Add support for if statements inline within lambdas

Author: NathanLovato

grammar.js

@@ -203,12 +203,21 @@ module.exports = grammar({
     body: ($) =>
       choice(
         $._simple_statements,
+        $._inline_compound_statement,
         $._newline,
         $._body_end,
         seq($._indent, repeat($._statement), choice($._body_end, $._dedent)),
       ),
 
-    // Simple statements
+    // Compound statements that are valid inline (same line after `:`). elif and
+    // else are not supported by the official compiler (tested in Godot 4.6)
+    _inline_compound_statement: ($) =>
+      choice(
+        alias($._inline_if_statement, $.if_statement),
+        $.for_statement,
+        $.while_statement,
+        $.match_statement,
+      ),
 
     _simple_statements: ($) =>
       seq(
@@ -403,6 +412,17 @@ module.exports = grammar({
         optional(field("alternative", $.else_clause)),
       ),
 
+    // This rule is used to implement inline if statements like `if condition:
+    // do_something()`. elif and else blocks are not supported inline, e.g. if
+    // true: pass elif false: pass does not parse. That's why this rule exists.
+    // Without it, on top of being off spec, the parser gets confused as inline
+    // cases are ambiguous (if a: if b: elif c -> what does elif belong to?).
+    //
+    // We alias this as if_statement so that the AST node type is the same
+    // whether it's an inline if or a block.
+    _inline_if_statement: ($) =>
+      seq("if", field("condition", $._expression), ":", field("body", $.body)),
+
     elif_clause: ($) =>
       seq(
         "elif",

test/corpus/lambdas.txt

@@ -462,3 +462,29 @@ func x():
                       (pass_statement))))))
             (string)))))))
 
+================================
+Lambda with inline if statement
+================================
+
+visibility_changed.connect(func() -> void: if visible: _update())
+
+---
+
+(source
+  (expression_statement
+    (attribute
+      (identifier)
+      (attribute_call
+        (identifier)
+        arguments: (arguments
+          (lambda
+            parameters: (parameters)
+            return_type: (type (identifier))
+            body: (body
+              (if_statement
+                condition: (identifier)
+                body: (body
+                  (expression_statement
+                    (call
+                      (identifier)
+                      arguments: (arguments))))))))))))