Fix rustc --unpretty=expanded emit of [const] [unsafe] impl Trait for

[Arnavion]

Fixes #158599

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[Arnavion]

FYI this is my first contribution to this repo. I've done x build library and x test per https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html , and used the stage1 rustc to test the repro in the linked issue as well as my real code that triggered this.