Function kernel::errorcode::into_statuscode

pub fn into_statuscode(r: Result<(), ErrorCode>) -> usize

Convert a Result<(), ErrorCode> to a StatusCode (usize) for userspace.

StatusCode is a useful “pseudotype” (there is no actual Rust type called StatusCode in Tock) for three reasons:

1. It can be represented in a single usize. This allows StatusCode to be easily passed across the syscall interface between the kernel and userspace.

2. It extends ErrorCode, but keeps the same error-to-number mappings as ErrorCode. For example, in both StatusCode and ErrorCode, the SIZE error is always represented as 7.

3. It can encode success values, whereas ErrorCode can only encode errors. Number 0 in ErrorCode is reserved, and is used for SUCCESS in StatusCode.

This helper function converts the Tock and Rust convention for a success/error type to a StatusCode. StatusCode is represented as a usize which is sufficient to send to userspace via an upcall.

The key to this conversion and portability between the kernel and userspace is that ErrorCode, which only expresses errors, is assigned fixed values, but does not use value 0 by convention. This allows us to use 0 as success in ReturnCode.