Keep a block's brackets when its folds merge by lines

crates/diffr-core/src/parse/fold_tests.rs

@@ -325,7 +325,8 @@ mod folds {
         // A function whose body is one `match`: the block the shared query
         // folds and the `match` the context query folds cover the same
         // lines, so they are one fold. It is the `match`'s, the innermost
-        // node whose extent that region is, and it carries both tags.
+        // node whose extent that region is, and it carries both tags and
+        // the block's braces, since the `match` has no opener of its own.
         let src =
             "fn f(x: u32) -> u32 {\n    match x {\n        1 => 2,\n        _ => 3,\n    }\n}\n";
         let params = crate::config::Config::from_toml("")
@@ -349,6 +350,11 @@ mod folds {
             "{:?}",
             same_lines[0].tags
         );
+        let syntax = same_lines[0].syntax.expect("the block's opener and closer");
+        assert_eq!(
+            text(src, &syntax),
+            "\n    match x {\n        1 => 2,\n        _ => 3,\n    }\n"
+        );
     }
 
     #[test]

crates/diffr-core/src/parse/folds.rs

@@ -232,7 +232,10 @@ fn depth(node: tree_sitter::Node<'_>) -> usize {
 /// Merge folds of different nodes that cover the same whole lines, such as a
 /// Rust `Self { … }` tail expression and the `{ … }` field list inside it.
 /// The merged fold keeps the innermost node's range, indent and syntax (the
-/// deepest among equal ranges), plus every fold's tags and relations.
+/// deepest among equal ranges), plus every fold's tags and relations. When
+/// the innermost node has no opener, as a lone statement in a block has none,
+/// the fold keeps the opener and closer of the node it merged: they delimit
+/// the same lines, so frontends still find the brackets and the rail.
 /// A fold with no whole line merges with nothing.
 fn merge_by_lines(
     mut folds: Vec<(usize, usize, FoldMetadata)>,
@@ -275,6 +278,7 @@ fn merge_by_lines(
         } else {
             (owner, held, metadata)
         };
+        kept.syntax = kept.syntax.or(lost.syntax);
         kept.tags.extend(lost.tags);
         kept.tags.sort();
         kept.tags.dedup();