document args for -Zbuild-std when folks aren't building everything

[robertbastian]

I tried this code:

$ cargo init repro
$ cd repro
$ cargo +nightly check --target thumbv8m.main-none-eabihf -Zbuild-std=std

I expected to see this happen: builds successfully

Instead, this happened:

   Compiling std v0.0.0 (.../.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/std)
error[E0432]: unresolved import `crate::sys::thread_local::key::StaticKey`
 --> .../.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/sys/thread_local/os.rs:5:5
  |
5 | use crate::sys::thread_local::key::StaticKey as OsKey;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `StaticKey` in `sys::thread_local::key`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `std` (lib) due to 1 previous error

Meta

$ rustc +nightly --version --verbose
rustc 1.81.0-nightly (fda509e81 2024-06-25)
binary: rustc
commit-hash: fda509e817abeeecb5b76bc1de844f355675c81e
commit-date: 2024-06-25
host: aarch64-apple-darwin
release: 1.81.0-nightly
LLVM version: 18.1.7

[bjorn3]

Isn't thumbv8m.main-none-eabihf (supposed to be) a no_std target? There is no OS such code runs on.

[robertbastian]

We're using -Zbuild-std=std to get -Zbuild-std-features=panic_immediate_abort. This works on older nightlies but might have changed?

[robertbastian]

We probably need to use -Zbuild-std=core,alloc -Zbuild-std-features=panic_immediate_abort

[workingjubilee]

probably in the unstable book for the option?

[workingjubilee]

sorry for absconding with your issue @robertbastian but it seems to me that not being able to build std is uhhh expected on a no_std target, even if it might have slightly funny results, whereas "how to steer -Zbuild-std correctly" seems to be inadequately documented for your particular usage, based on your remarks in unicode-org/icu4x#5132

[Enselic]

Triage: The job to fix a nicer "UI" for -Zbuild-std belongs to the cargo project. Can someone with sufficient privileges move this to the cargo project please? Thanks!

[workingjubilee]

@rustbot transfer cargo

[weihanglo]

There was a discussion on Zulip about why people building std for no_std targets. I saw this issue but still confused about the actual use case. Is it just for a simpler CI script setup?

[workingjubilee]

@weihanglo Despite the name -Zbuild-std, it's also quite useful for building libcore, and that's basically the main reason.

Embedded often wants this. For instance, there are cases where there is a target, but its baseline features don't include a feature your CPU has, so you want to rebuild the stdlib, which was compiled against that baseline, against your actual available features.