This came up in https://github.com/rust-lang/rust/pull/127682: the build scripts [invokes RUSTC](https://github.com/rust-lang/libc/blob/0e28c864c25d2e9b0ab082947445efccef213da4/build.rs#L123) without wrapping it in RUSTC_WRAPPER. Bypassing wrappers isn't great, so the build script should be fixed. See https://github.com/cuviper/autocfg/pull/56, https://github.com/djc/rustc-version-rs/pull/46 for similar changes in other crates, and https://github.com/rust-lang/cargo/pull/13659 for the change in cargo.