Cargo will lose inter-process file locking on Solaris on with 1.98-nightly

[weihanglo]

Problem

Cargo will lose inter-process file locking on Solaris starting with 1.98-nightly tomorrow.

std::fs::File::lock and friends were emulated on Solaris via fcntl from 1.91 (rust-lang/rust#146269) but
removed in 1.98 (rust-lang/rust#157509) because the emulation is process-scoped instead of handle-scoped. It
cannot honor the documented semantics of File::lock. On Solaris these will return ErrorKind::Unsupported.

Solaris had working cargo locking from 2022-12 (#11439) through 1.97. This is a regression on a tier-2 target.

Possible Solution(s)

1. Accept the regression and treat Solaris like other Unixes without flock(2). Cargo will silently lock nothing on Solaris.
2. Re-add a Solaris-only fcntl shim in Cargo, with the caveat that it is process-scoped and every call site must avoid opening the same lock path twice in one process.

Notes

• File::lock (probably) behaves incorrectly on Solaris rust#157390
• remove solaris implementation for File::lock, it has the wrong semantics rust#157509
• feat(std): emulate flock for solaris via fcntl rust#146269
• refactor: replace flock with std flock #15935
• Implements cargo file locking using fcntl on Solaris. #11439

Version

The upcoming nightly (2026-06-07 I guess)