rank)
-        } else if node_a.rank < node_b.rank {
+            self.redirect_root(root_a.rank, root_b, root_a, new_value);
+        } else if root_a.rank < root_b.rank {
             // b has greater rank, so a should redirect to b.
-            self.set(node_a.key.clone(), Redirect(node_b.key.clone()));
-            (node_b.key.clone(), node_b.rank)
+            self.redirect_root(root_b.rank, root_a, root_b, new_value);
         } else {
             // If equal, redirect one to the other and increment the
             // other's rank.
-            assert_eq!(node_a.rank, node_b.rank);
-            self.set(node_b.key.clone(), Redirect(node_a.key.clone()));
-            (node_a.key.clone(), node_a.rank + 1)
-        };
+            self.redirect_root(root_a.rank + 1, root_a, root_b, new_value);
+        }
+    }
 
-        self.set(new_root, Root(new_value, new_rank));
+    fn redirect_root(&mut self,
+                     new_rank: u32,
+                     old_root: VarValue<K>,
+                     new_root: VarValue<K>,
+                     new_value: K::Value) {
+        let old_root_key = old_root.key();
+        let new_root_key = new_root.key();
+        self.set(old_root_key, old_root.redirect(new_root_key));
+        self.set(new_root_key, new_root.root(new_rank, new_value));
     }
 }
 
@@ -213,8 +237,31 @@ impl<K:UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
     type Value = VarValue<K>;
     type Undo = ();
 
-    fn reverse(_: &mut Vec<VarValue<K>>, _: ()) {
-        panic!("Nothing to reverse");
+    fn reverse(_: &mut Vec<VarValue<K>>, _: ()) {}
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Base union-find algorithm, where we are just making setes
+
+impl<'tcx,K> UnificationTable<K>
+    where K : UnifyKey<Value=()>,
+{
+    pub fn union(&mut self, a_id: K, b_id: K) {
+        let node_a = self.get(a_id);
+        let node_b = self.get(b_id);
+        let a_id = node_a.key();
+        let b_id = node_b.key();
+        if a_id != b_id {
+            self.unify(node_a, node_b, ());
+        }
+    }
+
+    pub fn find(&mut self, id: K) -> K {
+        self.get(id).key()
+    }
+
+    pub fn unioned(&mut self, a_id: K, b_id: K) -> bool {
+        self.find(a_id) == self.find(b_id)
     }
 }
 
@@ -226,7 +273,6 @@ impl<K:UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
 impl<'tcx,K,V> UnificationTable<K>
     where K: UnifyKey<Value=Option<V>>,
           V: Clone+PartialEq,
-          Option<V>: UnifyValue,
 {
     pub fn unify_var_var(&mut self,
                          a_id: K,
@@ -235,8 +281,8 @@ impl<'tcx,K,V> UnificationTable<K>
     {
         let node_a = self.get(a_id);
         let node_b = self.get(b_id);
-        let a_id = node_a.key.clone();
-        let b_id = node_b.key.clone();
+        let a_id = node_a.key();
+        let b_id = node_b.key();
 
         if a_id == b_id { return Ok(()); }
 
@@ -257,7 +303,7 @@ impl<'tcx,K,V> UnificationTable<K>
             }
         };
 
-        Ok(self.unify(&node_a, &node_b, combined))
+        Ok(self.unify(node_a, node_b, combined))
     }
 
     /// Sets the value of the key `a_id` to `b`. Because simple keys do not have any subtyping
@@ -267,12 +313,12 @@ impl<'tcx,K,V> UnificationTable<K>
                            b: V)
                            -> Result<(),(V,V)>
     {
-        let node_a = self.get(a_id);
-        let a_id = node_a.key.clone();
+        let mut node_a = self.get(a_id);
 
         match node_a.value {
             None => {
-                self.set(a_id, Root(Some(b), node_a.rank));
+                node_a.value = Some(b);
+                self.set(node_a.key(), node_a);
                 Ok(())
             }
 
