Experiment: do the new nightly rustc flags replace lib-patcher?

[claude]

Requested by Tobias Gerber · Slack thread

Two nightly rustc flags recently landed that look like they do lib-patcher's job inside the compiler. This PR is the empirical proof + CI to see whether they resolve the original problem (rust#104707) — it does not touch lib-patcher's own code.

Before / After

Before — link two Rust C staticlibs that each embed std into one Rust binary and it collides:

rust-lld: error: duplicate symbol: rust_eh_personality
rust-lld: error: duplicate symbol: std::panicking::EMPTY_PANIC
rust-lld: error: duplicate symbol: std::sys::args::unix::imp::ARGV_INIT_ARRAY

...and each archive leaks its internals as externally-visible (GLOBAL DEFAULT) symbols — the namespace pollution lib-patcher exists to clean up.

After — building both staticlibs with the two nightly flags, the exact same setup links cleanly and runs:

proda_add(10, 5) = 15 ok
proda_roundtrip(42) = 42 ok
prodb_mul(6, 7) = 42 ok
prodb_roundtrip(99) = 99 ok
All tests passed!

The internals are now GLOBAL HIDDEN and renamed with a distinct per-crate suffix, while the public C API is untouched:

rust_eh_personality.rsa59ee01d1713d366   (producer-a)   GLOBAL HIDDEN
rust_eh_personality.rs95a5502f661d55cc   (producer-b)   GLOBAL HIDDEN
proda_add / prodb_mul                                   GLOBAL DEFAULT  (unrenamed)

Isolating the roles: hide-only still fails the link (STV_HIDDEN alone does not deduplicate two definitions); rename-only links (the per-crate rename is what removes the two-staticlib clash). So on ELF you need rename for the clash and hide for the exported-surface hygiene.

How

• -Zstaticlib-hide-internal-symbols (rust#155338) — sets ELF STV_HIDDEN / Mach-O equivalent on every defined symbol not in rustc's C-ABI export set.
• -Zstaticlib-rename-internal-symbols (rust#156950) — suffixes those non-exported symbols per-crate so two staticlibs don't clash.

New crates under tests/native-hiding/ (producer-a, producer-b, consumer) reproduce the problem: two staticlibs (C API + std + serde_json, built with LTO so std folds into the crate object) linked into one Rust bin. tests/native-hiding/run.sh builds without and with the flags and asserts the before/after; .github/workflows/native-symbol-hiding.yml runs it on ubuntu-latest and macos-latest with nightly.

Verified locally on nightly 1.99.0 (af3d95584 2026-07-09) — both flags present and working on ELF/Linux. On macOS (Apple) ld64 is first-definition-wins so the unpatched clash can't be reproduced (as the repo's own matrix already documents); there the workflow only asserts the flagged build links and runs.

Verdict

The flags natively cover lib-patcher's core job on ELF and Apple: rename removes the two-staticlib duplicate-symbol clash, hide keeps internal + dependency symbols out of the exported surface. They do not yet replace:

• Windows / COFF — the flags warn and no-op there; lib-patcher still renames COFF symbols itself.
• Stable toolchains — both flags are nightly-only.
• Arbitrary --keep-prefix — the flags key off rustc's own C-ABI export set, not a user-chosen prefix, and only work on sources you compile (not a vendored third-party .a).

So lib-patcher can be simplified (lean on the compiler where the flags apply) but not retired. This PR is the proof + CI; it deliberately does not remove any lib-patcher code.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KLEWk521QTtCbR2UzYq6du