Closed
Description
Reproduction
lib.rs
macro_rules! helper {
($expr:expr) => {
#[cfg(feature = $expr)]
fn foo() {}
}
}
helper!{concat!("thing")}This should be an error, as concat!("thing") is not allowed in this position. But instead, the cfg attribute in this example is silently ignored, and fn foo gets compiled.
I tested also with The previous statement was an overgeneralization. When used on #[doc], hence why the title claims that this applies to "attributes with 'name = $expr'" in general.#[doc], this technique apparently sometimes even works!
This is a regression from stable 1.17 to 1.18.
Behavior in various versions
- rustc
1.0–1.17: parse error (expected behavior) - rustc
1.18–1.22: ICE - rustc
1.23–nightly 1.30: silently ignored! 🙈
Output in 1.0–1.17 (expected)
src/lib.rs:3:25: 3:30 error: unexpected token: `concat!("thing")`
src/lib.rs:3 #[cfg(feature = $expr)]
^~~~~
Output in 1.18–1.22 (ICE)
$ RUST_BACKTRACE=1 cargo +1.18.0 build --example=macro-concat
Compiling concat v0.1.0 (file:///home/lampam/cpp/throwaway/concat)
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'no entry found for key', /checkout/src/libcore/option.rs:794
stack backtrace:
0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
...
9: core::option::expect_failed
10: rustc_resolve::macros::<impl rustc_resolve::Resolver<'a>>::resolve_macro_to_def
11: rustc_resolve::macros::<impl syntax::ext::base::Resolver for rustc_resolve::Resolver<'a>>::resolve_invoc
12: syntax::ext::expand::MacroExpander::expand
13: syntax::ext::expand::MacroExpander::expand_crate
14: rustc_driver::driver::phase_2_configure_and_expand::{{closure}}
15: rustc_driver::driver::phase_2_configure_and_expand
16: rustc_driver::driver::compile_input
17: rustc_driver::run_compiler
18: std::panicking::try::do_call
19: __rust_maybe_catch_panic
at /checkout/src/libpanic_unwind/lib.rs:98
20: <F as alloc::boxed::FnBox<A>>::call_box
21: std::sys::imp::thread::Thread::new::thread_start
at /checkout/src/liballoc/boxed.rs:650
at /checkout/src/libstd/sys_common/thread.rs:21
at /checkout/src/libstd/sys/unix/thread.rs:84
22: start_thread
23: clone
Output in 1.23–nightly 1.30 (silently ignored)
Compiling concat v0.1.0 (file:///home/lampam/cpp/throwaway/concat)
warning: function is never used: `foo`
--> examples/macro-concat.rs:4:9
|
4 | fn foo() {}
| ^^^^^^^^
...
8 | helper!{concat!("thing")}
| ------------------------- in this macro invocation
|
= note: #[warn(dead_code)] on by default