How to allow optional trailing commas in macros?

by

Make the comma optional

As DK. points out, the trailing comma can be made optional.

Rust 1.32

You can use the ? macro repeater to write this and disallow multiple trailing commas:

($Name:ident { $($Variant:ident),* $(,)? }) => { 
//                                 ^^^^^

Previous versions

This allows multiple trailing commas:

($Name:ident { $($Variant:ident),* $(,)* }) => { 
//                                 ^^^^^

Handle both cases

Or you can handle both cases by… handling both cases:

macro_rules! define_enum {
    ($Name:ident { $($Variant:ident,)* }) => {
        pub enum $Name {
            None,
            $($Variant),*,
        }
    };
    ($Name:ident { $($Variant:ident),* }) => {
        define_enum!($Name { $($Variant,)* });
    };
}

define_enum!(Foo1 { A, B });
define_enum!(Foo2 { A, B, });

fn main() {}

We’ve moved the main implementation to the version that expects the trailing comma. We then added a second clause that matches the case with the missing comma and rewrites it to the version with a comma.

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