[ENG-8] Add metadata()

[akhilles]

Example code:

load("@github/diodeinc/stdlib:v0.2.13/units.zen", "Voltage", "Current", "Resistance")

Powa = interface(
    NET = using(Net),
    voltage_meta = metadata(Voltage),
)

powa = io("POWA", Powa)

# Test 1: Basic metadata container creation and type storage
voltage_meta = powa.voltage_meta
current_meta = metadata(Current) 
resistance_meta = metadata(Resistance)

# Test 2: Storing values with complex record types (including enums)
v1 = Voltage(value=5.0, tolerance=0.05)     # 5V ±5%

c1 = Current(value=0.1, tolerance=0.05)     # 100mA ±5%
c2 = Current(value=0.02, tolerance=0.01)    # 20mA ±1%

r1 = Resistance(value=1000.0, tolerance=0.05)   # 1kΩ ±5%
r2 = Resistance(value=10000.0, tolerance=0.01)  # 10kΩ ±1%

voltage_meta.push(v1)
current_meta.push(c1)
current_meta.push(c2)
resistance_meta.push(r1)
resistance_meta.push(r2)

# Test 3: Automatic deserialization with get()
latest_voltage = voltage_meta.get()
latest_current = current_meta.get()
latest_resistance = resistance_meta.get()

print("Voltages:", voltage_meta.list())
print("Latest current:", latest_current)
print("Latest resistance:", latest_resistance)

# Test 4: Automatic deserialization with list()
all_voltages = voltage_meta.list()
print("Found", len(all_voltages), "voltage entries")

[LK]

can we also add spec for metadata here?

[akhilles]

done

[LK]

What happens if we have types where expected_typ is needed (e.g. enums, records, interfaces)?

[akhilles]

I couldn't find a way to convert those types to values without explicitly requiring the type to be provided at the call site. There's no easy way to extract + carry the necessary type information in Rust.

Maybe we can have an optional type hint argument? e.g. metadata.get(EnumTy). It's a bit awkward.

[akhilles]

I was able to get simple records and enums sorta working by hacking around in starlark-rust, but it was very brittle, and still failed on stuff like records containing enums.

[LK]

I'm ok to say they are not supported right now especially now with native units — what's the behavior if you try to construct a metadata container of type enum(...)?

[akhilles]

You can construct and push values into a metadata container with enum(). On get(), list(), it'll fail with "enum input requires expected_typ".

[LK]

Should we just disallow building a container with unsupported types for now?

[akhilles]

done

[LK]

this is very nice, I like this more than a bunch of global functions too