Use outline (#29687)

## Before

![Screenshot 2025-04-30 at 10 56
36 AM](https://github.com/user-attachments/assets/3a435f4c-ad45-4f26-a847-2d5c9d03648e)

## After

![Screenshot 2025-04-30 at 10 55
27 AM](https://github.com/user-attachments/assets/cc3a8144-b6fe-4a15-8a47-b2487ce4f66e)

Release Notes:

- Context picker and `@`-mentions now work with very large files.

crates/assistant_tool/Cargo.toml

@@ -24,6 +24,7 @@ language.workspace = true
 language_model.workspace = true
 parking_lot.workspace = true
 project.workspace = true
+regex.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 text.workspace = true

crates/assistant_tool/src/assistant_tool.rs

@@ -1,4 +1,5 @@
 mod action_log;
+pub mod outline;
 mod tool_registry;
 mod tool_schema;
 mod tool_working_set;

crates/assistant_tool/src/outline.rs

@@ -0,0 +1,132 @@
+use crate::ActionLog;
+use anyhow::{Result, anyhow};
+use gpui::{AsyncApp, Entity};
+use language::{OutlineItem, ParseStatus};
+use project::Project;
+use regex::Regex;
+use std::fmt::Write;
+use text::Point;
+
+/// For files over this size, instead of reading them (or including them in context),
+/// we automatically provide the file's symbol outline instead, with line numbers.
+pub const AUTO_OUTLINE_SIZE: usize = 16384;
+
+pub async fn file_outline(
+    project: Entity<Project>,
+    path: String,
+    action_log: Entity<ActionLog>,
+    regex: Option<Regex>,
+    cx: &mut AsyncApp,
+) -> anyhow::Result<String> {
+    let buffer = {
+        let project_path = project.read_with(cx, |project, cx| {
+            project
+                .find_project_path(&path, cx)
+                .ok_or_else(|| anyhow!("Path {path} not found in project"))
+        })??;
+
+        project
+            .update(cx, |project, cx| project.open_buffer(project_path, cx))?
+            .await?
+    };
+
+    action_log.update(cx, |action_log, cx| {
+        action_log.track_buffer(buffer.clone(), cx);
+    })?;
+
+    // Wait until the buffer has been fully parsed, so that we can read its outline.
+    let mut parse_status = buffer.read_with(cx, |buffer, _| buffer.parse_status())?;
+    while *parse_status.borrow() != ParseStatus::Idle {
+        parse_status.changed().await?;
+    }
+
+    let snapshot = buffer.read_with(cx, |buffer, _| buffer.snapshot())?;
+    let Some(outline) = snapshot.outline(None) else {
+        return Err(anyhow!("No outline information available for this file."));
+    };
+
+    render_outline(
+        outline
+            .items
+            .into_iter()
+            .map(|item| item.to_point(&snapshot)),
+        regex,
+        0,
+        usize::MAX,
+    )
+    .await
+}
+
+pub async fn render_outline(
+    items: impl IntoIterator<Item = OutlineItem<Point>>,
+    regex: Option<Regex>,
+    offset: usize,
+    results_per_page: usize,
+) -> Result<String> {
+    let mut items = items.into_iter().skip(offset);
+
+    let entries = items
+        .by_ref()
+        .filter(|item| {
+            regex
+                .as_ref()
+                .is_none_or(|regex| regex.is_match(&item.text))
+        })
+        .take(results_per_page)
+        .collect::<Vec<_>>();
+    let has_more = items.next().is_some();
+
+    let mut output = String::new();
+    let entries_rendered = render_entries(&mut output, entries);
+
+    // Calculate pagination information
+    let page_start = offset + 1;
+    let page_end = offset + entries_rendered;
+    let total_symbols = if has_more {
+        format!("more than {}", page_end)
+    } else {
+        page_end.to_string()
+    };
+
+    // Add pagination information
+    if has_more {
+        writeln!(&mut output, "\nShowing symbols {page_start}-{page_end} (there were more symbols found; use offset: {page_end} to see next page)",
+        )
+    } else {
+        writeln!(
+            &mut output,
+            "\nShowing symbols {page_start}-{page_end} (total symbols: {total_symbols})",
+        )
+    }
+    .ok();
+
+    Ok(output)
+}
+
+fn render_entries(
+    output: &mut String,
+    items: impl IntoIterator<Item = OutlineItem<Point>>,
+) -> usize {
+    let mut entries_rendered = 0;
+
+    for item in items {
+        // Indent based on depth ("" for level 0, "  " for level 1, etc.)
+        for _ in 0..item.depth {
+            output.push(' ');
+        }
+        output.push_str(&item.text);
+
+        // Add position information - convert to 1-based line numbers for display
+        let start_line = item.range.start.row + 1;
+        let end_line = item.range.end.row + 1;
+
+        if start_line == end_line {
+            writeln!(output, " [L{}]", start_line).ok();
+        } else {
+            writeln!(output, " [L{}-{}]", start_line, end_line).ok();
+        }
+        entries_rendered += 1;
+    }
+
+    entries_rendered
+}