FEATURE: Use offsets for errors when parsing an assert.

example_errors/tuple_errors_test.ucg

@@ -0,0 +1,2 @@
+assert |macro |;
+assert |{ foo = hello world}|;

src/build/mod.rs

@@ -243,9 +243,8 @@ impl<'a> Builder<'a> {
         Ok(())
     }
 
-    /// Evaluate an input string as UCG.
+    fn eval_span(&mut self, input: Span) -> Result<Rc<Val>, Box<Error>> {
-    pub fn eval_string(&mut self, input: &str) -> Result<Rc<Val>, Box<Error>> {
+        match parse(input) {
-        match parse(Span::new(input)) {
             Ok(stmts) => {
                 //panic!("Successfully parsed {}", input);
                 let mut out: Option<Rc<Val>> = None;
@@ -268,6 +267,11 @@ impl<'a> Builder<'a> {
         }
     }
 
+    /// Evaluate an input string as UCG.
+    pub fn eval_string(&mut self, input: &str) -> Result<Rc<Val>, Box<Error>> {
+        self.eval_span(Span::new(input))
+    }
+
     /// Builds a ucg file at the named path.
     pub fn build_file(&mut self, name: &'a str) -> BuildResult {
         self.curr_file = Some(name);
@@ -988,7 +992,12 @@ impl<'a> Builder<'a> {
         let expr = &tok.fragment;
         expr_as_stmt.push_str(expr);
         expr_as_stmt.push_str(";");
-        let ok = match self.eval_string(&expr_as_stmt) {
+        let assert_input = Span {
+            fragment: &expr_as_stmt,
+            line: tok.pos.line as u32,
+            offset: tok.pos.column,
+        };
+        let ok = match self.eval_span(assert_input) {
             Ok(v) => v,
             Err(e) => {
                 // failure!