Closed
Description
Given a proc-macro crate named demo_macros, which contains a single nop_attribute procedural macro:
#![feature(proc_macro)]
extern crate proc_macro;
use proc_macro::TokenStream;
#[proc_macro_attribute]
pub fn nop_attribute(_: TokenStream, input: TokenStream) -> TokenStream {
input
}Given a lib crate named demo, which attempts to use the nop_attribute procedural macro on a function within an extern block:
#![feature(proc_macro)]
extern crate demo_macros;
use demo_macros::nop_attribute;
extern {
#[nop_attribute]
pub fn fabs(_: f64) -> f64;
}Attempting to cargo +nightly build this results in the following error:
Compiling demo_macros v0.1.0 (file:///private/tmp/demo/demo_macros)
Compiling demo v0.1.0 (file:///private/tmp/demo)
error[E0658]: The attribute `nop_attribute` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
--> src/lib.rs:6:3
|
6 | #[nop_attribute]
| ^^^^^^^^^^^^^^^^
|
= help: add #![feature(custom_attribute)] to the crate attributes to enable
error: aborting due to previous error
If you want more information on this error, try using "rustc --explain E0658"
error: Could not compile `demo`.
To learn more, run the command again with --verbose
If I delete the extern block and change the code to just be #[nop_attribute] pub fn fabs(_: f64) -> f64 { 0.0 } then it compiles just fine. For some reason, attempting to apply a procedural macro attribute to an extern function doesn't work.
My rustc --version:
rustc 1.26.0-nightly (e026b59cf 2018-03-03)
You can fully reproduce the issue by executing the following sequence of shell commands:
$ cargo +nightly new demo_macros
$ cat >> demo_macros/Cargo.toml << EOF
[lib]
proc-macro = true
EOF
$ cat >| demo_macros/src/lib.rs << EOF
#![feature(proc_macro)]
extern crate proc_macro;
use proc_macro::TokenStream;
#[proc_macro_attribute]
pub fn nop_attribute(_: TokenStream, input: TokenStream) -> TokenStream {
input
}
EOF
$ cargo +nightly init .
$ echo 'demo_macros = { path = "demo_macros" }' >> Cargo.toml
$ cat >| src/lib.rs << EOF
#![feature(proc_macro)]
extern crate demo_macros;
use demo_macros::nop_attribute;
extern {
#[nop_attribute]
pub fn fabs(_: f64) -> f64;
}
EOF
$ cargo +nightly build