Tracking Issue for warning on unused lifetimes (by default)

[Mark-Simulacrum]

This issue is tracking the transition of the unused_lifetimes lint to warn-by-default (or higher).

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.

Steps

Several concerns were raised as blocking in #92386:

• Permit underscores to silence the lint
• Avoid warning on semver-exposed lifetimes

Additionally, a plausible half step was suggested that enabling the lint for impl lifetime declarations would likely have no semver compatibility concerns and so be feasible to land more quickly.

Known bugs (list may be incomplete)

• unused_lifetimes warning misses unused lifetime #74165
• unused-lifetimes lint duplicates "parameter is never used" error #72587
• unused_lifetimes lint has false positives with GATs #94307

Prior "tracking" issue: #41960