Where do we want to use associated consts?

[killercup]

Rust 1.20 stabilized associated consts!

…and, in contrast to every other hipster unstable Rust feature under the sun, we apparently have neither code comments nor commit messages in diesel's code base telling us what places could benefit from this new feature. But not to worry, we can easily[^1] find some!

Wait—should we? It makes the API a bit more idiomatic at the cost of requiring at least 1.12 to build diesel. Additionally, we could declare the current methods as #[deprecated] and not break any builds. The constants should be in SCREAMING_SNAKE_CASE anyway, so the names won't conflict. I'd say: Go for it, and actually replace these static methods. Right now, before 1.0, we can afford these breaking changes, after that we need to live with the status quo.

Here are the places[^1] that I found that could benefit from associated consts:

• diesel::query_builder::QueryId: const HAS_STATIC_QUERY_ID: bool instead of fn has_static_query_id() ->bool
• diesel::query_source::Column: const NAME: &'static str instead of fn name() -> &'static str
• diesel::types::FromSqlRow: const FIELDS_NEEDED: usize (with default) instead of fn fields_needed() -> usize { ... }

I'm not entirely sure if we want/can/should turn methods like foo() -> Self::Foo into associated consts. This would add a few more cases (<Foo as diesel::associations::HasTable>::table(), <Foo as diesel::query_source::Table>::all_columns(), and <Foo as diesel::associations::BelongsTo>::foreign_key_column()).

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[^1]: rg -i ' (pub | ) fn (.*?)\(\) ->'

[sgrif]

👍 for using associated constants for those 3 cases

[willmurphyscode]

Is anyone presently working on this? If not, I'd like to volunteer to give this a try.

[Eijebong]

I don't think anyone is working on it so feel free to work on it.
If you have any question don't hesitate to ask here or on our gitter room (link it the README)