Infinite type-level recursion in ForEachKey impl in Policy

[apoelstra]

In this code we have a method for_each which takes a F: FnMut and calls itself with an &mut F. This causes the compiler to try to instantiate the method with F, &mut F, &mut &mut F, etc., forever. Throughout our codebase, we've avoided this infinite recursion by having an internal real_for_each method which directly takes a &mut F, so that it can call itself with the same type that it was called with. Then our public-facing for_each simply sticks a &mut on its argument and calls real_for_each.

In the code in question, though, we didn't do this, and for some reason the Rust compiler has accepted it, until tonight. The latest nightly, 2023-04-21, does not compile the code.

Unfortunately this code has been present for a long time (I think since 1.0 of this crate) so to fix it, we will need to backport fixes to ever released version. Fortunately, there are a couple potential fixes, both of which are pretty simple:

• Add a real_for_each internal funciton like we do elsewhere
• Change &mut pred to &mut |key| pred(key) in two places (thanks GPT4 for this one)

See rust-lang/rust#110656

[sanket1729]

Interesting. I don't think we should back-port it to every release. How about just the releases in the last two years.

[apoelstra]

That sounds good to me.

[sanket1729]

Actually, @apoelstra the fact the downstream crates did not report any errors means that we should don't need any backports.

Using this function still results would have still compile error for all versions? I guess that now it results in compile error early on in this library?

[apoelstra]

@sanket1729 downstream crates haven't reported any errors because only recent Rust nightlies expose errors. I worry that the next stable version will expose the error and then we'll have trouble.