Portable definition of set_precision()?

[he32]

I am trying to build pydantic-core, and as part of that, the lexical-parse-float-1.0.5 rust crate on NetBSD/i386 using the i586-unknown-netbsd rust target. This fails with:

error[E0308]: mismatched types
  --> /usr/pkgsrc/devel/py-pydantic-core/work/vendor/lexical-parse-float-1.0.5/src/number.rs:77:23
   |
77 |         let _cw: () = set_precision::<F>();
   |                  --   ^^^^^^^^^^^^^^^^^^^^ expected `()`, found `FPUControlWord`
   |                  |
   |                  expected due to this

For more information about this error, try `rustc --explain E0308`.
error: could not compile `lexical-parse-float` (lib) due to 1 previous error

I expected this to build.

This possibly goes back to the set_precision() function not having a consistent type depending on whether "sse2" is in the target feature set (which it isn't in this case, ref. i586).

In ${PREFIX}/lib/rustlib/src/rust/library/core/src/num/dec2flt/fpu.rs I find:

#[cfg(all(target_arch = "x86", not(target_feature = "sse2")))]
mod fpu_precision {
...
    pub(crate) fn set_precision<T>() -> FPUControlWord {
...
#[cfg(any(not(target_arch = "x86"), target_feature = "sse2"))]
mod fpu_precision {
    pub(crate) fn set_precision<T>() {}
}

Even though my familiarity with rust is sub-par, those two function signatures surely are not the same, and hence, code which expects one but gets the other bombs out as above.

What's the appropriate fix or workaround?

Meta

rustc --version --verbose:

$ rustc --version --verbose
rustc 1.88.0 (6b00bc388 2025-06-23) (built from a source tarball)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: i586-unknown-netbsd
release: 1.88.0
LLVM version: 20.1.5
$ 

[bjorn3]

The definition of the set_precision function that lexical-parse-float calls can be found elsewhere in lexical-parse-float. The set_precision inside libcore is not exposed to users. lexical-parse-float should have used let _cw = ... rather than let _cw: () = ... here. In force_fast_path it does do this correctly. In any case this issue was already fixed in lexical-parse-float in Alexhuszagh/rust-lexical#219, but there hasn't been a release since.

[he32]

Thanks for the hints.
Just doing let _cw = ... apparently also doesn't work:

error: expected type, found `=`
  --> /usr/pkgsrc/devel/py-pydantic-core/work/vendor/lexical-parse-float-1.0.5/src/number.rs:77:18
   |
77 |         let _cw: = set_precision::<F>();
   |                - ^ expected type
   |                |
   |                while parsing the type for `_cw`

I'll visit that PR to see what other needs to be done.

[he32]

My lack of rust knowledge exposed again. The : also needs to go.

[he32]

Thanks, that fixes it.