[swift-evolution] [Proposal] Adjusting `inout` Declarations for Type Decoration

[Erica Sadun]

https://github.com/apple/swift-evolution/pull/127 <https://github.com/apple/swift-evolution/pull/127>

Adjusting inout Declarations for Type Decoration

Proposal: TBD
Author(s): Joe Groff <https://github.com/jckarter>, Erica Sadun <http://github.com/erica>
Status: TBD
Review manager: TBD
Introduction

The inout keyword indicates copy-in/copy-out argument behavior. In its current implementation
the keyword prepands argument names. We propose to move the inout keyword to the right
side of the colon to decorate the type instead of the parameter label. 

The initial Swift-Evolution discussion of this topic took place in the “Replace ‘inout’ with &” thread.

Motivation

In Swift 2, the inout parameter lives on the label side rather than the type side of the colon
although the keyword isn’t modifying the label but its type. Decorating
types instead of labels offers identifiable advantages: 

It enables the inout keyword to properly integrate into full type syntax, for example: 
(x: inout T) -> U // => (inout T) -> U
It avoids notational similarity with arguments labeled inout, for example:
func foo(inOut x: T) // foo(inOut:), type (T) -> Void
func foo(inout x: T) // foo(_:), type (inout T) -> Void
It better matches similar patterns in other languages such as borrowing in Rust, that may be later introduced back to Swift 
Detailed design

parameter → external-parameter-name optlocal-parameter-name : type-annotation
type-annotation → inout type-annotation
Alternatives Considered

Decorations using @inout (either @inout(T) or @inout T) were considered and discarded
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