explicitly provide type argument to avoid breakage in future Rust versions

[lcnr]

D1Backend has multiple where-bounds for HasSqlType due to the super-traits of diesel::backend::Backend: HasSqlType<SmallInt>, HasSqlType<Integer>, and others. It also has one where-bound which references a generic parameter, i.e. which is non-global: HasSqlType<T>.

The compiler previously preferred such non-global where-bounds over global¹ where-bounds in some cases, guiding type inference. This behavior was quite subtle and inconsistent, so we've reworked the implementation in rust-lang/rust#132325.

Because of this, we no longer infer the type argument of <D1Backend as HasSqlType<_>>::metadata to T as using SmallInt or Integer is just as valid and would also allow this code to compile. The change in inference behavior will likely be stable in version 1.85. This PR modifies your code to make the used type explicit, allowing your crate to compile in future Rust versions.

Footnotes

1. pretty much: a where-bound which does not mention any generic parameters ↩