Instrumentation backends

The change

dunolint now supports stanzas with several instrumentation fields, one per backend, as dune allows. Previously, it didn't consider that case. The (dune (instrumentation _)) selector now considers these fields as a collection, which comes with the following changes to the backend predicate.

Flags are matched as a subset.

• Before: (backend NAME FLAGS...) held when the field had exactly these flags, and enforcing it replaced the flags of the field.
• After: (backend NAME FLAGS...) holds when there is a field for NAME with at least these flags, in any order. Enforcing it keeps the existing flags of the field, and adds the missing ones.

Negating a backend without flags removes the field.

• Before: (not (backend NAME)) held when the field had NAME with some flags, and enforcing it never suggested any change.
• After: (not (backend NAME)) holds when there is no field for NAME, regardless of its flags. Enforcing it removes the field, like (absent NAME).

Fields are not renamed.

• Before: Enforcing (backend NAME) on a stanza with a field for another backend replaced the backend of that field.
• After: Enforcing (backend NAME) adds a field for NAME, and leaves the fields of other backends untouched.

Do I need to migrate?

Case 1: You use backend to require a backend

Rules such as the one below keep working, and no migration is needed:

(rule
 (enforce (dune (instrumentation (backend bisect_ppx)))))

The difference is that a bisect_ppx field with flags now satisfies the rule, and its flags are kept.

Case 2: You relied on backend to remove flags

Enforcing (backend NAME) doesn't remove the flags of an existing field anymore. If you need these flags removed, you have to do it manually.

Case 3: You relied on backend to replace a backend with another one

Enforcing (backend NAME) now adds a field for NAME next to the existing one. To replace a backend with another one, require the new backend with present, and remove the old one with absent:

(rule
 (enforce
  (dune
   (instrumentation
    (and
     (present bisect_ppx)
     (absent landmarks))))))

See Migrating to another backend for details.

Case 4: You use (not (backend NAME))

This now forbids any field for NAME, regardless of its flags, and enforcing it removes such a field. We recommend writing (absent NAME) instead, which is equivalent.