fix: Ensure mod unwind_unimplemented works without nightly feature enabled

[polarathene]

TL;DR: The PR now only reverts a recent change, so that origin will have mod unwind_unimplemented when the nightly / unwinding feature is not enabled. Details here.

Originally this PR was intended to fix building unwinding with nightly feature for panic = "abort" but I was mistaken as the update below notes. That build issue will be resolved with release of unwinding after 0.2.3 via: nbdd0121/unwinding#40

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Original PR message (with update)

This restores the Eyra hello-world-small crate example to build with panic = "abort".

References:

• https://github.com/sunfishcode/origin/pull/149/files#r1808139103
• Regression: error: this directive must appear between .cfi_startproc and .cfi_endproc directives nbdd0121/unwinding#39 (comment)

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This PR isn't quite ready to merge, when built there is many warnings now emitted about the feature = "unwinding" no longer being valid as it is no longer created implicitly from the nightly feature requirements.

UPDATE: I was mistaken about nightly = ["dep:unwinding"] vs nightly = ["unwinding"]. Apart from the difference in implicit feature being dropped, the difference did not actually affect the cfg condition on the unwinding dependency itself, or at least I cannot reproduce such.

I did notice that panic = "abort" in Cargo.toml has no affect on the dependency condition (only -C panic=abort does):

# This is only prevented by `-C panic=abort`, not by equivalent `Cargo.toml` setting
[target.'cfg(panic = "unwind")'.dependencies.some-crate]
version = "0.1.0"

[profile.release]
panic = "abort"

// Either approach for setting abort works here though
fn main() {
    #[cfg(panic = "unwind")]
    println!("unwind");

    #[cfg(panic = "abort")]
    println!("abort");
}

RUSTFLAGS='-C panic=abort' cargo run --release

[polarathene]

I reverted this back to what it was, as I think it's valid to enable for either condition vs only unwinding enabled on arm?

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Errors without this change

For x86_64 this change allowed the hello-world-small Eyra example to build successfully (if the nightly feature of origin was not enabled by c-scape):

• This was with eyra => c-gull => c-scape => origin crates all cloned locally where I removed the nightly feature from c-scape's origin feature list.
• I tried to reproduce with just origin but no luck. Presumably something eyra/c-scape was doing with origin and it's unwind modules here?

Without the change, these are the two kinds of failures:

With -C target-feature=+crt-static:

  = note: rust-lld: error: undefined symbol: _dl_find_object
          >>> referenced by unwind-dw2-fde-dip.o:(_Unwind_Find_FDE) in archive /usr/lib/gcc/x86_64-redhat-linux/14/libgcc_eh.a
          >>> referenced by unwind-dw2-fde-dip.o:(_Unwind_Find_FDE) in archive /usr/lib/gcc/x86_64-redhat-linux/14/libgcc_eh.a
          collect2: error: ld returned 1 exit status

Without -C target-feature=+crt-static, all the symbols that the module provides are listed as errors:

  = note: rust-lld: error: undefined symbol: _Unwind_Resume

...

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RUSTFLAGS='-C panic=abort' vs panic = "abort" (Cargo.toml)

If building with -C panic=abort (regardless of panic setting in Cargo.toml profile), with the nightly feature you'll get:

error[E0463]: can't find crate for `unwinding`
  --> /origin/src/lib.rs:39:1
   |
39 | extern crate unwinding;
   | ^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

That could be prevented by matching the unwinding dep cfg requirement for panic = "unwind", but you'd then also need the opposite for the mod unwind_unimplemented condition.

Not possible to use cfg with features yet: rust-lang/cargo#1197

[polarathene]

I figured given the context it was better to co-locate with the other unwinding related logic?

mod stubs was previously toggled by the nightly feature but has since lost that context with the switch to unwinding feature. Thus technically unwind_unimplemented and stubs are similar beyond the arm conditional.

Not sure how you want to approach this.

[sunfishcode]

Thanks!