New @convention(c) in Swift 2: How can I use it?

Passing a Swift closure to a C function taking a function pointer
parameter is now supported in Swift 2, and, as you noticed, function
types are specified with the @convention(c) attribute.

If you pass a closure directly as an argument to the C function then
this attribute is inferred automatically.

As a simple example, if you have this C function

CGFloat myCFunction(CGFloat (callback)(CGFloat x, CGFloat y)) {
    return callback(1.1, 2.2);
}

then you can call it from Swift as

let result = myCFunction( {
    (x, y) -> CGFloat in
    return x + y
} )
print(result) // 3.3

which does exactly the same as the more verbose

let swiftCallback : @convention(c) (CGFloat, CGFloat) -> CGFloat = {
    (x, y) -> CGFloat in
    return x + y
} 

let result = myCFunction( swiftCallback )
print(result) // 3.3