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A relevant old issue - #63221.

🎉 Experiment pr-158816 is completed!
📊 15145 regressed and 5 fixed (1012225 total)
📊 5239 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

There aren't t that many root regressions actually, there are just some old versions of big ecosystem crates that mess this up. This isn't exhaustive:

#[repr]

used on macro that expands on enum, doesnt look like anyone is actually relying on layout https://crates.io/crates/dmap/0.2.1/code/src/lib.rs#L20
used on bitflags macro, also declared as u8 repr inside macro itself JeromeHen/max30101-rs#1

#[target_feature]

v_escape zzau13/v_escape#182

#[non_exhaustive]

crossterm https://github.com/crossterm-rs/crossterm/blob/9033a483f80a9e549744cb9851ae3815abb2fa5a/src/style/types/attribute.rs#L91

#[inline]

many old versions of crates like itertools and hashbrown etc do e.g.
https://github.com/rust-lang/hashbrown/blob/34c11891e13fa3c0d08b0540e869aace9d347c26/src/raw/mod.rs#L1250-L1257
where

#[cfg(feature = "nightly")]
macro_rules! default_fn {
	(#[$($a:tt)*] $($tt:tt)*) => {
        #[$($a)*] default $($tt)*
    }
}
#[cfg(not(feature = "nightly"))]
macro_rules! default_fn {
	($($tt:tt)*) => {
        $($tt)*
    }
}

https://docs.rs/af_packet/latest/src/af_packet/tpacket3.rs.html#110
https://github.com/qwertz19281/rust_utils/blob/833f581b5e63117fb87a7dad9f41a6532113fd88/src/static_stor/mod.rs#L30

#[deprecated]

raw-cpuid https://github.com/gz/rust-cpuid/blob/6554bb914214934b15cccb9d8be4d0f71ac69232/src/lib.rs#L2510
gtk https://github.com/gtk-rs/gtk/blob/d6233591b3994cee74efe90ccf3e2dd76693c941/src/auto/constants.rs#L159-L162
trust-dns-resolver https://crates.io/crates/trust-dns-resolver/0.18.0-alpha.2/code/src/async_resolver/mod.rs#L326

#[automatically_derived]

Applied to inventory's submit!:
cucumber https://github.com/cucumber-rs/cucumber/blob/d95872541ee2a43d71089c64bd3fa8848e24c922/codegen/src/attribute.rs#L126
pyo3-stub-gen https://github.com/Jij-Inc/pyo3-stub-gen/blob/087ecfbe87758549efe94eb569cc847b28600a0d/pyo3-stub-gen-derive/src/gen_stub.rs#L190

https://github.com/vtio-rs/vtio/blob/49e0aee188bb05599346782e07b069406a65ef2b/crates/vtansi_derive/src/macros/ansi_control.rs#L166

#[cold]

Applied to panic! in generator https://github.com/Xudong-Huang/generator-rs/blob/9ede1477f615787e8e214c40a685363efd52da92/src/yield_.rs#L50

#[must_use]

Applied over bitflags! in naga https://docs.rs/naga/0.5.0/src/naga/valid/mod.rs.html#50

#[macro_use]

Used on cfg_if invocation https://crates.io/crates/xpsupport/0.2.2/code/src/lib.rs#L60

#[no_std]

Mistakenly used as outer attr and applied to first item (happens to be macro invoc) in crate root in trunkver https://github.com/urothis/trunkver/blob/main/src/lib.rs

#[ignore]

https://github.com/ajzaff/substrate-mfm-engine/blob/63222eb29832f04118f8cdc72a11fbb05f2643f5/src/code.rs#L15

#[should_panic]

So can one divide by zero or not? https://github.com/JesterOrNot/MathExpression/blob/master/tests/test.rs#L24-L33
https://github.com/conwayste/netwayste/blob/986b6eb3f09744651524a628ec74dbb47d7063f1/src/server.rs#L1471

So the big crates that make this mistake are (old versions of?) cgmath, crossterm, gtk, hashbrown, itertools, ndarray, raw-cpuid, rkyv, trust-dns-resolver, v_htmlescape.

That's a lot of breakage, probably too much to merge this, but also that so many big crates are making this mistake is worrying