[swift-evolution] Immutable Structures

[Brent Royal-Gordon]

> However I could probably still agree with you if mutating was also used in classes. What's the point of having it only on structures?

In a regular method, `self` is a regular (constant) parameter, but in a `mutating` method, `self` is `inout` so that it can be mutated and those mutations will reach the caller's copy of the instance. There's no need for an `inout` `self` on reference types, because the mutations are performed on the shared instance which both the caller and callee have a reference to, so `mutating` only really makes sense for value types.

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Brent Royal-Gordon
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