Make macro syntax similar to function

[cole-miller]

@CAD97

I believe @matklad has suggested this before, and it has analogues to Kotlin's inline fun . It is basically halfway between macros and functions: it has the API semantics of a function, but the code is evaluated in the caller's scope.

@petertodd also suggested roughly the same thing in this thread, which has some good discussion:

Mandatory inlined functions for unsized types and 'super lifetime?

Proposal Allow functions to be declared as inline fn, which would require them to be inlined into the callee. This makes it easy to do two key things: return unsized types, and return references to the stack. Here's an example of both capabilities at the same time. I'm using the 'super reserved keyword for the special "callee" reference: impl<T: ?Sized> Pin<T> { pub inline const fn value(mut value: T) -> Pin<&'super mut T> { // SAFETY: value is on a fixed position in the stack …