[ruby/prism] Fix in handling

in is a unique keyword because it can be the start of a clause or
an infix keyword. We need to be explicitly sure that even though in
_could_ close an expression context (the body of another in clause)
that we are not also parsing an inline in. The exception is the
case of a command call, which can never be the LHS of an expression,
and so we must immediately exit.

[Bug #21925]
[Bug #21674]

https://github.com/ruby/prism/commit/20374ced51

prism/prism.c

@@ -21678,12 +21678,6 @@ parse_expression(pm_parser_t *parser, pm_binding_power_t binding_power, bool acc
      ) {
         node = parse_expression_infix(parser, node, binding_power, current_binding_powers.right, accepts_command_call, (uint16_t) (depth + 1));
 
-        if (context_terminator(parser->current_context->context, &parser->current)) {
-            // If this token terminates the current context, then we need to
-            // stop parsing the expression, as it has become a statement.
-            return node;
-        }
-
         switch (PM_NODE_TYPE(node)) {
             case PM_MULTI_WRITE_NODE:
                 // Multi-write nodes are statements, and cannot be followed by
@@ -21796,6 +21790,17 @@ parse_expression(pm_parser_t *parser, pm_binding_power_t binding_power, bool acc
                     break;
             }
         }
+
+        if (context_terminator(parser->current_context->context, &parser->current)) {
+            pm_binding_powers_t next_binding_powers = pm_binding_powers[parser->current.type];
+            if (
+                !next_binding_powers.binary ||
+                binding_power > next_binding_powers.left ||
+                (PM_NODE_TYPE_P(node, PM_CALL_NODE) && pm_call_node_command_p((pm_call_node_t *) node))
+            ) {
+                return node;
+            }
+        }
     }
 
     return node;

test/prism/fixtures/case_in_in.txt

@@ -0,0 +1,4 @@
+case args
+in [event]
+  context.event in ^event
+end