@@ -295,46 +341,3 @@ impl<'tcx,K,V> UnificationTable<K>
     }
 }
 
-///////////////////////////////////////////////////////////////////////////
-
-// Integral type keys
-
-pub trait ToType<'tcx> {
-    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx>;
-}
-
-impl UnifyKey for ty::IntVid {
-    type Value = Option<IntVarValue>;
-    fn index(&self) -> u32 { self.index }
-    fn from_index(i: u32) -> ty::IntVid { ty::IntVid { index: i } }
-    fn tag(_: Option<ty::IntVid>) -> &'static str { "IntVid" }
-}
-
-impl<'tcx> ToType<'tcx> for IntVarValue {
-    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
-        match *self {
-            ty::IntType(i) => ty::mk_mach_int(tcx, i),
-            ty::UintType(i) => ty::mk_mach_uint(tcx, i),
-        }
-    }
-}
-
-impl UnifyValue for Option<IntVarValue> { }
-
-// Floating point type keys
-
-impl UnifyKey for ty::FloatVid {
-    type Value = Option<ast::FloatTy>;
-    fn index(&self) -> u32 { self.index }
-    fn from_index(i: u32) -> ty::FloatVid { ty::FloatVid { index: i } }
-    fn tag(_: Option<ty::FloatVid>) -> &'static str { "FloatVid" }
-}
-
-impl UnifyValue for Option<ast::FloatTy> {
-}
-
-impl<'tcx> ToType<'tcx> for ast::FloatTy {
-    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
-        ty::mk_mach_float(tcx, *self)
-    }
-}
diff --git a/src/librustc_data_structures/unify/test.rs b/src/librustc_data_structures/unify/test.rs
new file mode 100644
index 00000000000..d662842a37a
--- /dev/null
+++ b/src/librustc_data_structures/unify/test.rs
@@ -0,0 +1,185 @@
+#![allow(non_snake_case)]
+
+extern crate test;
+use self::test::Bencher;
+use std::collections::HashSet;
+use unify::{UnifyKey, UnificationTable};
+
+#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
+struct UnitKey(u32);
+
+impl UnifyKey for UnitKey {
+    type Value = ();
+    fn index(&self) -> u32 { self.0 }
+    fn from_index(u: u32) -> UnitKey { UnitKey(u) }
+    fn tag(_: Option<UnitKey>) -> &'static str { "UnitKey" }
+}
+
+#[test]
+fn basic() {
+    let mut ut: UnificationTable<UnitKey> = UnificationTable::new();
+    let k1 = ut.new_key(());
+    let k2 = ut.new_key(());
+    assert_eq!(ut.unioned(k1, k2), false);
+    ut.union(k1, k2);
+    assert_eq!(ut.unioned(k1, k2), true);
+}
+
+#[test]
+fn big_array() {
+    let mut ut: UnificationTable<UnitKey> = UnificationTable::new();
+    let mut keys = Vec::new();
+    const MAX: usize = 1 << 15;
+
+    for _ in 0..MAX {
+        keys.push(ut.new_key(()));
+    }
+
+    for i in 1..MAX {
+        let l = keys[i-1];
+        let r = keys[i];
+        ut.union(l, r);
+    }
+
+    for i in 0..MAX {
+        assert!(ut.unioned(keys[0], keys[i]));
+    }
+}
+
+#[bench]
+fn big_array_bench(b: &mut Bencher) {
+    let mut ut: UnificationTable<UnitKey> = UnificationTable::new();
+    let mut keys = Vec::new();
+    const MAX: usize = 1 << 15;
+
+    for _ in 0..MAX {
+        keys.push(ut.new_key(()));
+    }
+
+
+    b.iter(|| {
+        for i in 1..MAX {
+            let l = keys[i-1];
+            let r = keys[i];
+            ut.union(l, r);
+        }
+
+        for i in 0..MAX {
+            assert!(ut.unioned(keys[0], keys[i]));
+        }
+    })
+}
+
+#[test]
+fn even_odd() {
