Find cycles in rewrite constraints without performing the full exponential expansion of the RHS

[danakj]

Substitute one associated constant reference at a time on the RHS. If there's a cycle in the rewrite constraints, this allows it to be found without expanding the entire RHS, which can involve exponential type expansion, and repeating this growth a number of times equal to the number of steps in the cycle.

Fixes #5672

[danakj]

I couldn't come up with a counter example yet that would still be exponential with a power of the size of the cycle, before finding said cycle, under the solution proposed here.

[zygoloid]

Wouldn't we also get exponential runtime if we tweaked this example to not have a cycle? Eg

Suggested change

	T:! Z where
	.T1 = (.T2, .T2, .T2) and
	.T2 = (.T3, .T3, .T3) and
	.T3 = (.T4, .T4, .T4) and
	.T4 = (.T5, .T5, .T5) and
	.T5 = (.T6, .T6, .T6) and
	.T6 = (.T7, .T7, .T7) and
	.T7 = (.T1, .T1, .T1)
	);
	T:! Z where
	.T1 = (.T2, .T2, .T2) and
	.T2 = (.T3, .T3, .T3) and
	.T3 = (.T4, .T4, .T4) and
	.T4 = (.T5, .T5, .T5) and
	.T5 = (.T6, .T6, .T6) and
	.T6 = (.T7, .T7, .T7) and
	.T7 = (i32, i32, i32)
	);

If we want that case to also be sub-exponential, I think we'd want a different strategy. A couple of options come to mind:

• Cache the type that each rewrite substitutes into and reuse it rather than rebuilding it.
• Precompute the dependencies between rewrites and substitute bottom-up. (But we'd need to handle the .A = .B.A and .B = .Self case somehow.)

[danakj]

Noting here that with #5692 and #5693 this particular example becomes much faster (17ms). But it's still exponential, so we just need to add more to make it slow.

This version of it, which doesn't have a cycle, and is thus not impacted by those changes, takes minutes (at least) to complete:

fn F(
    T:! Z where
      .T0 = (.T1, .T1, .T1, .T1, .T1, .T1, .T1, .T1, .T1, .T1, .T1, .T1) and
      .T1 = (.T2, .T2, .T2, .T2, .T2, .T2, .T2, .T2, .T2, .T2, .T2, .T2) and
      .T2 = (.T3, .T3, .T3, .T3, .T3, .T3, .T3, .T3, .T3, .T3, .T3, .T3) and
      .T3 = (.T4, .T4, .T4, .T4, .T4, .T4, .T4, .T4, .T4, .T4, .T4, .T4) and
      .T4 = (.T5, .T5, .T5, .T5, .T5, .T5, .T5, .T5, .T5, .T5, .T5, .T5) and
      .T5 = (.T6, .T6, .T6, .T6, .T6, .T6, .T6, .T6, .T6, .T6, .T6, .T6) and
      .T6 = (.T7, .T7, .T7, .T7, .T7, .T7, .T7, .T7, .T7, .T7, .T7, .T7) and
      .T7 = (.T8, .T8, .T8, .T8, .T8, .T8, .T8, .T8, .T8, .T8, .T8, .T8) and
      .T8 = (.T9, .T9, .T9, .T9, .T9, .T9, .T9, .T9, .T9, .T9, .T9, .T9) and
      .T9 = ()
    );```