Tracking issue for RFC#1685: Deprecate anonymous parameters

[aturon]

This is a tracking issue for the RFC "Deprecate anonymous parameters " (rust-lang/rfcs#1685).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)

Unresolved questions:

• The precise deprecation strategy

[est31]

I could work on this. From reading the RFC discussion, I think the "soft" deprecation strategy has been chosen? So on the compiler side, we'll need an allow by default (deprecation) lint for anonymous parameters, right?

[aturon]

@est31 Awesome! And yes, that's correct. This will be most effective if we add the lint to Clippy as well.

(The open question is around the criteria for moving to a warn-by-default strategy in the future.)

[est31]

About the clippy lint, I'll leave that to someone else. Given #41692, it should be mostly copy paste, but maybe with official rust claiming the lint name one has to chose another one. No idea.

[matklad]

IJR inspection: intellij-rust/intellij-rust#1211

I'll try to add an automatic fix to rustfmt as well. Not sure it's a great idea, but I want to try :)