Suggest correct place to add self parameter when inside closure

[blyxxyz]

It would incorrectly suggest adding it as a parameter to the closure instead of the containing function.

For example:

help: add a `self` receiver parameter to make the associated `fn` a method
  |
5 |         let _ = || self&self;
  |                        ^^^^^

DiagnosticMetadata.current_function is only used for these messages so tweaking its behavior should be ok.

[rust-highfive]

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[lcnr]

Thanks, LGTM

[lcnr]

cc @estebank

@bors r+ rollup

[bors]

📌 Commit e701ae3 has been approved by lcnr