Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/assistant/src/slash_command/project_command.rs

@@ -0,0 +1,151 @@
+use super::{SlashCommand, SlashCommandOutput};
+use anyhow::{anyhow, Context, Result};
+use fs::Fs;
+use gpui::{AppContext, Model, Task, WeakView};
+use language::LspAdapterDelegate;
+use project::{Project, ProjectPath};
+use std::{
+    fmt::Write,
+    path::Path,
+    sync::{atomic::AtomicBool, Arc},
+};
+use ui::{prelude::*, ButtonLike, ElevationIndex};
+use workspace::Workspace;
+
+pub(crate) struct ProjectSlashCommand;
+
+impl ProjectSlashCommand {
+    async fn build_message(fs: Arc<dyn Fs>, path_to_cargo_toml: &Path) -> Result<String> {
+        let buffer = fs.load(path_to_cargo_toml).await?;
+        let cargo_toml: cargo_toml::Manifest = toml::from_str(&buffer)?;
+
+        let mut message = String::new();
+        writeln!(message, "You are in a Rust project.")?;
+
+        if let Some(workspace) = cargo_toml.workspace {
+            writeln!(
+                message,
+                "The project is a Cargo workspace with the following members:"
+            )?;
+            for member in workspace.members {
+                writeln!(message, "- {member}")?;
+            }
+
+            if !workspace.default_members.is_empty() {
+                writeln!(message, "The default members are:")?;
+                for member in workspace.default_members {
+                    writeln!(message, "- {member}")?;
+                }
+            }
+
+            if !workspace.dependencies.is_empty() {
+                writeln!(
+                    message,
+                    "The following workspace dependencies are installed:"
+                )?;
+                for dependency in workspace.dependencies.keys() {
+                    writeln!(message, "- {dependency}")?;
+                }
+            }
+        } else if let Some(package) = cargo_toml.package {
+            writeln!(
+                message,
+                "The project name is \"{name}\".",
+                name = package.name
+            )?;
+
+            let description = package
+                .description
+                .as_ref()
+                .and_then(|description| description.get().ok().cloned());
+            if let Some(description) = description.as_ref() {
+                writeln!(message, "It describes itself as \"{description}\".")?;
+            }
+
+            if !cargo_toml.dependencies.is_empty() {
+                writeln!(message, "The following dependencies are installed:")?;
+                for dependency in cargo_toml.dependencies.keys() {
+                    writeln!(message, "- {dependency}")?;
+                }
+            }
+        }
+
+        Ok(message)
+    }
+
+    fn path_to_cargo_toml(project: Model<Project>, cx: &mut AppContext) -> Option<Arc<Path>> {
+        let worktree = project.read(cx).worktrees().next()?;
+        let worktree = worktree.read(cx);
+        let entry = worktree.entry_for_path("Cargo.toml")?;
+        let path = ProjectPath {
+            worktree_id: worktree.id(),
+            path: entry.path.clone(),
+        };
+        Some(Arc::from(
+            project.read(cx).absolute_path(&path, cx)?.as_path(),
+        ))
+    }
+}
+
+impl SlashCommand for ProjectSlashCommand {
+    fn name(&self) -> String {
+        "project".into()
+    }
+
+    fn description(&self) -> String {
+        "insert current project context".into()
+    }
+
+    fn tooltip_text(&self) -> String {
+        "insert current project context".into()
+    }
+
+    fn complete_argument(
+        &self,
+        _query: String,
+        _cancel: Arc<AtomicBool>,
+        _cx: &mut AppContext,
+    ) -> Task<Result<Vec<String>>> {
+        Task::ready(Err(anyhow!("this command does not require argument")))
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _argument: Option<&str>,
+        workspace: WeakView<Workspace>,
+        _delegate: Arc<dyn LspAdapterDelegate>,
+        cx: &mut WindowContext,
+    ) -> Task<Result<SlashCommandOutput>> {
+        let output = workspace.update(cx, |workspace, cx| {
+            let project = workspace.project().clone();
+            let fs = workspace.project().read(cx).fs().clone();
+            let path = Self::path_to_cargo_toml(project, cx);
+            let output = cx.background_executor().spawn(async move {
+                let path = path.with_context(|| "Cargo.toml not found")?;
+                Self::build_message(fs, &path).await
+            });
+
+            cx.foreground_executor().spawn(async move {
+                let text = output.await?;
+
+                Ok(SlashCommandOutput {
+                    text,
+                    render_placeholder: Arc::new(move |id, unfold, _cx| {
+                        ButtonLike::new(id)
+                            .style(ButtonStyle::Filled)
+                            .layer(ElevationIndex::ElevatedSurface)
+                            .child(Icon::new(IconName::FileTree))
+                            .child(Label::new("Project"))
+                            .on_click(move |_, cx| unfold(cx))
+                            .into_any_element()
+                    }),
+                })
+            })
+        });
+        output.unwrap_or_else(|error| Task::ready(Err(error)))
+    }
+}