update patterns.md for const pattern RFC

[RalfJung]

Updates the reference for rust-lang/rfcs#3535.
Tracking issue: rust-lang/rust#120362.

This is not 100% implemented yet:

• parts of it are pending on make matching on NaN a hard error, and remove the rest of illegal_floating_point_literal_pattern rust#116284
• also the (recursive) structural equality checker in rustc currently sometimes issues a future-compat lint instead of a hard error; this may still take some tome to phase out (we need to first bump that to a "future-compat lint that is also shown in dependencies")
• violations of the T: PartialEq currently also just give a future-compat lint, not a hard error

[ehuss]

@RalfJung Is this ready to merge? Were there any changes since this was written?

[RalfJung]

We are slowly staging the deployment of this in rustc. Not sure how closely the reference wants to match that.

If you are okay with the reference stating things as required that are just future-compat lints in rustc (rather than hard errors), then this is ready to go, yes.

[RalfJung]

FYI, when rust-lang/rust#124661 lands rustc will fully behave as documented here. I hope to land that in this release cycle.

[ehuss]

I don't understand this part. Path patterns can be associated consts. Can you clarify what it is referring to?

[RalfJung]

This is intended to explain why this code does not compile:

trait Trait {
    const C: i32;
}

fn f<T: Trait>(x: i32) -> bool {
    matches!(x, T::C) //~ERROR: associated consts cannot be referenced in patterns
}

[ehuss]

And strings?

[RalfJung]

Ah right, they should also be added here, good catch.

[ehuss]

Thanks!