Question about `std::error::repr_bitpacked::Repr::new_os`

Balthild · January 25, 2026, 1:01am

The first line of this function is

let utagged = ((code as usize) << 32) | TAG_OS;

Would this work on 32-bit targets, where usize is 32 bit? There'll be nothing left in the number after shifting it for 32 bit, at first glance.

mbrubeck · January 25, 2026, 1:15am

That code is only built for certain 64-bit targets:

github.com/rust-lang/rust

library/std/src/io/error.rs

f134bbc78


      
          // On 64-bit platforms, `io::Error` may use a bit-packed representation to
          // reduce size. However, this representation assumes that error codes are
          // always 32-bit wide.
          //
          // This assumption is invalid on 64-bit UEFI, where error codes are 64-bit.
          // Therefore, the packed representation is explicitly disabled for UEFI
          // targets, and the unpacked representation must be used instead.
          #[cfg(all(target_pointer_width = "64", not(target_os = "uefi")))]
          mod repr_bitpacked;
          #[cfg(all(target_pointer_width = "64", not(target_os = "uefi")))]
          use repr_bitpacked::Repr;
          
          #[cfg(any(not(target_pointer_width = "64"), target_os = "uefi"))]
          mod repr_unpacked;
          #[cfg(any(not(target_pointer_width = "64"), target_os = "uefi"))]
          use repr_unpacked::Repr;

Balthild · January 25, 2026, 1:17am

Oh! That make sense. I didn't notice these conditions. Thank you.

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Question about `std::error::repr_bitpacked::Repr::new_os`

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