[swift-evolution] Idea: change "@noreturn func f()" to "func f() noreturn"

[Chris Lattner]

> On Feb 25, 2016, at 1:44 PM, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Premise: there are multiple ways of describing control-/data-flow when a function is called:
> 
>     func x() -> T   // x returns a value, of type T, to the caller
>     func x() throws  // x may throw an error to the caller
>     func x() throws -> T  // x may throw an error, and also return a value
>     func x(...) rethrows ... // if a closure argument can throw an error, x may also throw
>     @noreturn func x()  // x never returns to the caller
> 
> "noreturn" is currently spelled as an attribute, but like "throws" / "rethrows" / "-> T", it's really describing how control flow works.
> 
> IMO this calls for consistency: whatever happens "after" the function is called should appear "after" the parameter list.
> 
>     func x() noreturn {
>         ...
>     }
> 
> 
> Thoughts?

This would require taking noreturn as a keyword (so we could use it in the type grammar).  @noreturn is uncommon enough to be not worth doing that IMO.  It is also non-primal to the decl it is modifying, which makes it a perfect attribute.

-Chris

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