[Fix #7842] Add AcceptImplicitNamespaces to Lint/RaiseException

Fixes #7842.

This PR adds `AcceptImplicitNamespaces` option to `Lint/RaiseException`.

Users can specify and accept an implicit namespace as follows:

```yaml
Lint/RaiseException:
  AllowImplicitNamespaces:
    - 'Gem'
```

```ruby
# good
module Gem
  def self.foo
    raise Exception
  end
end

Gem.foo #=> Gem::Exception
```

`Gem` is specified by default.
https://ruby-doc.org/stdlib-2.7.0/libdoc/rubygems/rdoc/Gem/Exception.html

This way does not prevent all false positives, but it expect some cases
can be solved.

CHANGELOG.md

@@ -16,6 +16,7 @@
 * [#7842](https://github.com/rubocop-hq/rubocop/issues/7842): Fix a false positive for `Lint/RaiseException` when raising Exception with explicit namespace. ([@koic][])
 * [#7834](https://github.com/rubocop-hq/rubocop/issues/7834): Fix `Lint/UriRegexp` to register offense with array arguments. ([@tejasbubane][])
 * [#7841](https://github.com/rubocop-hq/rubocop/issues/7841): Fix an error for `Style/TrailingCommaInBlockArgs` when lambda literal (`->`) has multiple arguments. ([@koic][])
+* [#7842](https://github.com/rubocop-hq/rubocop/issues/7842): Fix a false positive for `Lint/RaiseException` when Exception without cbase specified under the namespace `Gem` by adding  `AllowedImplicitNamespaces` option. ([@koic][])
 
 ### Changes
 

config/default.yml

@@ -1580,6 +1580,8 @@ Lint/RaiseException:
   StyleGuide: '#raise-exception'
   Enabled: pending
   VersionAdded: '0.81'
+  AllowedImplicitNamespaces:
+    - 'Gem'
 
 Lint/RandOne:
   Description: >-

lib/rubocop/cop/lint/raise_exception.rb

@@ -6,32 +6,68 @@ module Lint
       # This cop checks for `raise` or `fail` statements which are
       # raising `Exception` class.
       #
+      # You can specify a module name that will be an implicit namespace
+      # using `AllowedImplicitNamespaces` option. The cop cause a false positive
+      # for namespaced `Exception` when a namespace is omitted. This option can
+      # prevent the false positive by specifying a namespace to be omitted for
+      # `Exception`. Alternatively, make `Exception` a fully qualified class
+      # name with an explicit namespace.
+      #
       # @example
       #   # bad
       #   raise Exception, 'Error message here'
       #
       #   # good
       #   raise StandardError, 'Error message here'
+      #
+      # @example AllowedImplicitNamespaces: ['Gem']
+      #   # good
+      #   module Gem
+      #     def self.foo
+      #       raise Exception # This exception means `Gem::Exception`.
+      #     end
+      #   end
       class RaiseException < Cop
         MSG = 'Use `StandardError` over `Exception`.'
 
         def_node_matcher :exception?, <<~PATTERN
-          (send nil? ${:raise :fail} (const {cbase nil?} :Exception) ... )
+          (send nil? {:raise :fail} (const ${cbase nil?} :Exception) ... )
         PATTERN
 
         def_node_matcher :exception_new_with_message?, <<~PATTERN
-          (send nil? ${:raise :fail}
-            (send (const {cbase nil?} :Exception) :new ... ))
+          (send nil? {:raise :fail}
+            (send (const ${cbase nil?} :Exception) :new ... ))
         PATTERN
 
         def on_send(node)
-          add_offense(node) if raise_exception?(node)
+          exception?(node, &check(node)) ||
+            exception_new_with_message?(node, &check(node))
         end
 
         private
 
-        def raise_exception?(node)
-          exception?(node) || exception_new_with_message?(node)
+        def check(node)
+          lambda do |cbase|
+            return if cbase.nil? && implicit_namespace?(node)
+
+            add_offense(node)
+          end
+        end
+
+        def implicit_namespace?(node)
+          return false unless (parent = node.parent)
+
+          if parent.module_type?
+            namespace = parent.identifier.source
+
+            return allow_implicit_namespaces.include?(namespace)
+          end
+
+          implicit_namespace?(parent)
+        end
+
+        def allow_implicit_namespaces
+          cop_config['AllowedImplicitNamespaces'] || []
         end
       end
     end

manual/cops_lint.md

@@ -1496,6 +1496,13 @@ Pending | Yes | No | 0.81 | -
 This cop checks for `raise` or `fail` statements which are
 raising `Exception` class.
 
+You can specify a module name that will be an implicit namespace
+using `AllowedImplicitNamespaces` option. The cop cause a false positive
+for namespaced `Exception` when a namespace is omitted. This option can
+prevent the false positive by specifying a namespace to be omitted for
+`Exception`. Alternatively, make `Exception` a fully qualified class
+name with an explicit namespace.
+
 ### Examples
 
 ```ruby
@@ -1505,6 +1512,22 @@ raise Exception, 'Error message here'
 # good
 raise StandardError, 'Error message here'
 ```
+#### AllowedImplicitNamespaces: ['Gem']
+
+```ruby
+# good
+module Gem
+  def self.foo
+    raise Exception # This exception means `Gem::Exception`.
+  end
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+AllowedImplicitNamespaces | `Gem` | Array
 
 ### References
 

spec/rubocop/cop/lint/raise_exception_spec.rb

@@ -1,7 +1,9 @@
 # frozen_string_literal: true
 
-RSpec.describe RuboCop::Cop::Lint::RaiseException do
-  subject(:cop) { described_class.new }
+RSpec.describe RuboCop::Cop::Lint::RaiseException, :config do
+  subject(:cop) { described_class.new(config) }
+
+  let(:cop_config) { { 'AllowedImplicitNamespaces' => ['Gem'] } }
 
   it 'registers an offense for `raise` with `::Exception`' do
     expect_offense(<<~RUBY)
@@ -87,4 +89,27 @@
       raise Foo::Exception
     RUBY
   end
+
+  context 'when under namespace' do
+    it 'does not register an offense when Exception without cbase specified' do
+      expect_no_offenses(<<~RUBY)
+        module Gem
+          def self.foo
+            raise Exception
+          end
+        end
+      RUBY
+    end
+
+    it 'does not register an offense when Exception with cbase specified' do
+      expect_offense(<<~RUBY)
+        module Gem
+          def self.foo
+            raise ::Exception
+            ^^^^^^^^^^^^^^^^^ Use `StandardError` over `Exception`.
+          end
+        end
+      RUBY
+    end
+  end
 end