Make and use a newtype for OptVariable

src/solve.rs

@@ -2,7 +2,7 @@ use crate::can::ident::Lowercase;
 use crate::can::symbol::Symbol;
 use crate::collections::ImMap;
 use crate::region::Located;
-use crate::subs::{Content, Descriptor, FlatType, Mark, Rank, Subs, Variable};
+use crate::subs::{Content, Descriptor, FlatType, Mark, OptVariable, Rank, Subs, Variable};
 use crate::types::Constraint::{self, *};
 use crate::types::Problem;
 use crate::types::Type::{self, *};
@@ -662,7 +662,7 @@ fn register(subs: &mut Subs, rank: Rank, pools: &mut Pools, content: Content) ->
         content,
         rank,
         mark: Mark::NONE,
-        copy: Variable::NULL,
+        copy: OptVariable::NONE,
     });
 
     pools.get_mut(rank).push(var);

src/subs.rs

@@ -87,6 +87,35 @@ impl Into<Variable> for VarStore {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct OptVariable(usize);
+
+impl OptVariable {
+    pub const NONE: OptVariable = OptVariable(Variable::NULL.0);
+
+    pub fn is_none(&self) -> bool {
+        self == &OptVariable::NONE
+    }
+
+    pub fn is_some(&self) -> bool {
+        self != &OptVariable::NONE
+    }
+
+    pub fn into_variable(self) -> Option<Variable> {
+        if self.is_none() {
+            None
+        } else {
+            Some(Variable(self.0))
+        }
+    }
+}
+
+impl Into<Option<Variable>> for OptVariable {
+    fn into(self) -> Option<Variable> {
+        self.into_variable()
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Variable(usize);
 
@@ -95,7 +124,7 @@ impl Variable {
     // This lets us avoid using Option<Variable> for the Descriptor's
     // copy field, which is a relevant space savings because we make
     // a *ton* of Descriptors.
-    pub const NULL: Variable = Variable(0);
+    const NULL: Variable = Variable(0);
 
     const FIRST_USER_SPACE_VAR: Variable = Variable(1);
 
@@ -103,14 +132,6 @@ impl Variable {
         // This is a hack that should only ever be used for testing!
         Variable(num)
     }
-
-    pub fn is_null(&self) -> bool {
-        self == &Variable::NULL
-    }
-
-    pub fn is_not_null(&self) -> bool {
-        self != &Variable::NULL
-    }
 }
 
 impl fmt::Debug for Variable {
@@ -258,7 +279,7 @@ impl Subs {
     pub fn restore(&mut self, var: Variable) {
         let desc = self.get(var);
 
-        if desc.copy.is_not_null() {
+        if desc.copy.is_some() {
             let content = desc.content;
 
             self.set(var, content.clone().into());
@@ -326,7 +347,7 @@ pub struct Descriptor {
     pub content: Content,
     pub rank: Rank,
     pub mark: Mark,
-    pub copy: Variable,
+    pub copy: OptVariable,
 }
 
 impl fmt::Debug for Descriptor {
@@ -334,7 +355,10 @@ impl fmt::Debug for Descriptor {
         write!(
             f,
             "{:?}, r: {:?}, m: {:?} c: {:?}",
-            self.content, self.rank, self.mark, self.copy
+            self.content,
+            self.rank,
+            self.mark,
+            self.copy.into_variable()
         )
     }
 }
@@ -351,7 +375,7 @@ impl From<Content> for Descriptor {
             content,
             rank: Rank::NONE,
             mark: Mark::NONE,
-            copy: Variable::NULL,
+            copy: OptVariable::NONE,
         }
     }
 }

src/unify.rs

@@ -1,7 +1,7 @@
 use crate::can::ident::{Lowercase, ModuleName, Uppercase};
 use crate::collections::ImMap;
 use crate::subs::Content::{self, *};
-use crate::subs::{Descriptor, FlatType, Mark, Subs, Variable};
+use crate::subs::{Descriptor, FlatType, Mark, OptVariable, Subs, Variable};
 use crate::types::RecordFieldLabel;
 use crate::types::{Mismatch, Problem};
 
@@ -397,7 +397,7 @@ fn merge(subs: &mut Subs, ctx: &Context, content: Content) -> Outcome {
         content,
         rank,
         mark: Mark::NONE,
-        copy: Variable::NULL,
+        copy: OptVariable::NONE,
     };
 
     subs.union(ctx.first, ctx.second, desc);
@@ -420,7 +420,7 @@ fn fresh(subs: &mut Subs, pool: &mut Pool, ctx: &Context, content: Content) -> V
             content,
             rank: ctx.first_desc.rank.min(ctx.second_desc.rank),
             mark: Mark::NONE,
-            copy: Variable::NULL,
+            copy: OptVariable::NONE,
         },
         pool,
     )