Faillible Queryable - Non-generic Queryable version experiment

[Ten0]

One approach to fixing #2523

[weiznich]

Just to drop a note here: I'm currently busy with other work related stuff, so I will likely not be able to look at this for the next week or so.

[Ten0]

So the test fails there seems to highlight a compatibility break with type inference where the compiler cannot infer "oh there's a single type I know for deserializing BigInt, that's i64, let me just use that" anymore, though I'm not sure why it has this impact.

If going for this approach, readmes and diesel_cli would still have to be updated.

[weiznich]

Another important change here is that we cannot anymore write different impls depending on the type returned by some query or the backend. Doing that is basically required to port diesel_cli. As consequence I would like to see how that could be solved before looking any further into this.

[Ten0]

Another important change here is that we cannot anymore write different impls depending on the type returned by some query or the backend. Doing that is basically required to port diesel_cli. As consequence I would like to see how that could be solved before looking any further into this.

From #2523 (comment):

it would probably always be implemented without an additional layer of genericity, so it could probably use FromSqlRow instead of Queryable, as the conflicting impl error wouldn't trigger.

As it turns out, we're actually using a bit of genericity here:

diesel/diesel_cli/src/infer_schema_internals/data_structures.rs

Lines 78 to 82 in a312528

	impl<DB> FromSqlRow<(sql_types::Text, sql_types::Text, sql_types::Text), DB> for ColumnInformation
	where
	DB: Backend + UsesInformationSchema,
	(String, String, String):
	FromStaticSqlRow<(sql_types::Text, sql_types::Text, sql_types::Text), DB>,

but for some reason it still works. (Maybe thanks to the local UsesInformationSchema trait.)

[Ten0]

One more downside of the non-generic version: rust error messages suggest less correct approaches to fixing FromSqlRow<ST, DB> is not implemented for T:
https://github.com/diesel-rs/diesel/pull/2559/checks?check_run_id=1617158750#step:6:258

-error[E0277]: the trait bound `UserWithToFewFields: diesel::Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Text>), _>` is not satisfied
+error[E0277]: the trait bound `(i32, std::string::String): diesel::deserialize::FromSql<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Text>), _>` is not satisfied

[weiznich]

Closed as I would like to go with #2599