Avoid two uses of defer {}

[dabrahams]

Last time I checked, things done in defer were not well-optimized and could cause closure allocation.

[dabrahams]

@swift-ci please test and merge

[jckarter]

Closure allocation overhead should not be a problem for defer. The "closure" is immediately invoked so we'll pass any "captures" immediately instead of putting them on the heap. The call can then be trivially inlined. I just tried swiftc -O with a simple example and this seemed to be the case. If there is a problem, we ought to fix trivial optimizer bugs like this instead of making the code more brittle.

[Mazyod]

I apologize if this is irrelevant, but I can see that the PR subtly changes the semantics of the function, besides avoiding defer statement. Using defer will guarantee the _fixLifetime to be called in the event that an error is thrown, while using a temporary variable skips that call.

// prints "d" then "a"
do { 
    defer { print("d") }
    try throwSomething() 
} catch _ { 
    print("a") 
} 

[jckarter]

It's probably harmless in this case since _fixLifetime is primarily a compiler hint, but yeah, in general this is an unwise transformation, because defer more safely handles hidden control flow edges, and does not have the overhead Dave seems to think it does.

[dabrahams]

Dang. I’ll revert this as soon as I get to a keyboard