Derive Generic FromSql/ToSql

[matt-duch]

Allows for derivation of FromSql and ToSql for generic types. Borrowed types are allowed for ToSql, but not meaningful to support for FromSql (so not supported). (also noted in #570)

Would you be interested in this change? A test was added. Happy to iterate.

[sfackler]

Are these bounds actually required?

[matt-duch]

FromSqlOwned is required for #[derive(FromSql)] which is needed to run the test. Or were you asking about the derived Debug and PartialEq?

Edit: I also put the bounds in the two different positions to test that bounds and where clauses work as expected.

[sfackler]

Derive logic should normally insert those bounds on the generated impls as necessary - you don't need where T: Clone at the struct definition when using #[derive(Clone)] for example.

[matt-duch]

Just to make sure I understand the change request, the idea is add a FromSql<'a> bound on any generic type in the FromSql derive logic?

[sfackler]

Yep.

[matt-duch]

So I started implementing this, and ran into an ambiguity (where this differs from Clone, which is in the rust prelude). I need to create a path to the FromSql and ToSql traits, however it could be one of a few options:

1. From the postgres_derive crate (postgres_derive::FromSql<'a, T>, but I think most uses will not import this crate directly)
2. From the postgres_types crate (postgres_types::FromSql<'a, T>)
3. Some other crate that re-exports or renames those types (no way to use a path from a crate name)
4. Assume that it's used somewhere (FromSql<'a, T>)
   Any suggestions on which assumption to use?

[sfackler]

Here's how the path is already produced in the existing logic: https://github.com/sfackler/rust-postgres/blob/master/postgres-derive/src/fromsql.rs#L90

[matt-duch]

Thanks for pointing that out. Based on that, looks like it was option (2). Branch is updated, and tests pass locally.

[matt-duch]

Updated to fix the two issues (lint and failing test).

[AngusWaller]

Hope this gets in soon, derive is breaking on any struct thats borrowed/lifetimes

[sfackler]

Sorry for the delay - LGTM other than the clippy failures!

[matt-duch]

@sfackler Sorry about that. Fixed, clippy runs locally, and pushed.

[sfackler]

Thanks! I will cut a release tomorrow.