Code supporting both 32-bit and 64-bit platforms

[sunfishcode]

The following code compiles on 64-bit targets:

fn main() {
    let a: usize = 2;
    let b: u32 = cast::u32(a).unwrap();
    let c: u64 = cast::u64(a);
    let _d: usize = cast::usize(b);
    let _e: usize = cast::usize(c);
}

However, it doesn't compile on a 32-bit target:

error[E0599]: no method named `unwrap` found for type `u32` in the current scope
 --> src/main.rs:3:31
  |
3 |     let b: u32 = cast::u32(a).unwrap();
  |                               ^^^^^^

error[E0308]: mismatched types
 --> src/main.rs:6:21
  |
6 |     let _e: usize = cast::usize(c);
  |                     ^^^^^^^^^^^^^^ expected usize, found enum `std::result::Result`
  |
  = note: expected type `usize`
             found type `std::result::Result<usize, cast::Error>`

because the Output types use Result or not depending on the target's usize size.

Is there a way to write code that does checked (when needed) casts between fixed-sized types and usize that works on both 32-bit and 64-bit platforms?

I might be looking for something returning a Result on both, relying on inlining and optimization to eliminate the overhead in cases where the conversion turns out to be trivial.

[japaric]

Is there a way to write code that does checked (when needed) casts between fixed-sized types and usize that works on both 32-bit and 64-bit platforms?

Yes, using conditional compilation: #[cfg(target_pointer_width = "32")].

I might be looking for something returning a Result on both

Then you might want to use usize::try_from instead.

[sunfishcode]

And now, TryFrom is stable in Rust 1.34, so that covers the need here!