macro should be const fn

What it does

Checks macros which should instead be const fns.

Lint Name

declarative_macro_should_be_const_fn

Category

correctness

Advantage

A significant improvement to code readability & documentation.

I do not believe there are performance differences when compiled with optimizations although I think it may be worth evaluating.

Drawbacks

It makes the typing more explicit, requiring more changes if the developer later wanted to change types.

Example

Where:

mod ioc {
    use std::os::raw::c_uint;
    pub const _IOC_NRSHIFT: c_uint = 0;
    pub const _IOC_TYPESHIFT: c_uint = 8;
    pub const _IOC_SIZESHIFT: c_uint = 16;
    pub const _IOC_DIRSHIFT: c_uint = 30;
}

this:

#[macro_export]
macro_rules! ioctl_expr {
    ($dir:expr, $ty:expr, $nr:expr, $size:expr) => {
        (($dir << $crate::ioctl::_IOC_DIRSHIFT)
            | ($ty << $crate::ioctl::_IOC_TYPESHIFT)
            | ($nr << $crate::ioctl::_IOC_NRSHIFT)
            | ($size << $crate::ioctl::_IOC_SIZESHIFT)) as ::std::os::raw::c_ulong
    };
}

should be this:

const fn ioctl_expr(dir: c_uint, ty: c_uint, nr: c_uint, size: c_uint) -> ::std::os::raw::c_ulong {
    (
        dir << crate::ioctl::_IOC_DIRSHIFT |
        ty << crate::ioctl::_IOC_TYPESHIFT |
        nr << crate::ioctl::_IOC_NRSHIFT |
        size << crate::ioctl::_IOC_SIZESHIFT
    ) as ::std::os::raw::c_ulong
}

Start by locating the declarative_macro_should_be_const_fn lint entry point in rust-clippy and review how existing correctness lints identify applicable macros. Define the cases covered by the example and evaluate whether optimization affects the proposed change. Done means the lint reliably identifies eligible macros and has coverage for its behavior.