Add test.

src/handlers/graphical.rs

@@ -214,8 +214,7 @@ impl GraphicalReportHandler {
         let opts = textwrap::Options::new(width)
             .initial_indent(&initial_indent)
             .subsequent_indent(&rest_indent);
-        let mut message = diagnostic.to_string();
+        let message = self.apply_tags(diagnostic.to_string());
-
 
         writeln!(f, "{}", textwrap::fill(&message, opts))?;
 
@@ -805,6 +804,31 @@ impl GraphicalReportHandler {
         }
         Ok((context_data, lines))
     }
+
+    fn apply_tags(&self, message: String) -> String {
+        let mut message = message;
+
+        for (tag, style) in &self.theme.tags {
+            let open_tag = format!("<{}>", tag);
+            let close_tag = format!("</{}>", tag);
+
+            while let Some(open_index) = message.find(&open_tag) {
+                if let Some(close_index) = message.find(&close_tag) {
+                    let inner = &message[open_index + open_tag.len()..close_index];
+
+                    message = message.replace(
+                        &format!("{}{}{}", open_tag, inner, close_tag),
+                        &inner.style(*style).to_string(),
+                    );
+                } else {
+                    // No closing? Just remove the opening...
+                    message = message.replace(&open_tag, "");
+                }
+            }
+        }
+
+        message
+    }
 }
 
 impl ReportHandler for GraphicalReportHandler {

tests/graphical.rs

@@ -4,6 +4,7 @@ use miette::{
     Diagnostic, GraphicalReportHandler, GraphicalTheme, MietteError, NamedSource,
     NarratableReportHandler, Report, SourceSpan,
 };
+use owo_colors::Style;
 use thiserror::Error;
 
 fn fmt_report(diag: Report) -> String {
@@ -1214,3 +1215,32 @@ fn zero_length_eol_span() {
 
     assert_eq!(expected, out);
 }
+
+#[test]
+fn message_tags() -> Result<(), MietteError> {
+    #[derive(Debug, Diagnostic, Error)]
+    #[error("this <green>is</green> a <yellow>string <blue>with</blue></yellow> many <green>style</green> tags!")]
+    #[diagnostic(code(oops::my::bad))]
+    struct MyBad;
+
+    let mut theme = GraphicalTheme::unicode();
+    theme.tags.insert("green".to_string(), Style::new().green());
+    theme
+        .tags
+        .insert("yellow".to_string(), Style::new().yellow());
+    theme.tags.insert("blue".to_string(), Style::new().blue());
+
+    let err = MyBad;
+    let report: Report = err.into();
+    let mut out = String::new();
+
+    GraphicalReportHandler::new_themed(theme)
+        .with_width(80)
+        .render_report(&mut out, report.as_ref())
+        .unwrap();
+
+    println!("Error: {}", out);
+    let expected = "\u{1b}[31moops::my::bad\u{1b}[0m\n\n  \u{1b}[31m×\u{1b}[0m this \u{1b}[32mis\u{1b}[0m a \u{1b}[33mstring \u{1b}[34mwith\u{1b}[0m\u{1b}[0m many \u{1b}[32mstyle\u{1b}[0m tags!\n";
+    assert_eq!(expected, out);
+    Ok(())
+}