refactor(xatu): unify scope graph occurrence storage

src/xatu/scope_graph.rs

@@ -1,8 +1,7 @@
 #[derive(Debug)]
 pub struct ScopeGraph {
     scopes: Vec<Scope>,
-    declarations: Vec<Declaration>,
-    references: Vec<Reference>,
+    occurrences: Vec<Occurrence>,
 }
 
 impl Default for ScopeGraph {
@@ -15,8 +14,7 @@ impl ScopeGraph {
     pub fn new() -> Self {
         Self {
             scopes: vec![Scope::new(None)],
-            declarations: Vec::new(),
-            references: Vec::new(),
+            occurrences: Vec::new(),
         }
     }
 
@@ -34,22 +32,22 @@ impl ScopeGraph {
     }
 
     pub fn add_declaration(&mut self, scope: ScopeId, name: impl Into<String>) -> DeclarationId {
-        let declaration_id = DeclarationId(self.declarations.len());
-        self.declarations.push(Declaration {
+        let declaration_id = DeclarationId(self.occurrences.len());
+        self.occurrences.push(Occurrence::Declaration(Declaration {
             scope,
             name: name.into(),
-        });
+        }));
         self.scope_mut(scope).declarations.push(declaration_id);
 
         declaration_id
     }
 
     pub fn add_reference(&mut self, scope: ScopeId, name: impl Into<String>) -> ReferenceId {
-        let reference_id = ReferenceId(self.references.len());
-        self.references.push(Reference {
+        let reference_id = ReferenceId(self.occurrences.len());
+        self.occurrences.push(Occurrence::Reference(Reference {
             scope,
             name: name.into(),
-        });
+        }));
         self.scope_mut(scope).references.push(reference_id);
 
         reference_id
@@ -108,11 +106,19 @@ impl ScopeGraph {
     }
 
     fn declaration(&self, declaration: DeclarationId) -> &Declaration {
-        &self.declarations[declaration.0]
+        let Occurrence::Declaration(declaration) = &self.occurrences[declaration.0] else {
+            unreachable!("declaration IDs point to declarations")
+        };
+
+        declaration
     }
 
     fn reference(&self, reference: ReferenceId) -> &Reference {
-        &self.references[reference.0]
+        let Occurrence::Reference(reference) = &self.occurrences[reference.0] else {
+            unreachable!("reference IDs point to references")
+        };
+
+        reference
     }
 }
 
@@ -159,6 +165,12 @@ impl Scope {
     }
 }
 
+#[derive(Debug)]
+enum Occurrence {
+    Declaration(Declaration),
+    Reference(Reference),
+}
+
 #[derive(Debug)]
 struct Declaration {
     scope: ScopeId,