Fix an incorrect autocorrect for Rails/ContentTag cop

Follow up to rubocop#242.

This PR allows `content_tag` when the first argument is a variable because
`content_tag(name)` is simpler rather than `tag.public_send(name)`.

When `public_send` is used, it becomes complicated as follows.

First, `content_tag(name)` cannot be corrected to `tag.name`
when the first argument is a variable.

So use `public_send` and prevent the following errors in that case:

## Case 1: Using `public_send` to prevent the following `NoMethodError`

Original code:

```ruby
content_tag(name, 'foo', class: 'bar')
```

Auto-corrected code (before):

```ruby
tag(name, 'foo', class: 'bar')
#=> NoMethodError (undefined method `each_pair' for "foo":String)
```

Auto-corrected code (after):

```ruby
tag.public_send(name, 'foo', class: 'bar')
```

## Case 2: Using `symbolize_keys` to prevent the following `ArgumentError`

Original code:

```ruby
content_tag(name, 'foo', {'class' => 'bar'})
```

Auto-corrected code (before):

```ruby
tag.public_send(name, 'foo', {'class' => 'bar'})
#=> `ArgumentError (wrong number of arguments (given 3, expected 1..2))`
```

Auto-corrected code (after):

```ruby
tag.public_send(name, 'foo', {'class' => 'bar'}.symbolize_keys)
```

The `symbolize_keys` may not be needed, but for safe auto-correction
it will be added if optional argument keys are not all symbols.

## Case 3: Using `o ? o.symbolize_keys : {}` to prevent the following `ArgumentError`

Original code:

```ruby
content_tag(name, 'foo', options)
```

Auto-corrected code (before):

When the third argument is `nil`.

```ruby
options = nil
tag.public_send(name, 'foo', options)
#=> `ArgumentError (wrong number of arguments (given 3, expected 1..2))`
```

Auto-corrected code (after):

```ruby
tag.public_send(name, 'foo', options ? options.symbolize_keys : {})
```

Guard with the empty hash in case the third argument is `nil`.

lib/rubocop/cop/rails/content_tag.rb

@@ -6,6 +6,11 @@ module Rails
       # This cop checks that `tag` is used instead of `content_tag`
       # because `content_tag` is legacy syntax.
       #
+      # !!! Note
+      #
+      #    Allow `content_tag` when the first argument is a variable because
+      #    `content_tag(name)` is simpler rather than `tag.public_send(name)`.
+      #
       # @example
       #  # bad
       #  content_tag(:p, 'Hello world!')
@@ -14,6 +19,7 @@ module Rails
       #  # good
       #  tag.p('Hello world!')
       #  tag.br
+      #  content_tag(name, 'Hello world!')
       class ContentTag < Cop
         include RangeHelp
         extend TargetRailsVersion
@@ -25,6 +31,9 @@ class ContentTag < Cop
         def on_send(node)
           return unless node.method?(:content_tag)
 
+          first_argument = node.first_argument
+          return if first_argument.variable? || first_argument.send_type? || first_argument.const_type?
+
           add_offense(node)
         end
 

manual/cops_rails.md

@@ -452,6 +452,11 @@ Enabled | Yes | Yes  | 2.6 | -
 This cop checks that `tag` is used instead of `content_tag`
 because `content_tag` is legacy syntax.
 
+!!! Note
+
+   Allow `content_tag` when the first argument is a variable because
+   `content_tag(name)` is simpler rather than `tag.public_send(name)`.
+
 ### Examples
 
 ```ruby
@@ -462,6 +467,7 @@ content_tag(:br)
 # good
 tag.p('Hello world!')
 tag.br
+content_tag(name, 'Hello world!')
 ```
 
 ### References

spec/rubocop/cop/rails/content_tag_spec.rb

@@ -60,6 +60,71 @@
       RUBY
     end
 
+    context 'when the first argument is a method or a variable' do
+      it 'does not register an offence when the first argument is a method' do
+        expect_no_offenses(<<~RUBY)
+          content_tag(name, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      it 'does not register an offence when the first argument is a lvar' do
+        expect_no_offenses(<<~RUBY)
+          name = do_something
+          content_tag(name, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      it 'does not register an offence when the first argument is an ivar' do
+        expect_no_offenses(<<~RUBY)
+          content_tag(@name, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      it 'does not register an offence when the first argument is a cvar' do
+        expect_no_offenses(<<~RUBY)
+          content_tag(@@name, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      it 'does not register an offence when the first argument is a gvar' do
+        expect_no_offenses(<<~RUBY)
+          content_tag($name, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      it 'does not register an offence when the first argument is a constant' do
+        expect_no_offenses(<<~RUBY)
+          content_tag(CONST, "Hello world!", class: ["strong", "highlight"])
+        RUBY
+      end
+
+      context 'when options argument is a hash or a variable' do
+        it 'does not register an offence when the last argument is a variable' do
+          expect_no_offenses(<<~RUBY)
+            content_tag(name, "Hello world!", options)
+          RUBY
+        end
+
+        it 'does not register an offence when the last argument is a hash of string keys with brace' do
+          expect_no_offenses(<<~RUBY)
+            content_tag(name, "Hello world!", {'class' => 'foo'})
+          RUBY
+        end
+
+        it 'does not register an offence when the last argument is a hash of string keys without brace' do
+          expect_no_offenses(<<~RUBY)
+            content_tag(name, "Hello world!", 'class' => 'foo')
+          RUBY
+        end
+
+        it 'does not register an offence when the last argument is a hash of string and symbol keys without brace' do
+          expect_no_offenses(<<~RUBY)
+            content_tag(name, "Hello world!", 'class' => 'foo', id: 'baz')
+          RUBY
+        end
+      end
+    end
+
     it 'corrects an offence with nested content_tag' do
       expect_offense(<<~RUBY)
         content_tag(:div) { content_tag(:strong, 'Hi') }