Change `Shape.cast(range(1))` to `unsigned(0)`

Summary

Change Shape.cast(range(1)) to return unsigned(0) instead of unsigned(1).

Currently, Shape.cast(range(1)) returns unsigned(1). This is inconsistent with the expectation that casting range to a shape will return the minimal shape capable of representing all elements of that range, which is clearly unsigned(0).

This behavior is an accidental result of using bits_for internally to determine required width for both endpoints, which returns 1 for an input of 0.

The behavior introduces edge cases in unexpected places. For example, one may expect that Memory(depth=depth).read_port().addr.width == ceil_log2(depth). This is currently false for depth of 1 for no particularly good reason.

Guide-level explanation

Shape.cast(range(1)) is changed to return unsigned(0). The same applies to any other range whose only element is 0, like Shape.cast(range(0, 2, 2)).

Arguably, this change requires a negative amount of exlanation, since it removes an edge case and brings the behavior into alignment with the language reference.

Reference-level explanation

See above.

Drawbacks

This is a minor backwards compatibility hazard.

Rationale and alternatives

The change itself is simple enough that it cannot really be done any other way.

It would be possible to introduce a compatibility warning to the 0.4 branch. However, a range() signal having a shape that's slightly too large is unlikely to cause problems in the first place, so the warning would cause a mass of false positives without a nice way to turn it off.

Prior art

None.

Unresolved questions