Expose List.capacity function

[samnaughtonb]

This PR exposes the List.capacity builtin (mentioned in #9596)

[greptile-apps]

Greptile Summary

Exposes List.capacity as a public builtin by mapping it to the existing low-level operation and adds an evaluator test.

• Adds the List.capacity : List(_item) -> U64 declaration and documentation.
• Registers the declaration with canonicalization as .list_capacity.
• Tests reserved capacity for a List(U64).

Confidence Score: 4/5

The PR should not merge until List.capacity reports a contract-compliant, backend-consistent value for nonempty zero-width-element lists.

The newly exposed builtin returns the list length in evaluation but zero through LLVM for the same nonempty zero-width list, violating its documented capacity invariant.

Files Needing Attention: src/canonicalize/BuiltinLowLevel.zig, src/build/roc/Builtin.roc, and the backend-focused capacity tests

Important Files Changed

Filename	Overview
src/build/roc/Builtin.roc	Adds the public declaration and an invariant that LLVM does not satisfy for nonempty zero-width-element lists.
src/canonicalize/BuiltinLowLevel.zig	Exposes the existing .list_capacity backend discrepancy through the public builtin.
src/eval/test/eval_low_level_tests.zig	Adds a positive reserved-capacity test but does not exercise zero-width lists or cross-backend consistency.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A["Builtin.List.capacity(list)"] --> B["Canonicalize to .list_capacity"]
    B --> C{"Backend"}
    C --> D["Evaluator: max(stored capacity, len)"]
    C --> E["LLVM: decode stored capacity"]
    F["Nonempty zero-width list\nlen = N, stored capacity = 0"] --> D
    F --> E
    D --> G["Returns N"]
    E --> H["Returns 0"]

Loading

_{Reviews (1): Last reviewed commit: "Expose List.capacity across backends" | Re-trigger Greptile}

[greptile-apps]

Zero-width capacity diverges by backend

When List.capacity is called on a nonempty zero-width-element list such as [(), (), ()], this mapping exposes .list_capacity, for which LLVM returns the stored capacity of 0 while evaluation returns the list length. This violates the documented capacity >= len invariant and makes the same Roc program return different results across backends.