What is the difference between the magic comments "frozen_string_literal: true" and "immutable: string" in Ruby?

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Both "frozenstringliteral: true" and "immutable: string" are magic comments in Ruby that indicate that string literals (i.e. strings that are specified directly in the code, rather than being generated at runtime) should be treated as immutable.

The "frozenstringliteral: true" magic comment was introduced in Ruby 2.3 as a way to improve performance and prevent unintended modification of string literals. When this magic comment is included at the beginning of a file or script, it specifies that all string literals in that file should be frozen, or treated as immutable. This can improve performance because frozen strings can be cached and reused, rather than having to create new string objects every time.

The "immutable: string" magic comment is not a standard part of Ruby, but is used in some third-party libraries and tools, such as the Ruby RIP gem. Like "frozenstringliteral: true", it indicates that string literals should be treated as immutable. However, it is not as widely recognized or standardized as the "frozenstringliteral" magic comment.