[swift-evolution] Mutability inference

[Brent Royal-Gordon]

> You are right, I work in a similar way currently. I have my personal rule to commit into Git 100% error _and_ warning free. So I define every new variable as „let“ and if I get a compiler error I change it to "var“.

I think the intention is that, before you change it to `var`, you should examine your code and see if there's a way you could write it that would allow it to remain a `let`. Could you assign it to a new constant instead of changing the old variable? Could you use a nonmutating operation instead of a mutating one? It's not that you *have* to take the nonmutating option—`var`s exist in Swift because they're perfectly appropriate in many cases—but if you try to turn `var`-based code into `let`-based code, the `let`-based version is often better.

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Brent Royal-Gordon
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