[Fix rubocop#6778] Fix a false positive in Style/HashSyntax cop

The `Style/HashSyntax` cop would register an offense when `EnforcedStyle` was
`ruby19_no_mixed_keys` and a hash key was an interpolated string
like the following code:

```
{"#{foo}": 1}
```

This change fixes that.

CHANGELOG.md

@@ -16,6 +16,7 @@
 * [#6941](https://github.com/rubocop-hq/rubocop/issues/6941): Add missing absence validations to `Rails/Validation`. ([@jmanian][])
 * [#6926](https://github.com/rubocop-hq/rubocop/issues/6926): [Fix #6926] Allow nil values to unset config defaults. ([@dduugg][])
 * [#6946](https://github.com/rubocop-hq/rubocop/pull/6946): Allow `Rails/ReflectionClassName` to use string interpolation for `class_name`. ([@r7kamura][])
+* [#6778](https://github.com/rubocop-hq/rubocop/issues/6778): Fix a false positive in `Style/HashSyntax` cop when a hash key is an interpolated string and EnforcedStyle is ruby19_no_mixed_keys. ([@tatsuyafw][])
 
 ### Changes
 

lib/rubocop/cop/style/hash_syntax.rb

@@ -132,7 +132,7 @@ def sym_indices?(pairs)
         end
 
         def word_symbol_pair?(pair)
-          return false unless pair.key.sym_type?
+          return false unless pair.key.sym_type? || pair.key.dsym_type?
 
           acceptable_19_syntax_symbol?(pair.key.source)
         end

spec/rubocop/cop/style/hash_syntax_spec.rb

@@ -387,6 +387,10 @@
         RUBY
       end
 
+      it 'accepts new syntax when keys are interpolated string' do
+        expect_no_offenses('{"#{foo}": 1, "#{@foo}": 2, "#@foo": 3}')
+      end
+
       it 'auto-corrects old to new style' do
         new_source = autocorrect_source('{ :a => 1, :b => 2 }')
         expect(new_source).to eq('{ a: 1, b: 2 }')
@@ -504,6 +508,10 @@
         RUBY
       end
 
+      it 'accepts new syntax when keys are interpolated string' do
+        expect_no_offenses('{"#{foo}": 1, "#{@foo}": 2, "#@foo": 3}')
+      end
+
       it 'auto-corrects old to new style' do
         new_source = autocorrect_source('{ :a => 1, :b => 2 }')
         expect(new_source).to eq('{ a: 1, b: 2 }')