T23a: AI chat, messages

- messages: yours in a quiet bubble, the assistant's as prose beside its mark,
  system notes, errors; avatars, headers with model and time, footers
- MessageList over gpui's bottom-aligned list: drawn near the view, held at the
  newest, counting arrivals while away; a way back down; day separators
- streaming text and markdown reveal toward the text as it grows, with a caret;
  a blinking cursor before the first words
- CodeBlock with language, copy, run, apply, download and folding;
  MarkdownRenderer::code lets messages draw fences with it
- pictures, grids, files, voice notes, video over host frames, link previews
- quotes, actions, an editor that takes focus, versions, regenerate, stop and
  continue
- errors with retry, a rate limit counting down, feedback, thinking blocks
- gallery: the AI Chat page with scripted states

src/documents/render.rs

@@ -17,6 +17,7 @@ use crate::{
 };
 
 type OnLink = Rc<dyn Fn(&str, &mut Window, &mut App)>;
+type DrawCode = Rc<dyn Fn(ElementId, Option<SharedString>, SharedString) -> AnyElement>;
 
 /// Markdown as a document: headings, prose with its styles and links, lists and tasks, quotes, code colored as code, tables, rules, math and footnotes. A press on a link opens it, or hands it to the host.
 #[derive(IntoElement)]
@@ -26,6 +27,7 @@ pub struct MarkdownRenderer {
     on_link: Option<OnLink>,
     size: TextSize,
     sections: Option<Entity<Sections>>,
+    code: Option<DrawCode>,
 }
 
 impl MarkdownRenderer {
@@ -36,6 +38,7 @@ impl MarkdownRenderer {
             on_link: None,
             size: TextSize::Base,
             sections: None,
+            code: None,
         }
     }
 
@@ -51,6 +54,15 @@ impl MarkdownRenderer {
         self
     }
 
+    /// Draws code blocks with the owner's element, given an id, the language and the code.
+    pub fn code(
+        mut self,
+        draw: impl Fn(ElementId, Option<SharedString>, SharedString) -> AnyElement + 'static,
+    ) -> Self {
+        self.code = Some(Rc::new(draw));
+        self
+    }
+
     /// Takes link presses instead of the browser.
     pub fn on_link(mut self, handler: impl Fn(&str, &mut Window, &mut App) + 'static) -> Self {
         self.on_link = Some(Rc::new(handler));
@@ -65,6 +77,7 @@ struct Draw<'a> {
     next: std::cell::Cell<usize>,
     sections: &'a Option<Entity<Sections>>,
     headings: std::cell::Cell<usize>,
+    code: &'a Option<DrawCode>,
 }
 
 impl Draw<'_> {
@@ -192,6 +205,13 @@ impl Draw<'_> {
                 .children(self.nodes(inner, cx))
                 .into_any_element(),
             Node::Code { language, text } => {
+                if let Some(draw) = self.code {
+                    return draw(
+                        self.key("code"),
+                        language.clone().map(Into::into),
+                        text.clone().into(),
+                    );
+                }
                 let styles = code_colors(text, cx);
                 div()
                     .flex()
@@ -406,6 +426,7 @@ impl RenderOnce for MarkdownRenderer {
             next: std::cell::Cell::new(0),
             sections: &self.sections,
             headings: std::cell::Cell::new(0),
+            code: &self.code,
         };
         div()
             .flex()