Format macro

[bch29]

Includes changes from #896

Addresses #664

This is a preliminary PR to help diagnose why this isn't working. I am happy to accept discussion/changes to how the macro works.

[bch29]

One question I had: this works somewhat unpleasantly with strings at the moment, because show adds quotes.

let x = "hello"
format! "x is {x}"

becomes x is "hello".

I could look up type information and handle strings differently, but that feels ugly. Perhaps we should add a Display typeclass for this purpose, which would render strings without quotes?

[Marwes]

I could look up type information and handle strings differently, but that feels ugly. Perhaps we should add a Display typeclass for this purpose, which would render strings without quotes?

Type information isn't available at macro expansion (though I have thought about adding a second macro expansion step which does have that capability). Having a Debug and Display typeclass/trait like rust is probably the way to go (with Show being changed to Debug) 👍

[bch29]

Added a Display typeclass. I have left Show as-is: I think if it is changed to Debug it should be done in a separate PR.

I now support {x} for display and {x:?} for show, similar to rust.

Now, arbitrary expressions that do not contain braces or : are allowed in the format subexpressions. This was actually simpler for me to implement than restricting to single variable names and it allows some nice flexibility, for example:

let foo = { x = 1, y = 2}
format! "x is {foo.x}, y is {foo.y}"

Let me know whether you agree that this is a good idea.

[Marwes]

Some test failures related to display not being imported but otherwise LGTM 👍

[scarf005]

Hi, could there be any chance this PR could get traction? being able to interpolate strings sounds really useful