Cannot call a function-like procedural macro: cannot be expanded to statements

Question

I'm trying to get my head around function-like procedural macros and struggling with the basics.

For a starter, I tried to create a macro that just prints all the tokens and does nothing:

extern crate proc_macro;
extern crate syn;
use proc_macro::TokenStream;

#[proc_macro]
pub fn my_macro(input: TokenStream) -> TokenStream {
    println!( "{:?}", input );

    TokenStream::new()
}

I then tried to use it in another binary:

extern crate rust_procmacro_test;

fn main() {
    rust_procmacro_test::my_macro!( aaa );
}

When I do cargo build I get this:

Compiling rust_procmacro_test v0.1.0 
Compiling procmacro_user v0.1.0 
error[E0658]: procedural macros cannot be expanded to statements (see issue #54727)
 --> src\main.rs:5:5
  |
5 |     rust_procmacro_test::my_macro!( aaa );
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

TokenStream [Ident { ident: "aaa", span: #0 bytes(114..117) }]
error: aborting due to previous error

It tells me that this cannot be compiled, but println inside the macro works. What is happening?

Issue #54727 is very broad and I'm not sure how it relates to this.

I'm using stable-i686-pc-windows-gnu, rustc 1.31.1 (b6c32da9b 2018-12-18).

Quick question: Why do you develop for `i686`? No one should do that anymore... It is superseded by `x86_64`. — hellow, Jan 14 '19 at 07:09
@hellow because there is no 64-bit gcc compiler for windows and so there is no x86_64-pc-windows-gnu target (that I'm aware of) and I had difficulties debugging with x86_64-windows-msvc. — Amomum, Jan 14 '19 at 10:02
`rustup target list` will give you `x86_64-pc-windows-gnu` and `x86_64-pc-windows-msvc` :) [Here's](https://stackoverflow.com/questions/50707118/how-to-debug-rust-unit-tests-on-windows) a good question about visual debugging rust — hellow, Jan 14 '19 at 10:32
@hellow hmm, I guess you're right, my knowledge is outdated :) I'll look into that. — Amomum, Jan 14 '19 at 11:30

score 4 · Accepted Answer · answered Jan 14 '19 at 01:06

What is happening?

While it's possible to define a function-like procedural macro in stable Rust, it's not yet possible to call one as an expression / statement. This is because there are concerns about how hygiene should be applied. There are other locations, such as in item position, where it is available:

extern crate rust_procmacro_test;

rust_procmacro_test::my_macro!( aaa );

fn main() {}

but println inside the macro works

This is probably just some laziness in the implementation — the macro is eagerly evaluated and then the feature gate is checked later. While this might be inefficient at the moment, once the feature is fully stabilized it won't matter.

Oh, that's unfortunate. Thank you! – Amomum Jan 14 '19 at 10:08 — Amomum, Jan 14 '19 at 10:08

Cannot call a function-like procedural macro: cannot be expanded to statements

1 Answers1