Migrate CoordsIter::T to be an associated type.

[frewsxcv]

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Previously

With the previous design, this would compile:

impl<'a> CoordsIter<'a, f32> for Polygon<f32> {
    ...
}

impl<'a> CoordsIter<'a, f64> for Polygon<f32> {
    ...
}

Even though this is not what our implementations actually look like, to the compiler it's possible for there to be multiple implementations of CoordsIter for Polygon<f32>.

This was problematic when I started rewriting our ExtremeIndices trait to be generic over any type that implements CoordsIter:

impl<'a, T, G> ExtremeIndices for G
where
    T: CoordinateType,
    G: CoordsIter<'a, T>,
{
    fn extreme_indices(&self) -> Result<Extremes, ()> {
        ...
    }
}

This is the compilation error:

the type parameter `T` is not constrained by the impl trait, self type, or predicates

The issue is that if someone writes this with this new ExtremeIndices implementation:

let p: Polygon<f32> = polygon![...];
p.extreme_indices()

The compiler doesn't know which implementation to choose: impl<'a> CoordsIter<'a, f32> for Polygon<f32> or impl<'a> CoordsIter<'a, f64> for Polygon<f32>.

So we need a way to tell the compiler we only have one implementation of CoordsIter per type. The root issue being that there is a free generic parameter in the CoordsIter definition.

Now

To fix this, we will move the T type parameter to an associated type. Writing this would be a compiler error:

impl<'a> CoordsIter<'a> for Polygon<f32> {
    type Scalar = f32;
    ...
}

impl<'a> CoordsIter<'a> for Polygon<f32> {
    type Scalar = f64;
    ...
}

conflicting implementations of trait `algorithm::coords_iter::CoordsIter<'_>` for type `geo_types::Polygon<f32>`

This unblocks #592

[michaelkirk]

I've read about the distinction, but still haven't really internalized an intuitive grasp on when to apply associated types vs. generics, but this is a clear and helpful example. Thank you!

[michaelkirk]

bors r+

[bors]

Build succeeded:

• bench
• coverage
• geo
• geo-postgis
• geo-types