test: add more tests

src/write.rs

@@ -840,6 +840,64 @@ mod tests {
         "###)
     }
 
+    #[test]
+    fn span_numbers_are_char_offsets() {
+        let source = "🍎 == apple";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_label(Label::new(2..4).with_message("comparison operator"))
+            .finish()
+            .write_to_string(Source::from(source));
+        assert_snapshot!(msg, @r###"
+        Error: 
+           ,-[<unknown>:1:1]
+           |
+         1 | 🍎 == apple
+           |    ^|  
+           |     `-- comparison operator
+        ---'
+        "###);
+    }
+
+    #[test]
+    fn empty_label() {
+        let source = "apple";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_label(Label::new(0..0).with_message("first character"))
+            .finish()
+            .write_to_string(Source::from(source));
+        assert_snapshot!(msg, @r###"
+            Error: 
+               ,-[<unknown>:1:1]
+               |
+             1 | apple
+               | | 
+               | `- first character
+            ---'
+            "###);
+    }
+
+    #[test]
+    fn one_char_label() {
+        let source = "apple";
+        let msg = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
+            .with_config(no_color_and_ascii())
+            .with_label(Label::new(0..1).with_message("first character"))
+            .finish()
+            .write_to_string(Source::from(source));
+        // TODO: it would be nice if this rendered just like the empty_label test
+        assert_snapshot!(msg, @r###"
+            Error: 
+               ,-[<unknown>:1:1]
+               |
+             1 | apple
+               | |  
+               | `-- first character
+            ---'
+            "###);
+    }
+
     #[test]
     fn two_labels_without_messages() {
         let source = "apple == orange;";