encounter an `invalid pointer` error on build script while using `-Zhint-mostly-unused` with `opt-level=N>0`

[TypeLevelConsoli]

When I enable -Zhint-mostly-unused and opt-level=N>0 in .cargo/config.toml on a project using utoipa-swagger-ui, the project fails to compile with a free(): invalid pointer while attempting to download the UI package during the build script.

Interestingly, if I remove opt-level or set it to 0, the project compiles normally. But when I set it to 1, 2, or 3, it fails with the same error (I have [profile.dev.package."*"] opt-level = 3 on ~/.cargo/config.toml as many Rust devs do).

If I remove -Zhint-mostly-unused, the project also compiles as expected.

The error only occurs when the two settings are enabled, as I described.

---

I tried this code:

A simple hello world with utoipa-swagger-ui as dep and a minimal .cargo/config.toml:

https://github.com/TypeLevelConsoli/mostly-unused-vs-swagger--cargo-error

I expected to see this happen:

compilation success

Instead, this happened:

I received an error while running the build script.

error: failed to run custom build command for `utoipa-swagger-ui v9.0.2`

Caused by:
  process didn't exit successfully: `/tmp/mostly-unused-vs-swagger--cargo-error/target/debug/build/utoipa-swagger-ui-85f8212ea4e4aaa6/build-script-build` (signal: 6, SIGABRT: process abort signal)
  --- stdout
  OUT_DIR: /tmp/mostly-unused-vs-swagger--cargo-error/target/debug/build/utoipa-swagger-ui-7a7fd1f5154fde51/out
  SWAGGER_UI_DOWNLOAD_URL: https://github.com/swagger-api/swagger-ui/archive/refs/tags/v5.17.14.zip
  cargo:rerun-if-env-changed=SWAGGER_UI_DOWNLOAD_URL
  start download to : "/tmp/mostly-unused-vs-swagger--cargo-error/target/debug/build/utoipa-swagger-ui-7a7fd1f5154fde51/out/v5.17.14.zip"
  reqwest feature: Err(NotPresent)
  trying to download using `curl` system package

  --- stderr
  free(): invalid pointer

Meta

$ rustc --version --verbose
rustc --version --verbose
rustc 1.90.0-nightly (ace633090 2025-07-23)
binary: rustc
commit-hash: ace633090349fc5075b5b0d56294de985e7d1191
commit-date: 2025-07-23
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

$ cargo --version --verbose
cargo 1.90.0-nightly (6833aa715 2025-07-13)
release: 1.90.0-nightly
commit-hash: 6833aa715d724437dc1247d0166afe314ab6854e
commit-date: 2025-07-13
host: x86_64-unknown-linux-gnu
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.14.1-DEV (sys:0.4.82+curl-8.14.1 vendored ssl:OpenSSL/3.5.0)
ssl: OpenSSL 3.5.0 8 Apr 2025
os: Fedora 41.0.0 [64-bit]

---

I first reported this on rust-lang/cargo#15644 (comment), and @saethlin asked to also create an issue here :)

[saethlin]

I've been able to reduce the example to this Cargo.toml:

[package]
name = "mostly-unused-vs-swagger"
version = "0.1.0"
edition = "2021"

[dependencies]
zip = { version = "4.0", default-features = false, features = ["deflate"] }

[profile.dev]
opt-level = 1

And this src/main.rs:

fn main() {
    let swagger_ui_zip = std::fs::File::open("test.zip").unwrap();
    let mut zip = zip::ZipArchive::new(swagger_ui_zip).expect("failed to open downloaded Swagger UI");
    zip.extract("/tmp/").unwrap();
}

Then run with

RUSTFLAGS="-Zhint-mostly-unused" cargo r

Somewhere along the way I ended up with a different crash, but I don't think that matters. The one this provokes is:

thread 'main' panicked at /home/ben/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zlib-rs-0.5.1/src/allocate.rs:127:5:
assertion failed: !opaque.is_null()

And the related code is here:
https://github.com/trifectatechfoundation/zlib-rs/blob/c2f1360765be8534af9a16152950a99063060886/zlib-rs/src/allocate.rs#L122-L133

#[cfg(feature = "rust-allocator")]
unsafe extern "C" fn zfree_rust(opaque: *mut c_void, ptr: *mut c_void) {
    if ptr.is_null() {
        return;
    }

    // we can't really do much else. Deallocating with an invalid layout is UB.
    debug_assert!(!opaque.is_null());

This really looks like allocator confusion, though I can't explain why delaying codegen would cause that.

[tmiasko]

This function pointer comparison looks dubious:

https://github.com/trifectatechfoundation/zlib-rs/blob/c2f1360765be8534af9a16152950a99063060886/zlib-rs/src/allocate.rs#L197

https://github.com/trifectatechfoundation/zlib-rs/blob/c2f1360765be8534af9a16152950a99063060886/zlib-rs/src/allocate.rs#L167-L172

https://github.com/trifectatechfoundation/zlib-rs/blob/c2f1360765be8534af9a16152950a99063060886/zlib-rs/src/allocate.rs#L1

EDIT: Replacing const with static to obtain consistent results of comparison resolves the issue.

[tmiasko]

I forwarded the issue to trifectatechfoundation/zlib-rs#401.