migration failure: -W keyword-idents misfires on dyn Trait in a macro expansion

[ExpHP]

This code is valid in both edition 2015 and 2018, but cargo fix --edition tries to fix it and fails.

Cargo.toml

[package]
name = "error"
version = "0.1.0"
edition = "2015"

[dependencies]

src/main.rs

pub trait PotentialBuilder: DynCloneDetail {}

pub trait DynCloneDetail {
    fn box_clone(&self) -> Box<dyn PotentialBuilder>;

    fn _as_ref_dyn(&self) -> &dyn PotentialBuilder;
}

#[macro_export]
macro_rules! impl_dyn_clone_detail {
    (impl[$($bnd:tt)*] DynCloneDetail for $Type:ty { ... }) => {
        impl<$($bnd)*> DynCloneDetail for $Type {
            fn box_clone(&self) -> Box<dyn PotentialBuilder> {
                Box::new(<$Type as Clone>::clone(self))
            }
            fn _as_ref_dyn(&self) -> &dyn PotentialBuilder { self }
        }
    };
}

#[derive(Clone)]
struct Pot;
impl PotentialBuilder for Pot {}
impl_dyn_clone_detail!{
    impl[] DynCloneDetail for Pot { ... }
}

fn main() {}

Command & output

$ cargo +nightly fix --edition
    Checking error v0.1.0 (/home/lampam/cpp/throwaway/error)
warning: failed to automatically apply fixes suggested by rustc to crate `error`

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see 
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/cargo/issues
quoting the full output of this command we'd be very appreciative!

warning: `dyn` is a keyword in the 2018 edition
warning: `dyn` is a keyword in the 2018 edition
  --> src/main.rs:13:40
   |
13 |             fn box_clone(&self) -> Box<dyn PotentialBuilder> {
   |                                        ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
  --> src/main.rs:13:40
   |
13 |             fn box_clone(&self) -> Box<dyn PotentialBuilder> {
   |                                        ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
   |
   |
   = note: `-W keyword-idents` implied by `-W rust-2018-compatibility`
   = note: `-W keyword-idents` implied by `-W rust-2018-compatibility`
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>

   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>

warning: `dyn` is a keyword in the 2018 edition
  --> src/main.rs:16:39
   |
16 |             fn _as_ref_dyn(&self) -> &dyn PotentialBuilder { self }
   |                                       ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
warning: `dyn` is a keyword in the 2018 edition
  --> src/main.rs:16:39
   |
16 |             fn _as_ref_dyn(&self) -> &dyn PotentialBuilder { self }
   |                                       ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
   |
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>


    Finished dev [unoptimized + debuginfo] target(s) in 0.31s

---

$ cargo +nightly --version
cargo 1.32.0-nightly (b3d0b2e54 2018-11-15)

[ExpHP]

It also fails on dyn Trait usage in the invocation, inside a type matched by :Ty:

pub trait PotentialBuilder: DynCloneDetail {}

pub trait DynCloneDetail {
    fn box_clone(&self) -> Box<dyn PotentialBuilder>;

    fn _as_ref_dyn(&self) -> &dyn PotentialBuilder;
}

#[macro_export]
macro_rules! impl_dyn_clone_detail {
    (impl[$($bnd:tt)*] DynCloneDetail for $Type:ty { ... }) => {
        impl<$($bnd)*> DynCloneDetail for $Type {
            fn box_clone(&self) -> Box<PotentialBuilder> {
                Box::new(<$Type as Clone>::clone(self))
            }
            fn _as_ref_dyn(&self) -> &PotentialBuilder { self }
        }
    };
}

impl PotentialBuilder for Box<dyn PotentialBuilder> {}
impl Clone for Box<dyn PotentialBuilder> {
    fn clone(&self) -> Self { self.box_clone() }
}
impl_dyn_clone_detail!{
    impl[] DynCloneDetail for Box<dyn PotentialBuilder> { ... }
}
fn main() {}

warning: `dyn` is a keyword in the 2018 edition
  --> src/main.rs:26:35
   |
26 |     impl[] DynCloneDetail for Box<dyn PotentialBuilder> { ... }
   |                                   ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`

[alexcrichton]

Thanks for the report! I've moved this over to rust-lang/rust#56327 as this is a bug in rustc, and we can track it from there