Add `#[diagnostic::do_not_recommend]` to `impl AsExpression for T: Expression`

[weiznich]

We would like to add #[diagnostic::do_not_recommend] to this particular impl:

diesel/diesel/src/expression/mod.rs

Lines 245 to 255 in c9bbd85

	impl<T, ST> AsExpression<ST> for T
	where
	T: Expression<SqlType = ST>,
	ST: SqlType + TypedExpressionType,
	{
	type Expression = T;
	fn as_expression(self) -> T {
	self
	}
	}

This would significantly improve the error messages emitted by #[derive(Insertable)] for type mismatches. It's currently not possible to add this attribute as it regresses the error message by rustc loosing the relevant span. This was reported upstream as regression in rust-lang/rust#123503 but is still not resolved.

We would need investigate if:

• There is something we can to in the derive code to fix these spans
• It's an actual rustc issue and needs to be resolved there.

[sim-hash]

@weiznich I'd be happy to take it on!

[weiznich]

@sim-hash Any help would be welcome. Let me know if you need more details.

[Muktarsadiq]

is this still available, if so i'd be happy to contribute

[weiznich]

@Muktarsadiq I would assume that @sim-hash is still working on this? You can certainly coordinate with each other if they need help?

[sim-hash]

@weiznich Yes, I’m still working on this and would like to give it a shot on my own until tomorrow, at the latest Thursday. If I’m unable to make progress by then, I’ll reach out to @weiznich / @Muktarsadiq

[sim-hash]

@weiznich

I have taken a look at this.

1. It's an actual rustc issue and needs to be resolved there - It is indeed a rustc issue, but the workaround below bypasses it completely, so we don't have to wait for an upstream fix.

2. There is something we can do in the derive code to fix these spans - There is a way to preserve the spans by moving the trait bounds to the impl's where clause.

Instead of relying on the implicit checks inside the Values type, we can inject explicit T: AsExpression<...> bounds into the where clause of the generated impl Insertable. By applying quote_spanned! to these explicit bounds, we force the compiler to verify them before it attempts the deep trait resolution where the span gets lost.

I'm happy to open a PR with these changes.

[weiznich]

@sim-hash That are great news ❤️ Thanks again for digging through this. I think adding the explicit bounds is totally fine, we should just add a comment to the derive source code that this is done to improve the error messages. I'm looking forward to your PR.

[sim-hash]

I rushed the MR. Sorry for the mix-up!