[smitherfield]

C compilers must assume that a function pointer (that is, a function passed as an argument, or that is a property of an object) may write to any global variable.

C compilers must also assume that any two pointers to the same type may alias (refer to the same object). The programmer can assert to the compiler that a pointer does not alias any others used in the same scope by declaring it with the `restrict` keyword.

For most functions this won't have much effect on the generated code. Writing equivalent functions to the ones in the swift-evolution doc in C, both with and without `restrict` everywhere possible, it looks like `restrict` only has an effect on the generated code for `increaseByGlobal`: https://godbolt.org/g/W8s3BA