improve `newtype_index!` macro

[nikomatsakis]

We have this handy macro newtype_index! that creates a, well, newtype'd index:

newtype_index!(RegionAtLocationIndex { DEBUG_FORMAT = "RegionAtLocationIndex({})" });

However, I think it could be improved in four ways:

First, the syntax should change to include the keyword struct and pub. This would help with people trying to find the definition of the type. I'd probably switch to {} form at the same time, but that's not necessary:

newtype_index! {
  pub struct RegionAtLocationIndex { DEBUG_FORMAT = "RegionAtLocationIndex({})" }
}

Second, doing this would also allow us to support arbitrary visibilities. For example, I'd like to make crate struct RegionAtLocationIndex:

newtype_index! {
  crate struct RegionAtLocationIndex { DEBUG_FORMAT = "RegionAtLocationIndex({})" }
}

Third, we should change to incorporate NonZero. That is, the original example would currently expands to something like:

pub struct RegionAtLocationIndex(u32);

but I want it to expand to:

pub struct RegionAtLocationIndex { non_zero: NonZero<u32> }

Of course, 0 is a valid index, so the various initialization and accessor routines would have to add or subtract one as appropriate.

Using NonZero would mean that Option<T> would be represented still as a single u32.

Finally, fourth, as a convenience, it would be nice to define inherent (non-trait) methods, so that using these types did not require importing the Idx trait:

        impl $type {
            #[inline]
            fn new(value: usize) -> Self {..}
            #[inline]
            fn index(self) -> usize {..}
        }

cc @Nashenas88 @spastorino @sgrif

[spastorino]

I can tackle this!

[sgrif]

FWIW I'd prefer we do pub(crate) struct instead of crate struct, so we continue to map to the corresponding Rust syntax. We should be able to just the vis matcher

[petrochenkov]

newtype_index! macro

Oh, that thing stopping "Go To Definition" from working 😄

[nikomatsakis]

@sgrif

FWIW I'd prefer we do pub(crate) struct instead of crate struct, so we continue to map to the corresponding Rust syntax. We should be able to just the vis matcher

crate is Rust syntax =) -- the vis matcher would be fine

[sgrif]

Oh neat, I was not aware of #45388