edition idioms: Trickiness with $crate in local macros

[alexcrichton]

Originally reported with Cargo this code:

#![warn(rust_2018_compatibility)]
extern crate serde;
#[macro_use]
extern crate serde_derive;

use serde::{Deserialize, Deserializer};

macro_rules! event {
    (
        pub struct $name:ident() {}
    ) => {
        #[derive(Deserialize)]
        pub struct $name {
            pub event_type: $crate::EventType,
        }
    }
}

pub enum EventType {}

event! {
    pub struct CustomEvent() {}
}

impl<'de> Deserialize<'de> for EventType {
    fn deserialize<D>(_deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        unimplemented!()
    }
}

gives an erroneous suggestion

which causes it to fail to be migrated by cargo fix

[alexcrichton]

@ehuss also notes that #56622 may have an impact on the severity of this

[ehuss]

This appears to be fixed now, presumably by #57155.

[alexcrichton]

Yay!