[ksss]

Summary

Add class << self support to the inline parser by introducing an AST node for the singleton class body.

Approach

class << self in a class or module body becomes AST::Ruby::Declarations::SingletonClassDecl, a member of the enclosing ClassDecl / ModuleDecl. The defs inside are kept as written (instance methods of the singleton class), and the consumers interpret them:

• MethodBuilder#build_singleton builds them as singleton methods of the enclosing class
• rbs validate validates their method types
• Environment resolves type names in them, and registers no constant for the singleton class itself

The parser pushes SingletonClassDecl onto module_nesting, so current_module alone tells whether the current scope is a singleton class.

class Foo
  class << self
    # @rbs () -> Integer
    def zero = 0   # => Foo.zero
  end
end

Diagnostics

RBS has no notation for the singleton class of a singleton class, an anonymous singleton class, or constants on a singleton class. These are reported:

• NestedSingletonScope: nested class << self, and def self.x inside class << self
• TopLevelSingletonScope: class << self at top level (the singleton class of main)
• NonSelfSingletonScope: class << expr for non-self expr
• ConstantInSingletonScope: class, module, and constant assignments inside class << self. At runtime they are defined on the singleton class (Foo::Inner raises NameError; it is Foo.singleton_class::Inner), so they are not flattened into the enclosing namespace.

Annotations inside a rejected construct are also reported as UnusedInlineAnnotation, because they are not read.

Out of scope

Mixin (include / extend / prepend) and attribute (attr_*) calls inside class << self remain NotImplementedYet. They can be supported later by adding the members to SingletonClassDecl and handling them in the builders.

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