feat: flatten diag msg with style

crates/interface/src/diagnostics/emitter/human.rs

@@ -189,7 +189,7 @@ impl HumanEmitter {
             .children
             .iter()
             .filter(|sub| sub.span.is_dummy())
-            .map(OwnedMessage::from_subdiagnostic)
+            .map(|sub| OwnedMessage::from_subdiagnostic(sub, self.supports_color()))
             .collect();
 
         let snippet = title
@@ -282,8 +282,12 @@ impl OwnedMessage {
         Self { id: diag.id(), label: diag.label().into_owned(), level: to_as_level(diag.level) }
     }
 
-    fn from_subdiagnostic(sub: &SubDiagnostic) -> Self {
-        Self { id: None, label: sub.label().into_owned(), level: to_as_level(sub.level) }
+    fn from_subdiagnostic(sub: &SubDiagnostic, supports_color: bool) -> Self {
+        Self {
+            id: None,
+            label: sub.label_with_style(supports_color).into_owned(),
+            level: to_as_level(sub.level),
+        }
     }
 
     fn as_ref(&self) -> Message<'_> {

crates/interface/src/diagnostics/mod.rs

@@ -4,7 +4,11 @@
 
 use crate::Span;
 use anstyle::{AnsiColor, Color};
-use std::{borrow::Cow, fmt, panic::Location};
+use std::{
+    borrow::Cow,
+    fmt::{self, Write},
+    panic::Location,
+};
 
 mod builder;
 pub use builder::{DiagBuilder, EmissionGuarantee};
@@ -315,7 +319,12 @@ pub struct SubDiagnostic {
 impl SubDiagnostic {
     /// Formats the diagnostic messages into a single string.
     pub fn label(&self) -> Cow<'_, str> {
-        flatten_messages(&self.messages)
+        flatten_messages(&self.messages, false, self.level)
+    }
+
+    /// Formats the diagnostic messages into a single string with ANSI color codes if applicable.
+    pub fn label_with_style(&self, supports_color: bool) -> Cow<'_, str> {
+        flatten_messages(&self.messages, supports_color, self.level)
     }
 }
 
@@ -377,7 +386,12 @@ impl Diag {
 
     /// Formats the diagnostic messages into a single string.
     pub fn label(&self) -> Cow<'_, str> {
-        flatten_messages(&self.messages)
+        flatten_messages(&self.messages, false, self.level)
+    }
+
+    /// Formats the diagnostic messages into a single string with ANSI color codes if applicable.
+    pub fn label_with_style(&self, supports_color: bool) -> Cow<'_, str> {
+        flatten_messages(&self.messages, supports_color, self.level)
     }
 
     /// Returns the messages of this diagnostic.
@@ -554,11 +568,69 @@ impl Diag {
     }
 }
 
-// TODO: Styles?
-fn flatten_messages(messages: &[(DiagMsg, Style)]) -> Cow<'_, str> {
-    match messages {
-        [] => Cow::Borrowed(""),
-        [(message, _)] => Cow::Borrowed(message.as_str()),
-        messages => messages.iter().map(|(msg, _)| msg.as_str()).collect(),
+/// Flattens diagnostic messages, applying ANSI styles if requested.
+fn flatten_messages(messages: &[(DiagMsg, Style)], with_style: bool, level: Level) -> Cow<'_, str> {
+    if with_style {
+        match messages {
+            [] => Cow::Borrowed(""),
+            [(msg, Style::NoStyle)] => Cow::Borrowed(msg.as_str()),
+            [(msg, style)] => {
+                let mut res = String::new();
+                write_fmt(&mut res, msg, style, level);
+                Cow::Owned(res)
+            }
+            messages => {
+                let mut res = String::new();
+                for (msg, style) in messages {
+                    match style {
+                        Style::NoStyle => res.push_str(msg.as_str()),
+                        _ => write_fmt(&mut res, msg, style, level),
+                    }
+                }
+                Cow::Owned(res)
+            }
+        }
+    } else {
+        match messages {
+            [] => Cow::Borrowed(""),
+            [(message, _)] => Cow::Borrowed(message.as_str()),
+            messages => messages.iter().map(|(msg, _)| msg.as_str()).collect(),
+        }
+    }
+}
+
+fn write_fmt(output: &mut String, msg: &DiagMsg, style: &Style, level: Level) {
+    let ansi_style = style.to_color_spec(level);
+    write!(output, "{}{}{}", ansi_style.render(), msg.as_str(), ansi_style.render_reset()).unwrap();
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_styled_messages() {
+        // Create a diagnostic with styled messages
+        let mut diag = Diag::new(Level::Note, "test");
+
+        diag.highlighted_note(vec![
+            ("plain text ", Style::NoStyle),
+            ("removed", Style::Removal),
+            (" middle ", Style::NoStyle),
+            ("added", Style::Addition),
+        ]);
+
+        let sub = &diag.children[0];
+
+        // Without styles - just concatenated text
+        let plain = sub.label();
+        assert_eq!(plain, "plain text removed middle added");
+
+        // With styles - includes ANSI escape codes
+        let styled = sub.label_with_style(true);
+        assert_eq!(
+            styled.to_string(),
+            "plain text \u{1b}[91mremoved\u{1b}[0m middle \u{1b}[92madded\u{1b}[0m".to_string()
+        );
     }
 }