fix(printer): clamp highlight length to at least 1 (#32)

Fixes: #31

src/printer/graphical_printer.rs

@@ -382,12 +382,13 @@ impl GraphicalReportPrinter {
         let mut highest = 0;
         let chars = &self.theme.characters;
         for hl in single_liners {
+            let hl_len = std::cmp::max(1, hl.len());
             let local_offset = hl.offset() - line.offset;
-            let vbar_offset = local_offset + (hl.len() / 2);
+            let vbar_offset = local_offset + (hl_len / 2);
             let num_left = vbar_offset - local_offset;
-            let num_right = local_offset + hl.len() - vbar_offset - 1;
+            let num_right = local_offset + hl_len - vbar_offset - 1;
             let start = std::cmp::max(local_offset, highest);
-            let end = local_offset + hl.len();
+            let end = local_offset + hl_len;
             if start < end {
                 underlines.push_str(
                     &format!(
@@ -414,9 +415,10 @@ impl GraphicalReportPrinter {
             if let Some(label) = hl.label() {
                 self.write_no_linum(f, linum_width)?;
                 self.render_highlight_gutter(f, max_gutter, line, all_highlights)?;
+                let hl_len = std::cmp::max(1, hl.len());
                 let local_offset = hl.offset() - line.offset;
-                let vbar_offset = local_offset + (hl.len() / 2);
-                let num_right = local_offset + hl.len() - vbar_offset - 1;
+                let vbar_offset = local_offset + (hl_len / 2);
+                let num_right = local_offset + hl_len - vbar_offset - 1;
                 let lines = format!(
                     "{:width$}{}{} {}",
                     " ",

tests/printer.rs

@@ -65,6 +65,48 @@ fn single_line_highlight() -> Result<(), MietteError> {
     Ok(())
 }
 
+#[test]
+fn single_line_highlight_with_empty_span() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("oops!")]
+    #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
+    struct MyBad {
+        src: String,
+        #[snippet(src, "This is the part that broke")]
+        ctx: SourceSpan,
+        #[highlight(ctx)]
+        highlight: SourceSpan,
+    }
+
+    let src = "source\n  text\n    here".to_string();
+    let len = src.len();
+    let err = MyBad {
+        src,
+        ctx: ("bad_file.rs", 0, len).into(),
+        highlight: ("this bit here", 9, 0).into(),
+    };
+    let out = fmt_report(err.into());
+    println!("{}", out);
+    let expected = r#"
+────[oops::my::bad]────────────────────
+
+    × oops!
+
+   ╭───[bad_file.rs:1:1] This is the part that broke:
+ 1 │ source
+ 2 │   text
+   ·   ┬
+   ·   ╰─ this bit here
+ 3 │     here
+
+    ‽ try doing it better next time?
+"#
+    .trim_start()
+    .to_string();
+    assert_eq!(expected, out);
+    Ok(())
+}
+
 #[test]
 fn single_line_highlight_no_label() -> Result<(), MietteError> {
     #[derive(Debug, Diagnostic, Error)]