[ruby-core:111122] [Ruby master Feature#19107] Allow trailing comma in method signature

Issue #19107 has been updated by byroot (Jean Boussier).


> Is there an actual case where this proposal is convenient?

Yes, when replacing old APIs that took an "option hash" by explicit keyword arguments, it tend to create very large signature.

The last example I have in mind is `redis-client`: https://github.com/redis-rb/redis-client/blob/dcfe43abb83597bee129537464e20805658bf7a9/lib/redis_client/config.rb#L21-L41

```ruby
      def initialize(
        username: nil,
        password: nil,
        db: nil,
        id: nil,
        timeout: DEFAULT_TIMEOUT,
        read_timeout: timeout,
        write_timeout: timeout,
        connect_timeout: timeout,
        ssl: nil,
        custom: {},
        ssl_params: nil,
        driver: nil,
        protocol: 3,
        client_implementation: RedisClient,
        command_builder: CommandBuilder,
        inherit_socket: false,
        reconnect_attempts: false,
        middlewares: false,
        circuit_breaker: nil
      )
```

When adding a new argument, it cause these annoying diffs:
```diff
diff --git a/lib/redis_client/config.rb b/lib/redis_client/config.rb
index fc74367..6412171 100644
--- a/lib/redis_client/config.rb
+++ b/lib/redis_client/config.rb
@@ -36,7 +36,8 @@ class RedisClient
         command_builder: CommandBuilder,
         inherit_socket: false,
         reconnect_attempts: false,
-        middlewares: false
+        middlewares: false,
+        circuit_breaker: nil
       )
         @username = username
         @password = password
```


Also this inconsistency is the reason why some popular styleguides reverted back to not using trailing comma for multi-line enumerations:

  - https://github.com/testdouble/standard/pull/453#issuecomment-1234208705
  - https://github.com/fables-tales/rubyfmt/issues/154


----------------------------------------
Feature #19107: Allow trailing comma in method signature
https://bugs.ruby-lang.org/issues/19107#change-100396

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
A popular style for multiline arrays, hashes or method calls, is to use trailing commas:

```ruby
array = [
  1,
  2,
  3,
]

hash = {
  foo: 1,
  bar: 2,
  baz: 3,
} 

Some.method(
  1,
  2,
  foo: 3,
)
```

The main reason to do this is to avoid unnecessary noise when adding one extra element:

```diff
diff --git a/foo.rb b/foo.rb
index b2689a7e4f..ddb7dc3552 100644
--- a/foo.rb
+++ b/foo.rb
@@ -1,4 +1,5 @@
 Foo.bar(
   foo: 1,
-  bar: 2
+  bar: 2,
+  baz: 3
 )
```

However, this pattern doesn't work with method declarations:

```ruby
def foo(bar:,) # syntax error, unexpected ')'
``` 

### Proposal

For consistency and convenience I propose to allow trailing commas in method declarations.



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