Rollup merge of #111612 - ChayimFriedman2:collect-into-slice-ref, r=p…

…etrochenkov

Give better error when collecting into `&[T]`

The detection of slice reference of `{integral}` in `rustc_on_unimplemented` is hacky, but a proper solution requires changing `FmtPrinter` to add a parameter to print integers as `{integral}` and I didn't want to change it just for `rustc_on_unimplemented`. I can do that if requested, though.

I'm open to better wording; this is the best I could come up with.