Add support for Prism.parse(foo, version: "current")

The docs currently say to use `Prism.parse(foo, version: RUBY_VERSION)` for this.
By specifying "current" instead, we can have prism raise a more specifc error.

Note: Does not use `ruby_version` from `ruby/version.h` because writing a test for that is not really possible.
`RUBY_VERSION` is nicely stubbable for both the c-ext and FFI backend.

Author: Earlopain

ext/prism/extension.c

@@ -25,6 +25,7 @@ VALUE rb_cPrismLexResult;
 VALUE rb_cPrismParseLexResult;
 VALUE rb_cPrismStringQuery;
 VALUE rb_cPrismScope;
+VALUE rb_cPrismCurrentVersionError;
 
 VALUE rb_cPrismDebugEncoding;
 
@@ -199,7 +200,13 @@ build_options_i(VALUE key, VALUE value, VALUE argument) {
         if (!NIL_P(value)) {
             const char *version = check_string(value);
 
-            if (!pm_options_version_set(options, version, RSTRING_LEN(value))) {
+            if (RSTRING_LEN(value) == 7 && strncmp(version, "current", 7) == 0) {
+                VALUE current_ruby_value = rb_const_get(rb_cObject, rb_intern("RUBY_VERSION"));
+                const char *current_version = RSTRING_PTR(current_ruby_value);
+                if (!pm_options_version_set(options, current_version, 3)) {
+                    rb_exc_raise(rb_exc_new_str(rb_cPrismCurrentVersionError, current_ruby_value));
+                }
+            } else if (!pm_options_version_set(options, version, RSTRING_LEN(value))) {
                 rb_raise(rb_eArgError, "invalid version: %" PRIsVALUE, value);
             }
         }
@@ -888,7 +895,7 @@ parse_input(pm_string_t *input, const pm_options_t *options) {
  *       version of Ruby syntax (which you can trigger with `nil` or
  *       `"latest"`). You may also restrict the syntax to a specific version of
  *       Ruby, e.g., with `"3.3.0"`. To parse with the same syntax version that
- *       the current Ruby is running use `version: RUBY_VERSION`. Raises
+ *       the current Ruby is running use `version: "current"`. Raises
  *       ArgumentError if the version is not currently supported by Prism.
  */
 static VALUE
@@ -1364,6 +1371,8 @@ Init_prism(void) {
     rb_cPrismStringQuery = rb_define_class_under(rb_cPrism, "StringQuery", rb_cObject);
     rb_cPrismScope = rb_define_class_under(rb_cPrism, "Scope", rb_cObject);
 
+    rb_cPrismCurrentVersionError = rb_const_get(rb_cPrism, rb_intern("CurrentVersionError"));
+
     // Intern all of the IDs eagerly that we support so that we don't have to do
     // it every time we parse.
     rb_id_option_command_line = rb_intern_const("command_line");

lib/prism.rb

@@ -37,6 +37,27 @@ module Prism
   private_constant :LexCompat
   private_constant :LexRipper
 
+  # Raised when requested to parse as the currently running Ruby version but Prism has no support for it.
+  class CurrentVersionError < ArgumentError
+    # Initialize a new exception for the given ruby version string.
+    def initialize(version)
+      message = +"invalid version: Requested to parse as `version: 'current'`; "
+      gem_version =
+        begin
+          Gem::Version.new(version)
+        rescue ArgumentError
+        end
+
+      if gem_version && gem_version < Gem::Version.new("3.3.0")
+        message << " #{version} is below the minimum supported syntax."
+      else
+        message << " #{version} is unknown. Please update the `prism` gem."
+      end
+
+      super(message)
+    end
+  end
+
   # :call-seq:
   #   Prism::lex_compat(source, **options) -> LexCompat::Result
   #

lib/prism/ffi.rb

@@ -423,7 +423,9 @@ def dump_options_command_line(options)
 
     # Return the value that should be dumped for the version option.
     def dump_options_version(version)
-      case version
+      current = version == "current"
+
+      case current ? RUBY_VERSION : version
       when nil, "latest"
         0 # Handled in pm_parser_init
       when /\A3\.3(\.\d+)?\z/
@@ -433,7 +435,11 @@ def dump_options_version(version)
       when /\A3\.5(\.\d+)?\z/
         3
       else
-        raise ArgumentError, "invalid version: #{version}"
+        if current
+          raise CurrentVersionError, RUBY_VERSION
+        else
+          raise ArgumentError, "invalid version: #{version}"
+        end
       end
     end
 

templates/sig/prism.rbs.erb

@@ -2,6 +2,10 @@ module Prism
   BACKEND: :CEXT | :FFI
   VERSION: String
 
+  class CurrentVersionError < ArgumentError
+    def initialize: (String version) -> void
+  end
+
   # Methods taking a Ruby source code string:
   <%-
     {

test/prism/api/parse_test.rb

@@ -140,6 +140,25 @@ def test_version
       end
     end
 
+    def test_version_current
+      if RUBY_VERSION >= "3.3"
+        assert Prism.parse_success?("1 + 1", version: "current")
+      else
+        assert_raise(CurrentVersionError) { Prism.parse_success?("1 + 1", version: "current") }
+      end
+
+      version = RUBY_VERSION.split(".").tap { |segments| segments[0] = segments[0].succ }.join(".")
+      stub_ruby_version(version) do
+        error = assert_raise(CurrentVersionError) { Prism.parse("1 + 1", version: "current") }
+        assert_includes error.message, "unknown"
+      end
+
+      stub_ruby_version("2.7.0") do
+        error = assert_raise(CurrentVersionError) { Prism.parse("1 + 1", version: "current") }
+        assert_includes error.message, "minimum"
+      end
+    end
+
     def test_scopes
       assert_kind_of Prism::CallNode, Prism.parse_statement("foo")
       assert_kind_of Prism::LocalVariableReadNode, Prism.parse_statement("foo", scopes: [[:foo]])
@@ -167,5 +186,16 @@ def find_source_file_node(program)
         queue.concat(node.compact_child_nodes)
       end
     end
+
+    def stub_ruby_version(version)
+      old_version = RUBY_VERSION
+
+      Object.send(:remove_const, :RUBY_VERSION)
+      Object.const_set(:RUBY_VERSION, version)
+      yield
+    ensure
+      Object.send(:remove_const, :RUBY_VERSION)
+      Object.const_set(:RUBY_VERSION, old_version)
+    end
   end
 end