Switch providers to not use opaque types

[Mark-Simulacrum]

The Rust compiler deals poorly with the signature of our providers, leading to exponential compile times with additional providers. In one real-world case this patch changes cargo check compile times from 6 minutes to 20 seconds, which is a significant delta. The tradeoff with the current compiler is somewhat more verbose error messages, but given the potentially massive wins in compile time for users of this crate that seems worth it.

Testing:

No particular testing - expected to be a no-op at runtime.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[camshaft]

For context, I opened an issue for rustc related to this: rust-lang/rust#100886.

My only worry is reverting this would be a breaking change, right? It's probably unlikely but an application could potentially start using the concrete types in a signature and changing it back to an impl Trait would break that.

That being said, it's probably worth it.

[Mark-Simulacrum]

My only worry is reverting this would be a breaking change, right? It's probably unlikely but an application could potentially start using the concrete types in a signature and changing it back to an impl Trait would break that.

Yeah, that seems true. I think you have two cases:

• Specifying the concrete type by writing it out - only possible if you directly refer to Providers, which is doc(hidden) and folks really shouldn't do that. They can do that today already if they want to, too, and break (just not with the same interface as with_X).
• Constructing builders in parallel, e.g., in two branches of an if. This will break if we go back to impl Trait but doesn't seem very likely.