create terminals for explicit layout braces

Author: tek

grammar.js

@@ -86,10 +86,25 @@ module.exports = grammar({
     $._cmd_layout_start_if,
     $._cmd_layout_start_let,
     $._cmd_layout_start_quote,
+    // This variant is used in a `choice` with the others, and serves only to create a terminal node for explicit
+    // braces.
+    // If the scanner emitted the same symbol for virtual and explicit braces, we would either get an anonymous node
+    // ranging over the brace, or a terminal brace node even for virtual starts if we were to alias the symbol to '{'
+    // unconditionally.
+    // So we use separate symbols and alias only this one.
+    // The same reasoning applies to `_cond_layout_end_explicit`.
+    // The terminal could be ensured in different ways – adding an `optional('{')` after the start symbol, using
+    // `seq($._cmd_layout_start_explicit, '{')` instead of including the brace in the scanner range, or branching the
+    // entire layout on the start token to unconditionally use `_cmd_brace_close` instead of
+    // `_cond_layout_end_explicit`.
+    // However, these solutions are all very expensive, adding between 500 and 1000kB to the shared object size, and up
+    // to a second in generation time.
+    $._cmd_layout_start_explicit,
 
     // Emitted when a new line's indent mandates ending the current layout (depending on the layout sort), or when a
     // special inline layout-ending token is encountered, like an `in`.
     $._cond_layout_end,
+    $._cond_layout_end_explicit,
 
     // Instruct the scanner to push or pop a brace context.
     $._cmd_brace_open,

grammar/module.js

@@ -62,11 +62,16 @@ module.exports = {
   declarations: $ => seq(semis($, $._topdecl), semi_opt($)),
 
   _body: $ => seq(
-    $._cmd_layout_start,
+    choice($._cmd_layout_start, alias($._cmd_layout_start_explicit, '{')),
     semi_opt($),
     field('imports', optional($.imports)),
     field('declarations', optional($.declarations)),
+    $._layout_end,
+  ),
+
+  _layout_end: $ => choice(
     $._cond_layout_end,
+    alias($._cond_layout_end_explicit, '}'),
   ),
 
 }

grammar/util.js

@@ -33,23 +33,23 @@ semis = ($, rule) => sep1(semi($), rule),
 /**
  * Wrap a repeated rule in a layout.
  * This is used for `where`, `let`, `of`, `if` and `do`, and the toplevel module.
- * The `start` rule must be one of the externals starting with `_cond_layout_<type>`, which instruct the scanner to push
+ * The `start` rule must be one of the externals starting with `_cmd_layout_<type>`, which instruct the scanner to push
  * a layout context with the current column as its indentation.
  * When a `_cond_layout_end` or `_cond_layout_semicolon` is encountered by the scanner, the recorded indent is compared
  * to the current one to make a decision.
  */
 layout_sort = ($, start, rule) => seq(
-  start,
+  choice(start, alias($._cmd_layout_start_explicit, '{')),
   optional(seq(
     semi_opt($),
     semis($, rule),
     semi_opt($),
   )),
-  $._cond_layout_end,
+  $._layout_end,
 ),
 
 /**
- * Alias of `layout_sort` using the common layout type for the start token, which corresponds to declarations and GADT
+ * Alias for `layout_sort` using the common layout type for the start token, which corresponds to declarations and GADT
  * constructors.
  */
 layout = ($, rule) => layout_sort($, $._cmd_layout_start, rule)