`#[inline(always)]` and `-Zhint-mostly-unused` play not well with imported main

[usamoi]

I tried this code:

foo/main.rs:

use bar::main

bar/lib.rs:

pub fn main() {
    println!("Hello, world!");
}

RUSTFLAGS="-Zhint-mostly-unused" cargo +nighly run -p foo

I expected to see this happen: it compiles

Instead, this happened: it emits many linking errors

If #[inline(always)] is used, an error will occur even if this flag is not present.

cargo +stable run -p foo

linking errors

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "-m64" "/tmp/rustcs3Kiu4/symbols.o" "<5 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/usamoi/playground/easy/target/debug/deps/libbar-b6c95891f9ca7c1d.rlib" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/tmp/rustcs3Kiu4/raw-dylibs" "-B<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld" "-fuse-ld=lld" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "/usamoi/playground/easy/target/debug/deps/foo-8a2f36a70114e06d" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: rust-lld: error: undefined symbol: bar::main::h034083ec22b3bbff
          >>> referenced by 467h5aysby46ky3mqgnyk3nkw
          >>>               /usamoi/playground/easy/target/debug/deps/foo-8a2f36a70114e06d.467h5aysby46ky3mqgnyk3nkw.0gco3m2.rcgu.o:(main)
          collect2: error: ld returned 1 exit status
          

error: could not compile `foo` (bin "foo") due to 1 previous error

Meta

rustc +nightly --version --verbose:

rustc 1.90.0-nightly (5795086bd 2025-07-16)
binary: rustc
commit-hash: 5795086bdfe7ed988aa53a110bd0692c33d8755b
commit-date: 2025-07-16
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

rustc +stable --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

[theemathas]

Since it wasn't clear from the issue text, the following code also causes a linker error, even without the -Zhint-mostly-unused flag:

lib.rs:

#[inline(always)]
pub fn main() {}

main.rs:

use foo::main;  // replace "foo" with your crate's name

[Noratrieb]

Looks like we need a mono root for imported main the same way there is one for normal main. This makes perfect sense, but it's very funny that this has never come up, yay for all this new testing I guess?