feat: implements experimental/onEnter (#328)

* feat: implements `experimental/onEnter`

* docs: update readme

* dev: update snapshot

* dev: allow configuration

crates/tinymist-query/src/on_enter.rs

@@ -0,0 +1,98 @@
+//! <https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#on-enter>
+
+use crate::{prelude::*, SyntaxRequest};
+
+/// The [`experimental/onEnter`] request is sent from client to server to handle
+/// the <kbd>Enter</kbd> key press.
+///
+/// - kbd:[Enter] inside triple-slash comments automatically inserts `///`
+/// - kbd:[Enter] in the middle or after a trailing space in `//` inserts `//`
+/// - kbd:[Enter] inside `//!` doc comments automatically inserts `//!`
+///
+/// [`experimental/onEnter`]: https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#on-enter
+///
+/// # Compatibility
+///
+/// This request was introduced in specification version 3.10.0.
+#[derive(Debug, Clone)]
+pub struct OnEnterRequest {
+    /// The path of the document to get folding ranges for.
+    pub path: PathBuf,
+    /// The source code position to request for.
+    pub position: LspPosition,
+}
+
+impl SyntaxRequest for OnEnterRequest {
+    type Response = Vec<TextEdit>;
+
+    fn request(
+        self,
+        source: &Source,
+        position_encoding: PositionEncoding,
+    ) -> Option<Self::Response> {
+        let root = LinkedNode::new(source.root());
+        let cursor = lsp_to_typst::position(self.position, position_encoding, source)?;
+        let leaf = root.leaf_at(cursor)?;
+
+        let worker = OnEnterWorker {
+            source,
+            position_encoding,
+        };
+
+        if matches!(leaf.kind(), SyntaxKind::LineComment) {
+            return worker.enter_line_doc_comment(&leaf, cursor);
+        }
+
+        None
+    }
+}
+
+struct OnEnterWorker<'a> {
+    source: &'a Source,
+    position_encoding: PositionEncoding,
+}
+
+impl OnEnterWorker<'_> {
+    fn enter_line_doc_comment(&self, leaf: &LinkedNode, cursor: usize) -> Option<Vec<TextEdit>> {
+        let skipper = |n: &LinkedNode| {
+            matches!(
+                n.kind(),
+                SyntaxKind::Space | SyntaxKind::Linebreak | SyntaxKind::LineComment
+            )
+        };
+        let parent = leaf.parent()?;
+        let till_curr = parent.children().take(leaf.index());
+        let first_index = till_curr.rev().take_while(skipper).count();
+        let comment_group_cnt = parent
+            .children()
+            .skip(leaf.index().saturating_sub(first_index))
+            .take_while(skipper)
+            .filter(|e| matches!(e.kind(), SyntaxKind::LineComment))
+            .count();
+
+        let comment_prefix = {
+            let mut p = unscanny::Scanner::new(leaf.text());
+            p.eat_while('/');
+            p.eat_if('!');
+            p.before()
+        };
+
+        // Continuing single-line non-doc comments (like this one :) ) is annoying
+        if comment_group_cnt <= 1 && comment_prefix == "//" {
+            return None;
+        }
+
+        // todo: indent
+        let indent = "";
+        // todo: remove_trailing_whitespace
+
+        let rng = cursor..cursor;
+
+        let edit = TextEdit {
+            range: typst_to_lsp::range(rng, self.source, self.position_encoding),
+            new_text: format!("\n{indent}{comment_prefix} $0"),
+        };
+
+        Some(vec![edit])
+    }
+}