This PR adds an alternative implementation of `DerivingBEq` based on
comparing `.ctorIdx` and using a dedicated matcher for comparing same
constructors (added in #10152), to avoid the quadratic overhead of the
default match implementation. The new option
`deriving.beq.linear_construction_threshold` sets the constructor count
threshold (10 by default) for using the new construction. Such instances
also allow `deriving ReflBEq, LawfulBeq`, although these proofs for
these properties are still quadratic.
This PR introduces an alternative construction for `DecidableEq`
instances that avoids the quadratic overhead of the default
construction.
The usual construction uses a `match` statement that looks at each pair
of constructors, and thus is necessarily quadratic in size. For
inductive data type with dozens of constructors or more, this quickly
becomes slow to process.
The new construction first compares the constructor tags (using the
`.ctorIdx` introduced in #9951), and handles the case of a differing
constructor tag quickly. If the constructor tags match, it uses the
per-constructor-eliminators (#9952) to create a linear-size instance. It
does so by creating a custom “matcher” for a parallel match on the data
types and the `h : x1.ctorIdx = x2.ctorIdx` assumption; this behaves
(and delaborates) like a normal `match` statement, but is implemented in
a bespoke way. This same-constructor-matcher will be useful for
implementing other instances as well.
The new construction produces less efficient code at the moment, so we
use it only for inductive types with 10 or more constructors by default.
The option `deriving.decEq.linear_construction_threshold` can be used to
adjust the threshold; set it to 0 to always use the new construction.
