assert platform-minimum requirements at build time

[udoprog]

Adds assertions in build.rs which ensures that the platform being built for supports the minimum required by Tokio.

Motivation

Tokio currently makes a number of soft assumptions about the size of a usize (and isize), usually through the form of casting a u32 like:

let value: u32 = todo!();
value as usize

Arguably the preferable way to do this would be to perform a checked conversion using TryFrom, but it would be a lot of work to trace all of them down and it might not always be immediately feasible when it comes to things like bit flags.

It is unlikely that someone will ever attempt to build Tokio for a platform where usize is not at least 4 bytes, but at least when they do, they should be firmly shown that it's a bad idea until assumptions around usize et. al. have been fixed.

Solution

Add assertions to build.rs. I started tinkering with constant assertions, but Rust 1.45 is a bit too limiting to make them ergonomical. While it's possible that build.rs is run with a different rustc than the one used to compile the project, one would have to do something strange to have it do so.

[Darksonn]

What about cross-compilation? Then the build script is not built in the target architecture. Another option is this:

#[cfg_attr(target_pointer_width = "16", compile_error("Pointer width must be at least 32"))]

[udoprog]

Hm. While strictly speaking not exhaustive, that's probably good enough until an MSRV bump. If someone designs a 31-bit hobby platform today they only have themselves to blame. :D

[udoprog]

How about this until we have sufficient const eval?

#[cfg(not(any(target_pointer_width = "32", target_pointer_width = "64", target_pointer_width = "128")))]
compile_error! {
    "Tokio requires the platform pointer width to be 32, 64, or 128 bits"
}