Introduce a new /delta command (#17903)

Release Notes:

- Added a new `/delta` command to re-insert changed files that were
previously included in a context.

---------

Co-authored-by: Roy <roy@anthropic.com>

crates/assistant/src/slash_command/delta_command.rs

@@ -0,0 +1,109 @@
+use crate::slash_command::file_command::{FileCommandMetadata, FileSlashCommand};
+use anyhow::Result;
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+};
+use collections::HashSet;
+use futures::future;
+use gpui::{Task, WeakView, WindowContext};
+use language::{BufferSnapshot, LspAdapterDelegate};
+use std::sync::{atomic::AtomicBool, Arc};
+use text::OffsetRangeExt;
+use workspace::Workspace;
+
+pub(crate) struct DeltaSlashCommand;
+
+impl SlashCommand for DeltaSlashCommand {
+    fn name(&self) -> String {
+        "delta".into()
+    }
+
+    fn description(&self) -> String {
+        "re-insert changed files".into()
+    }
+
+    fn menu_text(&self) -> String {
+        "Re-insert Changed Files".into()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancellation_flag: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        unimplemented!()
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _arguments: &[String],
+        context_slash_command_output_sections: &[SlashCommandOutputSection<language::Anchor>],
+        context_buffer: BufferSnapshot,
+        workspace: WeakView<Workspace>,
+        delegate: Option<Arc<dyn LspAdapterDelegate>>,
+        cx: &mut WindowContext,
+    ) -> Task<Result<SlashCommandOutput>> {
+        let mut paths = HashSet::default();
+        let mut file_command_old_outputs = Vec::new();
+        let mut file_command_new_outputs = Vec::new();
+        for section in context_slash_command_output_sections.iter().rev() {
+            if let Some(metadata) = section
+                .metadata
+                .as_ref()
+                .and_then(|value| serde_json::from_value::<FileCommandMetadata>(value.clone()).ok())
+            {
+                if paths.insert(metadata.path.clone()) {
+                    file_command_old_outputs.push(
+                        context_buffer
+                            .as_rope()
+                            .slice(section.range.to_offset(&context_buffer)),
+                    );
+                    file_command_new_outputs.push(Arc::new(FileSlashCommand).run(
+                        &[metadata.path.clone()],
+                        context_slash_command_output_sections,
+                        context_buffer.clone(),
+                        workspace.clone(),
+                        delegate.clone(),
+                        cx,
+                    ));
+                }
+            }
+        }
+
+        cx.background_executor().spawn(async move {
+            let mut output = SlashCommandOutput::default();
+
+            let file_command_new_outputs = future::join_all(file_command_new_outputs).await;
+            for (old_text, new_output) in file_command_old_outputs
+                .into_iter()
+                .zip(file_command_new_outputs)
+            {
+                if let Ok(new_output) = new_output {
+                    if let Some(file_command_range) = new_output.sections.first() {
+                        let new_text = &new_output.text[file_command_range.range.clone()];
+                        if old_text.chars().ne(new_text.chars()) {
+                            output.sections.extend(new_output.sections.into_iter().map(
+                                |section| SlashCommandOutputSection {
+                                    range: output.text.len() + section.range.start
+                                        ..output.text.len() + section.range.end,
+                                    icon: section.icon,
+                                    label: section.label,
+                                    metadata: section.metadata,
+                                },
+                            ));
+                            output.text.push_str(&new_output.text);
+                        }
+                    }
+                }
+            }
+
+            Ok(output)
+        })
+    }
+}