+    let mut ut: UnificationTable<UnitKey> = UnificationTable::new();
+    let mut keys = Vec::new();
+    const MAX: usize = 1 << 10;
+
+    for i in 0..MAX {
+        let key = ut.new_key(());
+        keys.push(key);
+
+        if i >= 2 {
+            ut.union(key, keys[i-2]);
+        }
+    }
+
+    for i in 1..MAX {
+        assert!(!ut.unioned(keys[i-1], keys[i]));
+    }
+
+    for i in 2..MAX {
+        assert!(ut.unioned(keys[i-2], keys[i]));
+    }
+}
+
+#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
+struct IntKey(u32);
+
+impl UnifyKey for IntKey {
+    type Value = Option<i32>;
+    fn index(&self) -> u32 { self.0 }
+    fn from_index(u: u32) -> IntKey { IntKey(u) }
+    fn tag(_: Option<IntKey>) -> &'static str { "IntKey" }
+}
+
+/// Test unifying a key whose value is `Some(_)`  with a key whose value is `None`.
+/// Afterwards both should be `Some(_)`.
+#[test]
+fn unify_key_Some_key_None() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    let k2 = ut.new_key(None);
+    assert!(ut.unify_var_var(k1, k2).is_ok());
+    assert_eq!(ut.probe(k2), Some(22));
+    assert_eq!(ut.probe(k1), Some(22));
+}
+
+/// Test unifying a key whose value is `None`  with a key whose value is `Some(_)`.
+/// Afterwards both should be `Some(_)`.
+#[test]
+fn unify_key_None_key_Some() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    let k2 = ut.new_key(None);
+    assert!(ut.unify_var_var(k2, k1).is_ok());
+    assert_eq!(ut.probe(k2), Some(22));
+    assert_eq!(ut.probe(k1), Some(22));
+}
+
+/// Test unifying a key whose value is `Some(x)` with a key whose value is `Some(y)`.
+/// This should yield an error.
+#[test]
+fn unify_key_Some_x_key_Some_y() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    let k2 = ut.new_key(Some(23));
+    assert_eq!(ut.unify_var_var(k1, k2), Err((22, 23)));
+    assert_eq!(ut.unify_var_var(k2, k1), Err((23, 22)));
+    assert_eq!(ut.probe(k1), Some(22));
+    assert_eq!(ut.probe(k2), Some(23));
+}
+
+/// Test unifying a key whose value is `Some(x)` with a key whose value is `Some(x)`.
+/// This should be ok.
+#[test]
+fn unify_key_Some_x_key_Some_x() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    let k2 = ut.new_key(Some(22));
+    assert!(ut.unify_var_var(k1, k2).is_ok());
+    assert_eq!(ut.probe(k1), Some(22));
+    assert_eq!(ut.probe(k2), Some(22));
+}
+
+/// Test unifying a key whose value is `None` with a value is `x`.
+/// Afterwards key should be `x`.
+#[test]
+fn unify_key_None_val() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(None);
+    assert!(ut.unify_var_value(k1, 22).is_ok());
+    assert_eq!(ut.probe(k1), Some(22));
+}
+
+/// Test unifying a key whose value is `Some(x)` with the value `y`.
+/// This should yield an error.
+#[test]
+fn unify_key_Some_x_val_y() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    assert_eq!(ut.unify_var_value(k1, 23), Err((22, 23)));
+    assert_eq!(ut.probe(k1), Some(22));
+}
+
+/// Test unifying a key whose value is `Some(x)` with the value `x`.
+/// This should be ok.
+#[test]
+fn unify_key_Some_x_val_x() {
+    let mut ut: UnificationTable<IntKey> = UnificationTable::new();
+    let k1 = ut.new_key(Some(22));
+    assert!(ut.unify_var_value(k1, 22).is_ok());
+    assert_eq!(ut.probe(k1), Some(22));
+}
+