RFC #73: Stricter connections.

text/0073-stricter-connections.md

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+- Start Date: 2024-09-16
+- RFC PR: [amaranth-lang/rfcs#73](https://github.com/amaranth-lang/rfcs/pull/73)
+- Amaranth Issue: [amaranth-lang/amaranth#1511](https://github.com/amaranth-lang/amaranth/issues/1511)
+
+# Stricter connections
+
+## Summary
+[summary]: #summary
+
+Make it possible for `lib.wiring.connect()` to reject connections between ports with shapes that despite having equal widths are incompatible.
+
+## Motivation
+[motivation]: #motivation
+
+The primary motivating use case is to be able to prevent connecting stream interfaces with incompatible payload shapes.
+
+## Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+`lib.stream` defines a basic stream interface with `ready`, `valid` and `payload` members.
+Additional metadata signals can be added by making the `payload` shape an aggregate.
+This has the advantage that such streams can be passed through standard components like FIFOs without them having to know or care about these signals.
+
+Consider how we can make a byte-wide packetized stream by defining the `payload` shape as `StructLayout({"data": 8, "last": 1})`.
+The `data` field contains the data byte and the `last` field is a flag indicating that the current byte is the last in the packet.
+
+This is not the only common way to make a packetized stream.
+Instead of (or in addition to) the `last` flag, the payload could have a `first` flag, indicating that the current byte is the first in the packet.
+Both are valid options with different semantics for different usecases.
+
+The problem is that `lib.wiring.connect()` currently only checks that port members have matching widths, so a connection between a stream interface with `last` semantics and a stream interface with `first` semantics will not be rejected, despite being incompatible.
+
+Currently, `lib.data.View.eq()` does no checks on the passed value and immediately forwards to `self.as_value().eq()`, making this legal:
+
+```python
+>>> a = Signal(StructLayout({"data": 8, "last": 1}))
+>>> b = Signal(StructLayout({"data": 8, "first": 1}))
+>>> a.eq(b)
+(eq (sig a) (sig b))
+```
+
+This RFC proposes adding a check to `View.eq()` that would reject direct assignments from another aggregate data structure with a non-identical layout.
+If such an assignment is desired, the other aggregate data structure can be explicitly passed through `Value.cast()` first.
+
+Currently `lib.wiring.connect()` passes every signal through `Value.cast()` before assigning them.
+This results in a `ValueCastable`'s `.eq()` not being called, and thereby bypassing the check proposed above.
+
+This RFC proposes removing the `Value.cast()` so a `ValueCastable`'s `.eq()` will be called directly.
+
+This RFC proposes updating `lib.enum.EnumView` in the same manner, for the same reason, as `lib.data.View`.
+
+## Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Modify `lib.wiring.connect()` to not pass port members through `Value.cast()`, so that a `ValueCastable`'s `.eq()` will be called, allowing it to perform compatibility checks.
+- If a `ValueCastable` doesn't define `.eq()`, reject the assignment.
+
+Modify `lib.data.View.eq(other)` to add the following checks:
+- If `other` is a `ValueCastable`, do `Layout.cast(other.shape())`
+  - If a valid `Layout` is returned, reject the assignment if it doesn't match `self.shape()`.
+  - If `Layout.cast()` raises, reject the assignment.
+- Otherwise, proceed as normal.
+
+Modify `lib.enum.EnumView.eq(other)` to add the following checks:
+- If `other` is a `ValueCastable`, reject the assignment if `other.shape()` doesn't match `self.shape()`.
+- Otherwise, proceed as normal.
+
+Rejected assignments are a warning in Amaranth 0.6 and becomes a hard error in Amaranth 0.7.
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+- Increased language complexity.
+
+- This will add an implied requirement for a `ValueCastable` to implement `.eq()` to be usable with `lib.wiring`. Currently a `ValueCastable` is not required to implement `.eq()` at all.
+  - We could fall back to `Value.cast().eq()` when `.eq()` is not defined.
+
+## Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- Signals like `first` and `last` could be added as separate ports in the stream signature, preventing incompatible connections.
+  - This was already discussed in [RFC 61](0061-minimal-streams.md) and decided against.
+- An explicit hook for checking compatibility between interfaces could be added, instead of relying on `.eq()`.
+  - This RFC proposes picking the low-hanging fruits first, making use of already existing mechanisms.
+    If this is not enough, another RFC can propose an explicit hook later.
+
+## Prior art
+[prior-art]: #prior-art
+
+[RFC 61](0061-minimal-streams.md) already discussed the need to eventually make `lib.wiring.connect()` stricter to prevent the connection of stream interfaces with incompatible payloads.
+
+## Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+## Future possibilities
+[future-possibilities]: #future-possibilities
+
+- A hook can still be added to allow a signature/interface to perform an explicit compatibility check, for cases where signatures have identical members but still have metadata indicating they are incompatible.
+  - This hook could also allow overriding the existing checks, allowing connections where interfaces are compatible despite differences in members.