Language changes:

• Frozen string literals: By default, strings act as if frozen. Modifying a frozen string in a file with the magic comment will result in a deprecation warning instead of an error. A future version will raise an exception.
• Default block parameter: Ruby 3.4 introduces a default block parameter to make short code blocks cleaner. Instead of using numbered parameters or naming them, you can use a simple name like it.
• Keyword splatting nil: In Ruby 3.4, ** with nil implicitly converts nil into a hash.

Core classes updates:

• Exception#set_backtrace: In Ruby 3.4, Exception#set_backtrace accepts a Thread::Backtrace::Location array to rebuild a Backtrace instance.
• Range#size: If the range is not iterable, Ruby 3.4 will throw a TypeError when calling size.

Other changes:

• Methods passing unused blocks: In verbose mode, Ruby will throw a warning if a block is passed to a method that doesn't use it.
• Array.each performance improvement: Ruby 3.4 improves the performance of Array.each due to an implementation rewrite.
• Error message and backtrace display update: Ruby will use single quotes instead of backticks and display a class name before a method name.