chore: Make terminal_view own the TerminalSlashCommand (#31070)

This reduces 'touch crates/editor/src/editor.rs && cargo +nightly build'
from 8.9s to 8.5s. That same scenario used to take 8s less than a week
ago. :)
I'm measuring with nightly rustc, because it's compile times are better
than those of stable thanks to
https://github.com/rust-lang/rust/pull/138522

main (8.2s total):

![image](https://github.com/user-attachments/assets/767a2ac4-7bba-4147-bd16-9b09eed5b433)

[cargo-timing.html.zip](https://github.com/user-attachments/files/20364175/cargo-timing.html.zip)

#22be776 (7.5s total):

[cargo-timing-20250521T085303.892834Z.html.zip](https://github.com/user-attachments/files/20364391/cargo-timing-20250521T085303.892834Z.html.zip)

![image](https://github.com/user-attachments/assets/c4476df9-cb6e-4403-b0db-de00521f1fd0)


Release Notes:

- N/A

crates/terminal_view/Cargo.toml

@@ -18,6 +18,7 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 async-recursion.workspace = true
+assistant_slash_command.workspace = true
 breadcrumbs.workspace = true
 collections.workspace = true
 db.workspace = true

crates/terminal_view/src/terminal_slash_command.rs

@@ -0,0 +1,129 @@
+use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+
+use crate::{TerminalView, terminal_panel::TerminalPanel};
+use anyhow::Result;
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
+use gpui::{App, Entity, Task, WeakEntity};
+use language::{BufferSnapshot, CodeLabel, LspAdapterDelegate};
+use ui::prelude::*;
+use workspace::{Workspace, dock::Panel};
+
+use assistant_slash_command::create_label_for_command;
+
+pub struct TerminalSlashCommand;
+
+const LINE_COUNT_ARG: &str = "--line-count";
+
+const DEFAULT_CONTEXT_LINES: usize = 50;
+
+impl SlashCommand for TerminalSlashCommand {
+    fn name(&self) -> String {
+        "terminal".into()
+    }
+
+    fn label(&self, cx: &App) -> CodeLabel {
+        create_label_for_command("terminal", &[LINE_COUNT_ARG], cx)
+    }
+
+    fn description(&self) -> String {
+        "Insert terminal output".into()
+    }
+
+    fn icon(&self) -> IconName {
+        IconName::Terminal
+    }
+
+    fn menu_text(&self) -> String {
+        self.description()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn accepts_arguments(&self) -> bool {
+        true
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancel: Arc<AtomicBool>,
+        _workspace: Option<WeakEntity<Workspace>>,
+        _window: &mut Window,
+        _cx: &mut App,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        Task::ready(Ok(Vec::new()))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        arguments: &[String],
+        _context_slash_command_output_sections: &[SlashCommandOutputSection<language::Anchor>],
+        _context_buffer: BufferSnapshot,
+        workspace: WeakEntity<Workspace>,
+        _delegate: Option<Arc<dyn LspAdapterDelegate>>,
+        _: &mut Window,
+        cx: &mut App,
+    ) -> Task<SlashCommandResult> {
+        let Some(workspace) = workspace.upgrade() else {
+            return Task::ready(Err(anyhow::anyhow!("workspace was dropped")));
+        };
+
+        let Some(active_terminal) = resolve_active_terminal(&workspace, cx) else {
+            return Task::ready(Err(anyhow::anyhow!("no active terminal")));
+        };
+
+        let line_count = arguments
+            .get(0)
+            .and_then(|s| s.parse::<usize>().ok())
+            .unwrap_or(DEFAULT_CONTEXT_LINES);
+
+        let lines = active_terminal
+            .read(cx)
+            .entity()
+            .read(cx)
+            .last_n_non_empty_lines(line_count);
+
+        let mut text = String::new();
+        text.push_str("Terminal output:\n");
+        text.push_str(&lines.join("\n"));
+        let range = 0..text.len();
+
+        Task::ready(Ok(SlashCommandOutput {
+            text,
+            sections: vec![SlashCommandOutputSection {
+                range,
+                icon: IconName::Terminal,
+                label: "Terminal".into(),
+                metadata: None,
+            }],
+            run_commands_in_text: false,
+        }
+        .to_event_stream()))
+    }
+}
+
+fn resolve_active_terminal(
+    workspace: &Entity<Workspace>,
+    cx: &mut App,
+) -> Option<Entity<TerminalView>> {
+    if let Some(terminal_view) = workspace
+        .read(cx)
+        .active_item(cx)
+        .and_then(|item| item.act_as::<TerminalView>(cx))
+    {
+        return Some(terminal_view);
+    }
+
+    let terminal_panel = workspace.read(cx).panel::<TerminalPanel>(cx)?;
+    terminal_panel.read(cx).pane().and_then(|pane| {
+        pane.read(cx)
+            .active_item()
+            .and_then(|t| t.downcast::<TerminalView>())
+    })
+}

crates/terminal_view/src/terminal_view.rs

@@ -2,8 +2,10 @@ mod persistence;
 pub mod terminal_element;
 pub mod terminal_panel;
 pub mod terminal_scrollbar;
+mod terminal_slash_command;
 pub mod terminal_tab_tooltip;
 
+use assistant_slash_command::SlashCommandRegistry;
 use editor::{Editor, EditorSettings, actions::SelectAll, scroll::ScrollbarAutoHide};
 use gpui::{
     AnyElement, App, DismissEvent, Entity, EventEmitter, FocusHandle, Focusable, KeyContext,
@@ -29,6 +31,7 @@ use terminal::{
 use terminal_element::{TerminalElement, is_blank};
 use terminal_panel::TerminalPanel;
 use terminal_scrollbar::TerminalScrollHandle;
+use terminal_slash_command::TerminalSlashCommand;
 use terminal_tab_tooltip::TerminalTooltip;
 use ui::{
     ContextMenu, Icon, IconName, Label, Scrollbar, ScrollbarState, Tooltip, h_flex, prelude::*,
@@ -78,6 +81,7 @@ actions!(terminal, [RerunTask]);
 impl_actions!(terminal, [SendText, SendKeystroke]);
 
 pub fn init(cx: &mut App) {
+    assistant_slash_command::init(cx);
     terminal_panel::init(cx);
     terminal::init(cx);
 
@@ -87,6 +91,7 @@ pub fn init(cx: &mut App) {
         workspace.register_action(TerminalView::deploy);
     })
     .detach();
+    SlashCommandRegistry::global(cx).register_command(TerminalSlashCommand, true);
 }
 
 pub struct BlockProperties {