Failing to build on certain embedded platforms

[joshlf]

Progress

• Fix build failures: Support 16-bit target platforms #2401
• Add a regression test
  • Note that this is blocked on being able to roll our pinned nightly toolchain in order to bring in this upstream commit
  • The relevant build command is: RUSTFLAGS='-C target-cpu=atmega328p' cargo +nightly-2025-02-25 check --target=avr-none -Zbuild-std=core

---

Originally reported by @ErickTorresBrownU in rust-random/rand#1574 (comment):

For instance, I can no longer compile my embedded Rust code because rand/rand_core now depends on zerocopy.

error[E0080]: evaluation of constant value failed
   --> C:\Users\erick\.cargo\registry\src\index.crates.io-6f17d22bba15001f\zerocopy-0.8.20\src\layout.rs:120:80
    |
120 |     pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 28) {
    |                                                                                ^^^^^^^ attempt to shift left by `28_i32`, which would overflow

@ErickTorresBrownU, what platform are you building for here? We would like to support this use case and add this platform to our CI so we don't regress.

[ErickTorresBrownU]

Arduino Nano ~ avr-atmega328p

[joshlf]

EDIT: Nevermind, figured it out 🙂

---

Okay thanks. What build command are you using? I'm trying but getting this failure:

$ RUSTFLAGS='-C target-cpu=atmega328p' cargo +nightly-2025-02-25 check --target=avr-none -Zbuild-std
   Compiling gimli v0.31.1
   Compiling miniz_oxide v0.8.3
error[E0432]: unresolved import `alloc::sync`
 --> .../.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/gimli-0.31.1/src/read/dwarf.rs:2:12
  |
2 | use alloc::sync::Arc;
  |            ^^^^ could not find `sync` in `alloc`

error[E0432]: unresolved import `alloc::sync`
 --> .../.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/gimli-0.31.1/src/read/abbrev.rs:4:12
  |
4 | use alloc::sync::Arc;
  |            ^^^^ could not find `sync` in `alloc`

error: values of the type `[u8; 32768]` are too big for the target architecture
  --> .../.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/miniz_oxide-0.8.3/src/inflate/stream.rs:86:5
   |
86 |     fn default() -> Self {
   |     ^^^^^^^^^^^^^^^^^^^^

error: could not compile `miniz_oxide` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
For more information about this error, try `rustc --explain E0432`.
error: could not compile `gimli` (lib) due to 2 previous errors

By contrast, if I build without -Zbuild-std, I get:

$ RUSTFLAGS='-C target-cpu=atmega328p' cargo +nightly-2025-02-25 check --target=avr-none
    Checking zerocopy v0.8.20 (.../workspace/zerocopy)
error[E0463]: can't find crate for `core`
  |
  = note: the `avr-none` target may not be installed
  = help: consider downloading the target with `rustup target add avr-none`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `zerocopy` (lib) due to 1 previous error

[joshlf]

We've published a fix in 0.8.21. @ErickTorresBrownU can you confirm that this fixes your issue?

[ErickTorresBrownU]

Just applied the changes and yes, the issue is resolved with your changes. Good job! 🤗

[joshlf]

Okay great, thanks for testing it out!