false positive `unused-extern-crates`

[matthiaskrgr]

Using the following flags

--force-warn unused-extern-crates

this code:

// No error, panic handler is supplied by libstd linked though the empty library.

//@ check-pass
//@ aux-build: cfg_false_lib.rs

#![no_std]

extern crate proc_macro;

fn main() {
    panic!()
}

caused the following diagnostics:

    Checking _5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40 v0.1.0 (/tmp/icemaker_global_tempdir.ZzRBF4urpxex/icemaker_clippyfix_tempdir.aeQgi7GJI5WY/_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40)
warning: unused extern crate
 --> src/main.rs:8:1
  |
8 | extern crate proc_macro;
  | ^^^^^^^^^^^^^^^^^^^^^^^^ unused
  |
  = note: requested on the command line with `--force-warn unused-extern-crates`
help: remove the unused `extern crate`
  |
8 - extern crate proc_macro;
  |

warning: `_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40` (bin "_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40") generated 1 warning (run `cargo clippy --fix --bin "_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40" -p _5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.21s

However after applying these diagnostics, the resulting code:

// No error, panic handler is supplied by libstd linked though the empty library.

//@ check-pass
//@ aux-build: cfg_false_lib.rs

#![no_std]


fn main() {
    panic!()
}

no longer compiled:

    Checking _5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40 v0.1.0 (/tmp/icemaker_global_tempdir.ZzRBF4urpxex/icemaker_clippyfix_tempdir.aeQgi7GJI5WY/_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40)
error: `#[panic_handler]` function required, but not found

error: unwinding panics are not supported without std
  |
  = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
  = note: since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem

error: could not compile `_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40` (bin "_5ba66f8e62a4839c0ab39d6e4a771d7ada6b5b40") due to 2 previous errors
warning: build failed, waiting for other jobs to finish...

Version:

rustc 1.93.0-nightly (a8664a153 2025-10-26)
binary: rustc
commit-hash: a8664a1534913ccff491937ec2dc7ec5d973c2bd
commit-date: 2025-10-26
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.3

[osamakader]

I tried this change to compiler/rustc_resolve/src/check_unused.rs, so the above code issue is fixed, but it doesn't give a warning if the proc_macro is not really used.

 use rustc_ast as ast;
-use rustc_ast::visit::{self, Visitor};
+use rustc_ast::{attr, visit::{self, Visitor}};
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::MultiSpan;

     pub(crate) fn check_unused(&mut self, krate: &ast::Crate) {
         let tcx = self.tcx;
         let mut maybe_unused_extern_crates = FxHashMap::default();
+        let is_no_std = attr::contains_name(&krate.attrs, rustc_span::sym::no_std);
 
         for import in self.potentially_unused_imports.iter() {
             match import.kind {

                 ImportKind::ExternCrate { id, .. } => {
                     let def_id = self.local_def_id(id);
                     if self.extern_crate_map.get(&def_id).is_none_or(|&cnum| {
+                        let is_proc_macro_in_no_std =
+                            is_no_std && tcx.crate_name(cnum) == rustc_span::sym::proc_macro;
+
                         !tcx.is_compiler_builtins(cnum)
                             && !tcx.is_panic_runtime(cnum)
                             && !tcx.has_global_allocator(cnum)
                             && !tcx.has_panic_handler(cnum)
+                            && !is_proc_macro_in_no_std
                     }) {
                         maybe_unused_extern_crates.insert(id, import.span);
                     }