[PATCH v6 2/3] rust: io: mem: add a generic iomem abstraction

[Guangbo Cui]

> +/// A generic memory-mapped IO region.
> +///
> +/// Accesses to the underlying region is checked either at compile time, if the
> +/// region's size is known at that point, or at runtime otherwise.
> +///
> +/// # Invariants
> +///
> +/// `IoMem` always holds an `IoRaw` inststance that holds a valid pointer to the
> +/// start of the I/O memory mapped region.
> +pub struct IoMem<const SIZE: usize = 0> {
> +    io: IoRaw<SIZE>,
> +}

Compile-time checks are only possible when CONFIG_RUST_BUILD_ASSERT_ALLOW=y.
Otherwise, using compile-time check APIs of Io will cause a modpost error 
because the rust_build_error symbol is not exported. Details at the issue[1].

Maybe Io should expose compile-time check APIs only when CONFIG_RUST_BUILD_ASSERT_ALLOW=y?
The expectation is that a build error should occur when calling `Io::readX` and
`Io::writeX` due to a boundary check failure, rather than because the
`rust_build_error` symbol is not exported.

Link: https://github.com/Rust-for-Linux/linux/issues/1141 [1]

Best regards,
Guangbo Cui