Incorrect span / broken rustfix: help: use `dyn`: `dyn #[dom_struct]`

[SimonSapin]

One of the many new (or newly enabled by default) warnings in Servo in today’s Nightly:

warning: trait objects without an explicit `dyn` are deprecated
   --> components/script/dom/window.rs:169:1
    |
169 | #[dom_struct]
    | ^^^^^^^^^^^^^ help: use `dyn`: `dyn #[dom_struct]`

The suggestion is wrong, which means cargo fix also fails and therefore does not apply fixes for (the many) other warnings where the suggestion is correct.

warning: failed to automatically apply fixes suggested by rustc to crate `script`

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

  * components/script/dom/dedicatedworkerglobalscope.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/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error: expected item, found keyword `dyn`
   --> components/script/dom/dedicatedworkerglobalscope.rs:164:1
    |
164 | dyn #[dom_struct]
    | ^^^ expected item

error: aborting due to previous error

Original diagnostics will follow.

[Centril]

Triage: Preliminarily assigning P-high before Felix reviews this.

[Centril]

Also cc @pnkfelix since iirc you did some macro related work here.

[SimonSapin]

Possible solution: when a warning’s span is exactly a macro invocation, don’t emit a “suggested fix” that is likely wrong. (But still emit a warning.)

[estebank]

We have that check for most suggestions, but it might be missing for dyn. Was this in latest nightly? I know a couple of these have been fixed in the past few weeks.

[SimonSapin]

This is in rustc 1.37.0-nightly (04a3dd8 2019-06-18).

#![allow(bare_trait_objects)] works around this and allows rustfix’ing the rest of the warnings (namely old syntax for inclusive ranges) or warnings in other crates.

[SimonSapin]

Oh. I just noticed the --broken-code suggestion. I’d missed it in that blob of text.