Attribute hygiene

This module provides hygiene for attributes. The goal is to report misuses of attributes to the user as soon as possible so that no mistyped attribute get silently ignored.

`type ('context, 'payload) t`````

Type of declared attribute.

The `'context` type parameter describes where the attribute is expected and the `'payload` one what its payload should contain.

`type packed````=`

`|` `T : (_, _) t ‑> packed`

``
`module Context : sig ... end`
`val declare : Ppxlib__.Import.string ‑> 'a Context.t ‑> (Ppxlib__.Import.payload, 'b, 'c) Ast_pattern.t ‑> 'b ‑> ('a, 'c) t`

`declare fully_qualified_name context payload_pattern k` declares an attribute. `k` is used to build the value resulting from parsing the payload.

For instance if a rewriter named "foo" expect the attribute `@@default` on record field declaration with an expression as payload:

          let default =
            Attribute.declare "foo.default"
              Attribute.Context.label_declaration
              Ast_pattern.(pstr (pstr_eval __ nil))
              (fun x -> x)
          ;;

`fully_qualified_name` is expected to be a dot-separated list of names. When matching, any full suffix will be accepted. So for instance an attribute declared with name "foo.bar.default" will match exactly these attribute names: "default", "bar.default" and "foo.bar.default".

Additionally it is possible to prevent a suffix to be shortened by prefixing it with '@'. So for instance an attribute declared with name "foo.

- @bar.default " will match exactly these attribute names: "bar.default" and "foo.bar.default".

  When matching against a list of attributes on an item, if several matches are possible, the longest one is used. For instance using the attribute "foo.default" declared in the previous example, on this code it will match the `@foo.default 0` attribute:

            type t =
              { x : int [@default 42] [@foo.default 0]
              }

  This is to allow the user to specify a `@default` attribute for all re-writers that use it but still put a specific one for one specific re-writer.

  It is not allowed to declare an attribute with a name that matches a previously-defined one on the same context. For instance trying to declare the same attribute twice will fail.

`val name : (_, _) t ‑> Ppxlib__.Import.string`
`val context : ('a, _) t ‑> 'a Context.t`
`val get : ('a, 'b) t ‑> 'a ‑> 'b Ppxlib__.Import.option`
`val consume : ('a, 'b) t ‑> 'a ‑> ('a * 'b) Ppxlib__.Import.option`

`consume t x` returns the value associated to attribute `t` on `x` if present as well as `x` with `t` removed.

`val remove_seen : 'a Context.t ‑> packed Ppxlib__.Import.list ‑> 'a ‑> 'a`

`remove_seen x attrs` removes the set of attributes matched by elements of `attrs`. Only remove them if they where seen by get or consume.

`module Floating : sig ... end`
`val explicitly_drop : Ast_traverse.iter`

Code that is voluntarily dropped by a rewriter needs to be given to this object. All attributes inside will be marked as handled.

`val check_unused : Ast_traverse.iter`

Raise if there are unused attributes

`val freshen_and_collect : Ast_traverse.map`

Replace all attribute names by a `String.copy` and collect them. To be used in conjuction with check_all_seen.

`val check_all_seen : Ppxlib__.Import.unit ‑> Ppxlib__.Import.unit`

Check that all attributes collected by freshen_and_collect have been:

- matched at least once by one of: get, consume or Floating.convert
- seen by `check_unused` (to allow white-listed attributed to pass through)

This helps with faulty ppx rewriters that silently drop attributes.

`val mark_as_handled_manually : Ppxlib__.Import.attribute ‑> Ppxlib__.Import.unit`

Mark an attribute as seen and handled. This is only to make ppx rewriters that don't use ppxlib works well with the ones that do use it.

`val dropped_so_far_structure : Ppxlib__.Import.structure ‑> Ppxlib__.Import.string Loc.t Ppxlib__.Import.list`

Return the list of attributes that have been dropped so far: attributes that haven't been marked and are not present in the given AST. This is used to debug extensions that drop attributes.

`val dropped_so_far_signature : Ppxlib__.Import.signature ‑> Ppxlib__.Import.string Loc.t Ppxlib__.Import.list`
`val reset_checks : Ppxlib__.Import.unit ‑> Ppxlib__.Import.unit`
`val pattern : ('a, 'b) t ‑> ('a, 'c, 'd) Ast_pattern.t ‑> ('a, 'b Ppxlib__.Import.option ‑> 'c, 'd) Ast_pattern.t`