Add RFC for EnumView.Matches.

text/0071-enumview-matches.md

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+- Start Date: 2024-07-29
+- RFC PR: [amaranth-lang/rfcs#71](https://github.com/amaranth-lang/rfcs/pull/71)
+- Amaranth Issue: [amaranth-lang/amaranth#1468](https://github.com/amaranth-lang/amaranth/issues/1468)
+
+# `EnumView.matches`
+
+## Summary
+[summary]: #summary
+
+A `matches` method is added to `EnumView`, performing a similar function to `Value.matches`.
+
+## Motivation
+[motivation]: #motivation
+
+Amaranth currently has `Value.matches`, which checks whether a value matches any of the given patterns. An obvious application of such a feature is using it with an enumerated value. However, as it stands, `EnumView` doesn't provide a corresponding method natively, requiring a cast to `Value`. Additionally, casting the `EnumView` to `Value` erases the enumeration type, providing no protection against matching with values from the wrong type.
+
+This RFC proposes to fill that gap by adding a type-safe `EnumView.matches` method.
+
+## Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The `EnumView.matches` method can be used to check whether an enumerated value belongs to a given set:
+
+```
+class Color(enum.Enum, shape=3):
+    BLACK = 0
+    RED = 1
+    GREEN = 2
+    BLUE = 4
+    WHITE = 7
+
+s = Signal(Color)
+m.d.comb += is_grayscale.eq(s.matches(Color.BLACK, Color.WHITE))
+```
+
+## Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The following method is added:
+
+- `EnumView.matches(*values)`: equivalent to `self.as_value().matches(*values)`, but requires all `values` to be a member of the same enum as the `EnumView`, raising a `TypeError` otherwise
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+- more moving parts in the language
+- more general pattern matching facilities have been requested on multiple occasions, which this facility could end up redundant with
+
+## Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The proposed functionality is an obvious extension of `Value.matches` to the `EnumView` class. The proposed typechecking is consistent with current `EnumView` behavior.
+
+## Prior art
+[prior-art]: #prior-art
+
+`Value.matches`, Rust `matches!()`.
+
+## Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+## Future possibilities
+[future-possibilities]: #future-possibilities
+
+None for `EnumView`. However, there have been discussions of having more advanced pattern matching facilities in the language in general.