In [this
comment](https://github.com/bytecodealliance/wasmtime/pull/3545#discussion_r756284757)
I noted a potential subtle issue with the way that a few rules were
written that is fine now but could cause some unexpected pain when we
get around to verification.
Specifically, a set of rules of the form
```
(rule (A (B _)) (C))
(rule (A _) (D))
```
should, under any reasonable "default" rule ordering scheme, fire the
more specific rule `(A (B _))` when applicable, in preference to the
second "fallback" rule.
However, for future verification-specific applications of ISLE, we want
to ensure the property that a rule's meaning/validity is not dependent
on being overridden by more specific rules. In other words, if a rule
specifies a rewrite, that rewrite should always be correct; and choosing
a more specific rule can give a *better* compilation (better generated
code) but should not be necessary for correctness.
This is an admittedly under-documented part of the language, though in the
pending #3560 I added a note about rule ordering being a heuristic that
should hopefully make this slightly clearer. Ultimately I want to have
tests that choose non-default rule orderings and differentially fuzz in
order to be sure that we're following this principle; and of course once
we're actually doing verification, we'll catch issues like this upfront.
Apologies for the subtle footgun here and hopefully the reasoning is
clear enough :-)
bc0de464bc