Improved variable lifetime tracking

src/build/scan.go

@@ -260,7 +260,18 @@ func scanFile(path string, functions chan<- *Function) error {
 			if len(tokens) == 1 {
 				name := tokens[0].Text()
 				register := x64.CallRegisters[len(function.variables)]
-				variable := &Variable{Name: name, Register: register}
+				uses := countIdentifier(function.Body, name)
+
+				if uses == 0 {
+					return errors.New(&errors.UnusedVariable{Name: name}, file, tokens[0].Position)
+				}
+
+				variable := &Variable{
+					Name:     name,
+					Register: register,
+					Alive:    uses,
+				}
+
 				function.addVariable(variable)
 				return nil
 			}