Lint violation not caught when using macro_rules imported from another crate

Hello all,

I'm implementing a crate that has the purpose of wrapping some user code and make sure it is called in a certain context. I can summarize my workspace this way:

A "user_code" crate:
This contains a single "user" function, which I would have no control over. Notice that it returns a u32.

pub fn addition(a: u32, b: u32) -> u32 {
    a + b

A "wrapper" crate:
This contains a macro_rules. This macro creates some modules and a caller function, which will call the user code seen above. This is a highly simplified version. Notice that the return value of addition is ignored.

macro_rules! macro_wrapper {
    () => {
        pub mod my_user_code {
            use wrapper::*;
            use user_code::*;

            pub mod addition {
                use wrapper::*;
                use super::*;

                pub fn caller(a: u32, b: u32) {
                    addition(a, b);

An "example" crate:
Here I simply call the wrapper macro, in order to form the modules and functions I need and then use in the main function.


use wrapper::macro_wrapper;

macro_wrapper! {

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

This might seem like super strange code (it is) but it's a very simplified example of a legit project :slight_smile:
My goal now is to make sure that this code doesn't compile because the return value of addition is not used - it was forgotten. The lint #![deny(unused_results)] seems to achieve exactly what I want.

However, this works only if, instead of

use wrapper::macro_wrapper;

I have the macro directly defined in the In that case I get:

error: unused result of type `u32`
  --> example\src\
14 |                       addition(a, b);
   |                       ^^^^^^^^^^^^^^^
21 | / macro_wrapper! {
22 | |
23 | | }
   | |_- in this macro invocation
note: the lint level is defined here
  --> example\src\
1  | #![deny(unused_results)]
   |         ^^^^^^^^^^^^^^
   = note: this error originates in the macro `macro_wrapper` (in Nightly builds, run with -Z macro-backtrace for more info)

If the macro is defined in its crate "wrapper", the code compiles and runs just fine without any error. This is not the result I expect.

What am I missing here? Thank you in advance for reading!

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.