feat: converter = None as the sentinel (#996)

* feat: converter = None as the sentinel

1. Easier for users to access instead of a private sentinel.
2. simplifies later performance-related logic changes.
3. Broadens support to allow `dataclasses.field(metadata=dict(converter=None))`, not just `eqx.field`.

Signed-off-by: Nathaniel Starkman <nstarman@users.noreply.github.com>

* refactor: only set annotations if datclass init

Signed-off-by: Nathaniel Starkman <nstarman@users.noreply.github.com>

---------

Signed-off-by: Nathaniel Starkman <nstarman@users.noreply.github.com>

Author: nstarman

equinox/_module.py

@@ -48,12 +48,9 @@ def static_field(**kwargs):
     return field(**kwargs, static=True)
 
 
-_converter_sentinel: Any = doc_repr(object(), "lambda x: x")
-
-
 def field(
     *,
-    converter: Callable[[Any], Any] = _converter_sentinel,
+    converter: Callable[[Any], Any] | None = None,
     static: bool = False,
     **kwargs,
 ):
@@ -66,6 +63,7 @@ def field(
         `bool`/`int`/`float`/`complex` values to JAX arrays. This is ran after the
         `__init__` method (i.e. when using a user-provided `__init__`), and after
         `__post_init__` (i.e. when using the default dataclass initialisation).
+        If `converter` is `None`, then no converter is registered.
     - `static`: whether the field should not interact with any JAX transform at all (by
         making it part of the PyTree structure rather than a leaf).
     - `**kwargs`: All other keyword arguments are passed on to `dataclass.field`.
@@ -104,7 +102,7 @@ class MyModule(eqx.Module):
     except KeyError:
         metadata = {}
     if "converter" in metadata:
-        raise ValueError("Cannot use metadata with `static` already set.")
+        raise ValueError("Cannot use metadata with `converter` already set.")
     if "static" in metadata:
         raise ValueError("Cannot use metadata with `static` already set.")
     # We don't just use `lambda x: x` as the default, so that this works:
@@ -122,7 +120,7 @@ class MyModule(eqx.Module):
     # Oddities like the above are to be discouraged, of course, but in particular
     # `field(init=False)` was sometimes used to denote an abstract field (prior to the
     # introduction of `AbstractVar`), so we do want to support this.
-    if converter is not _converter_sentinel:
+    if converter is not None:
         metadata["converter"] = converter
     if static:
         metadata["static"] = True
@@ -302,14 +300,14 @@ def __new__(
         # checkers.
         # Note that mutating the `__init__.__annotations__` is okay, as it was created
         # by the dataclass decorator on the previous line, so nothing else owns it.
-        for f in dataclasses.fields(cls):
-            if f.name not in cls.__init__.__annotations__:
-                continue  # Odd behaviour, so skip.
-            try:
-                converter = f.metadata["converter"]
-            except KeyError:
-                pass
-            else:
+        if has_dataclass_init:
+            for f in dataclasses.fields(cls):
+                if not f.init:
+                    continue
+
+                if (converter := f.metadata.get("converter")) is None:
+                    continue  # No converter, so skip.
+
                 try:
                     signature = inspect.signature(converter)
                 except ValueError:
@@ -574,18 +572,16 @@ def __call__(cls, *args, **kwargs):
         assert not _is_abstract(cls)
         # [Step 3] Run converters
         for field in dataclasses.fields(cls):
+            if (converter := field.metadata.get("converter")) is None:
+                continue
+
             try:
-                converter = field.metadata["converter"]
-            except KeyError:
+                value = getattr(self, field.name)
+            except AttributeError:
+                # Let the all-fields-are-filled check handle the error.
                 pass
             else:
-                try:
-                    value = getattr(self, field.name)
-                except AttributeError:
-                    # Let the all-fields-are-filled check handle the error.
-                    pass
-                else:
-                    setattr(self, field.name, converter(value))
+                setattr(self, field.name, converter(value))
         # [Step 4] Check that all fields are occupied.
         missing_names = {
             field.name