fix(graphical): handle an empty source

In some cases the source can be completely empty -- handle that in a reasonable fashion.

src/handlers/graphical.rs

@@ -379,8 +379,9 @@ impl GraphicalReportHandler {
         // numbers need!
         let linum_width = lines[..]
             .last()
-            .expect("get_lines should always return at least one line?")
-            .line_number
+            .map(|line| line.line_number)
+            // It's possible for the source to be an empty string.
+            .unwrap_or(0)
             .to_string()
             .len();
 
@@ -402,7 +403,7 @@ impl GraphicalReportHandler {
                 contents.line() + 1,
                 contents.column() + 1
             )?;
-        } else if lines.len() == 1 {
+        } else if lines.len() <= 1 {
             writeln!(f, "{}", self.theme.characters.hbar.to_string().repeat(3))?;
         } else {
             writeln!(f, "[{}:{}]", contents.line() + 1, contents.column() + 1)?;

tests/graphical.rs

@@ -34,6 +34,39 @@ fn fmt_report(diag: Report) -> String {
     out
 }
 
+#[test]
+fn empty_source() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("oops!")]
+    #[diagnostic(code(oops::my::bad), help("try doing it better next time?"))]
+    struct MyBad {
+        #[source_code]
+        src: NamedSource,
+        #[label("this bit here")]
+        highlight: SourceSpan,
+    }
+
+    let src = "".to_string();
+    let err = MyBad {
+        src: NamedSource::new("bad_file.rs", src),
+        highlight: (0, 0).into(),
+    };
+    let out = fmt_report(err.into());
+    println!("Error: {}", out);
+    // For an empty string, the label cannot be rendered.
+    let expected = r#"oops::my::bad
+
+  × oops!
+   ╭─[bad_file.rs:1:1]
+   ╰────
+  help: try doing it better next time?
+"#
+    .trim_start()
+    .to_string();
+    assert_eq!(expected, out);
+    Ok(())
+}
+
 #[test]
 fn single_line_with_wide_char() -> Result<(), MietteError> {
     #[derive(Debug, Diagnostic, Error)]