10850bab

Report Eq and Ord instances that are written out

Two module-level rules, `HsNoCustomEq` and `HsNoCustomOrd`, for the standard
that Eq and Ord are derived.

A written one says which two of these count as the same, or which of them comes
first, in a body that names the fields it was written against. Add a field and
the derived version accounts for it while the written one goes on answering the
older question, so the type gains a field that equality ignores and sorting
cannot see.

## Two rules rather than one

`HsNoCustomShowRead` is one rule over two classes because Show and Read are an
inverse pair: a repository that has decided to write one has decided about the
round trip that is the reason for the rule. Eq and Ord do not sit like that. A
type that has earned a written Ord, ordered by a key rather than by its fields
in order, has usually not earned a written Eq, and one id for both would make
the suppression that allows the first allow the second without anyone saying so.

## No exception for methods that discard their arguments

That exception is on Show and Read because an instance printing a constant keeps
a secret out of every log line and has nothing to drift from. Equality that
cannot see the value is not a comparison anyone reaches for on purpose, so the
same shape is reported here. `HsNoCustomEq/bad-ignoring-arguments.hs` pins it.

## originIsWrittenOut

Whether a body was typed out is `originIsWrittenOut` in `Facts.Instance` rather
than a copy in each rule: a stock, newtype, anyclass, via or standalone
derivation is all the compiler writing against the fields. Enumerated rather
than matched with a catch-all, so a further `InstanceOrigin` has to be answered
for at the one site that asks.

## What to look at

- `hopinion/src/Hopinion/Check/Hs/NoCustomEq/Rule.hs` and
  `.../NoCustomOrd/Rule.hs`, mostly the `ruleWhy` of each.
- `originIsWrittenOut` in `hopinion/src/Hopinion/Facts/Instance.hs`.
- The resources, in particular each `good.hs`, which is where the deriving forms
  that must stay quiet are listed.

## Checks

`nix flake check` passes. Nothing in this repository is reported by either rule.

Total

16s

Created 1 month ago Started 1 month ago Finished 1 month ago

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