Function-like procedural macros in expression position as seen in 1.30 announcement are not stable yet

[dtolnay]

Only macros in statement position are stable in 1.30.

error[E0658]: procedural macros cannot be expanded to expressions (see issue #38356)                                           
 --> src/main.rs:5:15                                                                                                          
  |                                                                                                                            
5 |     let sql = sql!(SELECT * FROM posts WHERE id=1);                                                                        
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The stabilization hasn't landed in nightly yet either.

error[E0658]: procedural macros cannot be expanded to expressions (see issue #54727)                                           
 --> src/main.rs:5:15                                                                                                          
  |                                                                                                                            
5 |     let sql = sql!(SELECT * FROM posts WHERE id=1);                                                                        
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                         
  |                                                                                                                            
  = help: add #![feature(proc_macro_hygiene)] to the crate attributes to enable

Mentioning @steveklabnik.

[steveklabnik]

Whoops!

Any ideas on a decent example we could change it to?

[coder543]

Will this be stabilized for 1.31? Or is there an issue to track the status of proc macros in expression position?

[steveklabnik]

No, it's not clear when this will become stable.

As the error message says, rust-lang/rust#54727 is the tracking issue.

[dtolnay]

@steveklabnik the include_dir! macro from https://github.com/Michael-F-Bryan/include_dir would make a good example. It is currently implemented using proc-macro-hack and has some crates using it. Could now be written stably as an ordinary procedural macro.

[sgrif]

We're going to change the sql_function! macro in Diesel to a proc macro since parsing Rust function syntax + looking for special attributes in macro_rules! is not fun

[Ralith]

Any ideas on a decent example we could change it to?

Something that compiles a GLSL shader program, perhaps? e.g. vulkano has some really ugly hacks to accomplish this at present.

[rukai]

@Ralith Not anymore! :D vulkano-rs/vulkano#1062
Vulkano now uses the newly stabilised function-like macros, instead of hacking it via [#derive()]

[ubnt-intrepid]

I'm currently working on introducing a new helper macros in my project, based on #[proc_macro] , and found that the expression-style macros could be defined by doing as follows:

#[proc_macro]
pub fn sql_impl(input: TokenStream) -> TokenStream {
    let expr = derive_sql_expr(input);
    quote::quote!(
        fn output() -> impl sql::Sql {
            #output
        }
    ).into()
}

extern crate sql_impl;
#[doc(hidden)]
pub use sql_impl::*;

#[macro_export(local_inner_macros)]
macro_rules! sql {
    ($($t:tt)*) => {{
        struct __Dummy;
        impl __Dummy {
            sql_impl!($($t)*);
        }
        __Dummy::output()
    }}
}

Restrictions:

• The token streams cannot capture any identifiers contained in the caller's scope
  • let i=0; sql!(SELECT * FROM users WHERE id=i) should be rejected
• the type of generated expression must be written as a return type (or impl Trait)

[dtolnay]

Or use https://github.com/dtolnay/proc-macro-hack which lifts both of those restrictions.