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Merged
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Sep 26, 2025
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Switch to new panic=immediate-abort flag #86

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2 changes: 1 addition & 1 deletion .github/workflows/build.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -76,4 +76,4 @@ jobs:
run: >
rustup component add rust-src;
RUSTFLAGS="-Zlocation-detail=none -Zfmt-debug=none" cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features="optimize_for_size" --target x86_64-unknown-linux-gnu --release;
cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort --target x86_64-unknown-linux-gnu --release;
RUSTFLAGS="-Zunstable-options -Cpanic=immediate-abort" cargo +nightly build -Z build-std=std,panic_abort --target x86_64-unknown-linux-gnu --release;
14 changes: 7 additions & 7 deletions README.md
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Expand Up @@ -220,21 +220,21 @@ optimized for binary size. More information about it can be found in the

On macOS, the final stripped binary size is reduced to 51KB.

# Remove `panic` String Formatting with `panic_immediate_abort`
# Remove `panic` String Formatting with `panic=immediate-abort`

![Minimum Rust: Nightly](https://img.shields.io/badge/Minimum%20Rust%20Version-nightly-orange.svg)

Even if `panic = "abort"` is specified in `Cargo.toml`, `rustc` will still include panic strings
and formatting code in final binary by default.
[An unstable `panic_immediate_abort` feature](https://github.com/rust-lang/rust/pull/55011)
[An unstable `panic=immediate-abort` feature](https://github.com/rust-lang/rust/pull/146317)
has been merged into the `nightly` `rustc` compiler to address this.

To use this, repeat the instructions above to use `build-std`, but also pass the following
[`-Z build-std-features=panic_immediate_abort`](https://doc.rust-lang.org/cargo/reference/unstable.html#build-std-features)
option.
To use this, repeat the instructions above to use `build-std`, but also pass
[`-Zunstable-options -Cpanic=immediate-abort`](https://doc.rust-lang.org/rustc/command-line-arguments.html#-z-set-unstable-options)
to `rustc`.

```bash
$ cargo +nightly build -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort \
$ RUSTFLAGS="-Zunstable-options -Cpanic=immediate-abort" cargo +nightly build -Z build-std=std,panic_abort \
--target x86_64-apple-darwin --release
```

Expand All @@ -252,7 +252,7 @@ we will restrict our usage of `libstd` in order to reduce binary size further.

If you want an executable smaller than 20 kilobytes, Rust's string formatting code,
[`core::fmt`](https://doc.rust-lang.org/core/fmt/index.html) must
be removed. `panic_immediate_abort` only removes some usages of this code. There is a lot of other
be removed. `panic=immediate-abort` only removes some usages of this code. There is a lot of other
code that uses formatting in some cases. That includes Rust's "pre-main" code in `libstd`.

By using a C entry point (by adding the `#![no_main]` attribute) , managing stdio manually, and
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