regression: unsafe precondition(s) violated: slice::get_unchecked_mut requires that the index is within the slice

[Mark-Simulacrum]

[INFO] [stderr] error: failed to run custom build command for `libsodium-sys v0.2.4 (https://github.com/Dhole/sodiumoxidez?branch=extra#53c0fb16)`
[INFO] [stderr] note: To improve backtraces for build dependencies, set the CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG=true environment variable to enable debug information generation.
[INFO] [stderr] 
[INFO] [stderr] Caused by:
[INFO] [stderr]   process didn't exit successfully: `/opt/rustwide/target/debug/build/libsodium-sys-06e88a69c64e58f8/build-script-build` (signal: 6, SIGABRT: process abort signal)
[INFO] [stderr]   --- stdout
[INFO] [stderr]   cargo:rerun-if-env-changed=SODIUM_LIB_DIR
[INFO] [stderr]   cargo:rerun-if-env-changed=SODIUM_SHARED
[INFO] [stderr]   cargo:rerun-if-env-changed=SODIUM_USE_PKG_CONFIG
[INFO] [stderr]   cargo:rerun-if-env-changed=SODIUM_DISABLE_PIE
[INFO] [stderr] 
[INFO] [stderr]   --- stderr
[INFO] [stderr]   thread 'main' panicked at library/core/src/panicking.rs:156:5:
[INFO] [stderr]   unsafe precondition(s) violated: slice::get_unchecked_mut requires that the index is within the slice
[INFO] [stderr]   stack backtrace:

reverse-deps of libsodium-sys 0.2.5, pnet_packet 0.26:

• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/gh/Kuska-ssb.solar/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/gh/JJJollyjim.serenity-build-test/log.txt
• [... many more ...]

"unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed isize::MAX"

• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/gh/google.hat-backup/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/gh/ncsurobotics.SW8S-Rust/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.78-3/beta-2024-03-30/gh/tino-platform.tino/log.txt

[Mark-Simulacrum]

Most likely we will just close this as won't fix, but this has a lot of breakage -- probably ~300-400 crates in total across the Crater run (tests or build scripts). At minimum it'll be in release notes and possibly blog post as "hey, we're doing new checking"...

[Noratrieb]

@saethlin

[saethlin]

This is about what I expected 🤷

One of the biggest offenders is that we didn't have any documentation for the preconditions of slice::get_unchecked{_mut} at 1.0 so a lot of people who learned unsafe Rust around that era and didn't look back have published crates with increasingly dangerous code in them, as we did add documentation for what the precondition is (it should have raised a lot of eyebrows that we had unsafe code with no documented preconditions but I guess there wasn't such rigor back then) and last release we told LLVM about the UB we're checking for in slice::get_unchecked.

The PR that originally added these checks already has the relnotes label, it's #120594. The implementation has evolved a bit since that PR, but the observable behavior for most users is the same as in that PR.