Add handling of implicit hash arguments

Arguments that are passed as a hash need special consideration since in certain case they are not treated as keyword arguments. For example, if a call is passing `"a" => 1` as an argument, this will be turned into an implicit hash argument and not a keyword argument.

The existing compiler checks to see if all hash nodes can be treated as keyword arguments. If they can, then it will treat them as keyword arguments. If not, then it will treat them as implicit hash arguments.

This commit implements the same logic inside the Prism compiler.

prism_compile.c

@@ -993,16 +993,49 @@ pm_setup_args(pm_arguments_node_t *arguments_node, int *flags, struct rb_callinf
                       break;
                   }
                   else {
-                      *kw_arg = rb_xmalloc_mul_add(len, sizeof(VALUE), sizeof(struct rb_callinfo_kwarg));
-                      *flags = VM_CALL_KWARG;
-                      (*kw_arg)->keyword_len = (int) len;
+                    // We need to first figure out if all elements of the KeywordHashNode are AssocNodes
+                    // with static literal keys.
+                    // TODO: Figure this out from flags on the KeywordHashNode when Prism supports it
+                    bool all_keys_static_literals = true;
+
+                    for (size_t i = 0; i < len; i++) {
+                        pm_assoc_node_t *assoc = (pm_assoc_node_t *)keyword_arg->elements.nodes[i];
+                        pm_node_t *key = assoc->key;
+
+                        if (!key || !PM_NODE_TYPE_P(key, PM_ASSOC_NODE) || !pm_static_literal_p(key)) {
+                            all_keys_static_literals = false;
+                            break;
+                        }
+                    }
 
-                      // TODO: Method callers like `foo(a => b)`
-                      for (size_t i = 0; i < len; i++) {
-                          pm_assoc_node_t *assoc = (pm_assoc_node_t *)keyword_arg->elements.nodes[i];
-                          (*kw_arg)->keywords[i] = pm_static_literal_value(assoc->key, scope_node, parser);
-                          PM_COMPILE_NOT_POPPED(assoc->value);
-                      }
+                    if (all_keys_static_literals) {
+                        // If they are all static literal keys then we can pass them as keyword arguments.
+                        *kw_arg = rb_xmalloc_mul_add(len, sizeof(VALUE), sizeof(struct rb_callinfo_kwarg));
+                        *flags = VM_CALL_KWARG;
+                        VALUE *keywords = (*kw_arg)->keywords;
+                        (*kw_arg)->references = 0;
+                        (*kw_arg)->keyword_len = (int)len;
+
+                        for (size_t i = 0; i < len; i++) {
+                            pm_assoc_node_t *assoc = (pm_assoc_node_t *)keyword_arg->elements.nodes[i];
+                            pm_node_t *key = assoc->key;
+                            keywords[i] = pm_static_literal_value(key, scope_node, parser);
+                            PM_COMPILE_NOT_POPPED(assoc->value);
+                        }
+                    } else {
+                        // If they aren't all static literal keys then we need to construct a new hash
+                        // and pass that as an argument.
+                        orig_argc++;
+                        *flags |= VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT;
+
+                        for (size_t i = 0; i < len; i++) {
+                            pm_assoc_node_t *assoc = (pm_assoc_node_t *)keyword_arg->elements.nodes[i];
+                            PM_COMPILE_NOT_POPPED(assoc->key);
+                            PM_COMPILE_NOT_POPPED(assoc->value);
+                        }
+
+                        ADD_INSN1(ret, &dummy_line_node, newhash, INT2FIX(len * 2));
+                    }
                   }
                   break;
               }

test/ruby/test_compile_prism.rb

@@ -1329,6 +1329,16 @@ def self.foo(j)
         end
         foo(1)
       CODE
+
+      assert_prism_eval(<<-CODE)
+        def self.prism_opt_var_trail_hash(a = nil, *b, c, **d); end
+        prism_opt_var_trail_hash("a")
+        prism_opt_var_trail_hash("a", c: 1)
+        prism_opt_var_trail_hash("a", "b")
+        prism_opt_var_trail_hash("a", "b", "c")
+        prism_opt_var_trail_hash("a", "b", "c", c: 1)
+        prism_opt_var_trail_hash("a", "b", "c", "c" => 0, c: 1)
+      CODE
     end
 
     def test_CallAndWriteNode