ManagedBuffer.capacity depends on malloc introspection (e.g., malloc_size), which is not available on this platform -- and is thus marked as such.

Here, _HashNode.Storage is a ManagedBuffer. Thankfully, the uses of .capacity are fairly limited: one for statistic purposes, which we just completely omit on the platform, and the other, setting a minimum capacity in the header by the buffer's .capacity; the alternative being setting that minimum capacity directly.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate). n/a: no new code paths, just refactoring
• I've added benchmarks covering new functionality (if appropriate). n/a: no new functionality
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions. having difficulty running tests myself, will use CI if possible
• I've updated the documentation if necessary. no documentation changes necessary