Compilation error when compiling with nightly with `unstable` feature enabled

[jqnatividad]

What version of regex are you using?

1.10.5

Describe the bug at a high level.

Compilation error when compiling with rust 1.82.0-nightly (7120fdac7 2024-07-25)

What are the steps to reproduce the behavior?

Compile a program that uses regex 1.10.5 with latest rust nightly with the unstable feature enabled.

What is the actual behavior?

Running `/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name regex --edition=2021 /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/regex-1.10.5/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --cfg 'feature="default"' --cfg 'feature="pattern"' --cfg 'feature="perf"' --cfg 'feature="perf-backtrack"' --cfg 'feature="perf-cache"' --cfg 'feature="perf-dfa"' --cfg 'feature="perf-inline"' --cfg 'feature="perf-literal"' --cfg 'feature="perf-onepass"' --cfg 'feature="std"' --cfg 'feature="unicode"' --cfg 'feature="unicode-age"' --cfg 'feature="unicode-bool"' --cfg 'feature="unicode-case"' --cfg 'feature="unicode-gencat"' --cfg 'feature="unicode-perl"' --cfg 'feature="unicode-script"' --cfg 'feature="unicode-segment"' --cfg 'feature="unstable"' --check-cfg 'cfg(docsrs)' --check-cfg 'cfg(feature, values("default", "logging", "pattern", "perf", "perf-backtrack", "perf-cache", "perf-dfa", "perf-dfa-full", "perf-inline", "perf-literal", "perf-onepass", "std", "unicode", "unicode-age", "unicode-bool", "unicode-case", "unicode-gencat", "unicode-perl", "unicode-script", "unicode-segment", "unstable", "use_std"))' -C metadata=e15ac38c36a6cd7f -C extra-filename=-e15ac38c36a6cd7f --out-dir /home/runner/work/qsv/qsv/target/debug/deps -L dependency=/home/runner/work/qsv/qsv/target/debug/deps --extern aho_corasick=/home/runner/work/qsv/qsv/target/debug/deps/libaho_corasick-417a2760a2457731.rmeta --extern memchr=/home/runner/work/qsv/qsv/target/debug/deps/libmemchr-c7ff0286392b94eb.rmeta --extern regex_automata=/home/runner/work/qsv/qsv/target/debug/deps/libregex_automata-213236c3a36ea036.rmeta --extern regex_syntax=/home/runner/work/qsv/qsv/target/debug/deps/libregex_syntax-c1e3dd509c68fbb0.rmeta --cap-lints allow -C target-cpu=native`
error[E0107]: trait takes 0 lifetime arguments but 1 lifetime argument was supplied
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/regex-1.10.5/src/pattern.rs:13:14
    |
13  | impl<'r, 't> Pattern<'t> for &'r Regex {
    |              ^^^^^^^---- help: remove the unnecessary generics
    |              |
    |              expected 0 lifetime arguments
    |
note: trait defined here, with 0 lifetime parameters
   --> /home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/str/pattern.rs:100:11
    |
100 | pub trait Pattern: Sized {
    |           ^^^^^^^

error[E0207]: the lifetime parameter `'t` is not constrained by the impl trait, self type, or predicates
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/regex-1.10.5/src/pattern.rs:13:10
   |
13 | impl<'r, 't> Pattern<'t> for &'r Regex {
   |          ^^ unconstrained lifetime parameter

What is the expected behavior?

Compile with nightly with unstable feature enabled.

[BurntSushi]

That's interesting. I didn't know about a change to that trait. Patches are welcome, otherwise I'll do it soon.

But I generally wouldn't recommend relying on or even using unstable APIs. If I could go back in time, I wouldn't even offer Pattern trait integration in this crate. But it's been there for a long time and it isn't that costly to maintain.

[jqnatividad]

Thanks for the quick response!

I'm not actually using Pattern, I enabled unstable with nightly primarily because I wanted SIMD acceleration and there's an old changelog entry about the simd feature being dropped in lieu of unstable:

regex/CHANGELOG.md

Lines 1224 to 1230 in 8856fe3

	0.2.9 (2018-03-12)
	==================
	This release introduces a new nightly only feature, `unstable`, which enables
	SIMD optimizations for certain types of regexes. No additional compile time
	options are necessary, and the regex crate will automatically choose the
	best CPU features at run time. As a result, the `simd` (nightly only) crate
	dependency has been dropped.

And since pattern is enabled by the unstable feature, we get this error.

If I'm just interested in tapping regex's SIMD optimizations, how does one do it?

[BurntSushi]

If I'm just interested in tapping regex's SIMD optimizations, how does one do it?

You don't. It just happens automatically. There is nothing that unstable enables other than the Pattern trait. Search for "simd" in the CHANGELOG. In the regex 1.0.1 release on 2018-06-19, SIMD was enabled automatically on Rust 1.27+ stable. You are six years behind the eight ball. :P

Unlike some other crates in the ecosystem, the regex crate adopted the stable SIMD API as soon as it was released, many years ago. Other crates still rely on the std::simd API which is not stable (but is platform independent).

[jqnatividad]

LOL! I missed that entry!

Thanks @BurntSushi !