Tracking Issue for RFC 3955: Named Fn trait parameters

[JonathanBrouwer]

This is a tracking issue for the RFC 3955: Named Fn trait parameters
The feature gate for the issue is #![feature(named_fn_trait_parameters)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC (claimed by @JonathanBrouwer)
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• list all the "unresolved questions" found in the RFC to ensure they are
  not forgotten.

Implementation history

• Improve diagnostic for patterns in function pointer types #160282
• Initial implementation of named Fn trait parameters #158517
• Store the names of Fn trait parameters in the AST, fix rustfmt bug #160937
• Implement semantic analysis for named Fn trait params #162634