[swift-evolution] Allowing mutable members in structs

[Dan Raviv]

Would it make sense to allow mutable properties in structs, which could be
mutated even by non-mutating methods?

I've noticed this could be useful when implementing a non-mutating protocol
method. The method semantically doesn't change the state of the protocol's
implementor, and therefore shouldn't be declared as mutating. However,
*some* implementations might need to change some internal state for
implementing the functionality. Making specific struct properties mutable
for such a case could make sense.

Alternatively, Swift could allow implementing a non-mutating protocol
method as a mutating method in the implementor. Currently, this doesn't
seem to be  allowed; Swift doesn't recognize the mutating method as a match
for the non-mutating protocol method.

Cheers,
Dan
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