Rewrite "version: nearest" to require no maintenance

ext/prism/extension.c

@@ -205,18 +205,14 @@ build_options_i(VALUE key, VALUE value, VALUE argument) {
                     rb_exc_raise(rb_exc_new_cstr(rb_cPrismCurrentVersionError, ruby_version));
                 }
             } else if (RSTRING_LEN(value) == 7 && strncmp(version, "nearest", 7) == 0) {
-                const char *nearest_version;
-
-                if (ruby_version[0] < '3' || (ruby_version[0] == '3' && ruby_version[2] < '3')) {
-                    nearest_version = "3.3";
-                } else if (ruby_version[0] > '4' || (ruby_version[0] == '4' && ruby_version[2] > '1')) {
-                    nearest_version = "4.1";
-                } else {
-                    nearest_version = ruby_version;
-                }
-
-                if (!pm_options_version_set(options, nearest_version, 3)) {
-                    rb_raise(rb_eArgError, "invalid nearest version: %s", nearest_version);
+                if (!pm_options_version_set(options, ruby_version, 3)) {
+                    // Prism doesn't know this specific version. Is it lower?
+                    if (ruby_version[0] < '3' || (ruby_version[0] == '3' && ruby_version[2] < '3')) {
+                        options->version == PM_OPTIONS_VERSION_CRUBY_3_3;
+                    } else {
+                        // Must be higher.
+                        options->version == PM_OPTIONS_VERSION_LATEST;
+                    }
                 }
             } else if (!pm_options_version_set(options, version, RSTRING_LEN(value))) {
                 rb_raise(rb_eArgError, "invalid version: %" PRIsVALUE, value);

lib/prism/ffi.rb

@@ -423,27 +423,28 @@ def dump_options_command_line(options)
 
     # Return the value that should be dumped for the version option.
     def dump_options_version(version)
-      checking =
-        case version
-        when "current"
-          RUBY_VERSION
-        when "latest"
-          nil
-        when "nearest"
-          if RUBY_VERSION <= "3.3"
-            "3.3"
-          elsif RUBY_VERSION >= "4.1"
-            "4.1"
-          else
-            RUBY_VERSION
-          end
+      case version
+      when "current"
+        version_string_to_number(RUBY_VERSION) || raise(CurrentVersionError, RUBY_VERSION)
+      when "latest", nil
+        0 # Handled in pm_parser_init
+      when "nearest"
+        dump = version_string_to_number(RUBY_VERSION)
+        return dump if dump
+        if RUBY_VERSION < "3.3"
+          version_string_to_number("3.3")
         else
-          version
+          0 # Handled in pm_parser_init
         end
+      else
+        version_string_to_number(version) || raise(ArgumentError, "invalid version: #{version}")
+      end
+    end
 
-      case checking
-      when nil
-        0 # Handled in pm_parser_init
+    # Converts a version string like "4.0.0" or "4.0" into a number.
+    # Returns nil if the version is unknown.
+    def version_string_to_number(version)
+      case version
       when /\A3\.3(\.\d+)?\z/
         1
       when /\A3\.4(\.\d+)?\z/
@@ -452,12 +453,6 @@ def dump_options_version(version)
         3
       when /\A4\.1(\.\d+)?\z/
         4
-      else
-        if version == "current"
-          raise CurrentVersionError, RUBY_VERSION
-        else
-          raise ArgumentError, "invalid version: #{version}"
-        end
       end
     end
 

test/prism/api/parse_test.rb

@@ -154,6 +154,10 @@ def test_version_current
       end
     end
 
+    def test_nearest
+      assert Prism.parse_success?("1 + 1", version: "nearest")
+    end
+
     def test_scopes
       assert_kind_of Prism::CallNode, Prism.parse_statement("foo")
       assert_kind_of Prism::LocalVariableReadNode, Prism.parse_statement("foo", scopes: [[:foo]])