Tracking issue for DebugTuple::formatter

[alexcrichton]

This is a tracking issue for the unstable feature debug_builder_formatter in the standard library. This method was add relatively recently, and my thoughts on it are:

• It seems odd to have this function only on DebugTuple and no other builders.
• It does seem reasonable to allow reading out the formatter for flags and such in various situations perhaps.

cc @sfackler

[sfackler]

This exists due to some sketchiness in Debug impls for tuples. We should hide it behind privacy rather than a feature gate.

[alexcrichton]

🔔 This issue is now entering its final comment period for deprecation 🔔

@sfackler has claimed the fix for this :)

[alexcrichton]

The libs team discussed this in triage recently and the decision was to deprecate