deprecate: GeometryCollection::from(single_geom). It doesnt give us much and causes awkwardness downstream.

[michaelkirk]

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As with most blanket implementations, the problems are little convoluted to explain.

Maybe easier to explain is that, as is, this implementation is only marginally useful. Having to write GeometryCollection::new(vec![point]) isn't much more arduous than writing GeometryCollection::from(point), and arguably clearer.

But the actual trouble I hit was while working on the WKT crate (georust/wkt#95, see the second commit in particular).

Here's a simplified example of the kind of thing I was trying to do but can't (without jumping through some hoops):

// Any type that can be built from our serialized wkt representation can impl this trait.
trait FromSerialized<T> {
    fn from_wkt(wkt: &str) -> Result<Self>;
}

// A blanket implementation exists for all the things that can be (fallibly)
// built from a Geometry - i.e. all the inner variants (Point, Line, etc., but NOT a GeometryCollection)
impl<T: CoordNum, IG> FromSerialized<T> for IG
where
IG: TryFrom<Geometry<T>> + Into<Geometry<T>> + Default
{
    fn from_wkt(wkt: &str) -> Self {
        let g: Geometry<T> = parse_geometry_from_wkt(wkt);
        let specific = Self::try_from(g).map_err(|_| "e.g. Got LineString, expected Point").unwrap();
        specific
    }
}

fn parse_geometry_from_wkt<T: CoordNum>(wkt: &str) -> Geometry<T> {
    todo!("pretend this does something interesting, like parse WKT")
}

mod tests {
    use crate::{Point, LineString, GeometryCollection};
    use super::{FromWkt};

    #[test]
    fn example() {
        // All geometry variants *except* GeometryCollections can utilize the blanket implementation above.
        let point = Point::from_serialized("POINT(1 2)").unwrap();
        let line_string = LineString::from_wkt("LINESTRING(1 1, 2 2)").unwrap();

        // !!! But GeometryCollection::from_serialized will not compile...
        // ... because it has no `impl TryFrom<Geometry>`, because that impl would conflict (be ambiguous) with the existing
        // (proposed deprecation) `impl TryFrom<Into<Geometry>> for GeometryCollection`
        let geom_collection: GeometryCollection<f64> = GeometryCollection:: from_wkt("GEOMETRYCOLLECTION(("POINT(1 2)))");
    }
}

I worked around it in the WKT crate, but it was awkward. And I don't think many people would be sad if the conflicting implementation went away.

If people are commonly creating GeometryCollections from a single geometry, I think something like GeometryCollection::from_single(point) (or whatever) would be less obtrusive.

[michaelkirk]

rebased to fix conflicts in the change log

[michaelkirk]

bors r=frewsxcv

[bors]

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