Fix SourceFinder's constant evaluation issue (#869)

Currently, if the signature's constant part is not defined, a NameError
would be raised.

```
irb(main):001> show_source Foo
(eval):1:in `<top (required)>': uninitialized constant Foo (NameError)

Foo
^^^
        from (irb):1:in `<main>'
```

This commit fixes the issue and simplifies the `edit` command's implementation.

lib/irb/cmd/edit.rb

@@ -27,13 +27,7 @@ def execute(*args)
         if path.nil?
           path = @irb_context.irb_path
         elsif !File.exist?(path)
-          source =
-            begin
-              SourceFinder.new(@irb_context).find_source(path)
-            rescue NameError
-              # if user enters a path that doesn't exist, it'll cause NameError when passed here because find_source would try to evaluate it as well
-              # in this case, we should just ignore the error
-            end
+          source = SourceFinder.new(@irb_context).find_source(path)
 
           if source&.file_exist? && !source.binary_file?
             path = source.file

lib/irb/source_finder.rb

@@ -4,6 +4,8 @@
 
 module IRB
   class SourceFinder
+    class EvaluationError < StandardError; end
+
     class Source
       attr_reader :file, :line
       def initialize(file, line, ast_source = nil)
@@ -66,20 +68,19 @@ def initialize(irb_context)
     end
 
     def find_source(signature, super_level = 0)
-      context_binding = @irb_context.workspace.binding
       case signature
       when /\A(::)?[A-Z]\w*(::[A-Z]\w*)*\z/ # Const::Name
-        eval(signature, context_binding) # trigger autoload
-        base = context_binding.receiver.yield_self { |r| r.is_a?(Module) ? r : Object }
+        eval_receiver_or_owner(signature) # trigger autoload
+        base = @irb_context.workspace.binding.receiver.yield_self { |r| r.is_a?(Module) ? r : Object }
         file, line = base.const_source_location(signature)
       when /\A(?<owner>[A-Z]\w*(::[A-Z]\w*)*)#(?<method>[^ :.]+)\z/ # Class#method
-        owner = eval(Regexp.last_match[:owner], context_binding)
+        owner = eval_receiver_or_owner(Regexp.last_match[:owner])
         method = Regexp.last_match[:method]
         return unless owner.respond_to?(:instance_method)
         method = method_target(owner, super_level, method, "owner")
         file, line = method&.source_location
       when /\A((?<receiver>.+)(\.|::))?(?<method>[^ :.]+)\z/ # method, receiver.method, receiver::method
-        receiver = eval(Regexp.last_match[:receiver] || 'self', context_binding)
+        receiver = eval_receiver_or_owner(Regexp.last_match[:receiver] || 'self')
         method = Regexp.last_match[:method]
         return unless receiver.respond_to?(method, true)
         method = method_target(receiver, super_level, method, "receiver")
@@ -94,6 +95,8 @@ def find_source(signature, super_level = 0)
         source = RubyVM::AbstractSyntaxTree.of(method)&.source rescue nil
         Source.new(file, line, source)
       end
+    rescue EvaluationError
+      nil
     end
 
     private
@@ -112,5 +115,12 @@ def method_target(owner_receiver, super_level, method, type)
     rescue NameError
       nil
     end
+
+    def eval_receiver_or_owner(code)
+      context_binding = @irb_context.workspace.binding
+      eval(code, context_binding)
+    rescue NameError
+      raise EvaluationError
+    end
   end
 end

test/irb/cmd/test_show_source.rb

@@ -52,6 +52,19 @@ def test_show_source_with_missing_signature
       assert_match(%r[Couldn't locate a definition for foo], out)
     end
 
+    def test_show_source_with_missing_constant
+      write_ruby <<~'RUBY'
+        binding.irb
+      RUBY
+
+      out = run_ruby_file do
+        type "show_source Foo"
+        type "exit"
+      end
+
+      assert_match(%r[Couldn't locate a definition for Foo], out)
+    end
+
     def test_show_source_string
       write_ruby <<~'RUBY'
         binding.irb