Tracking Issue for `debug_closure_helpers`

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Feature gate: #![feature(debug_closure_helpers)]

This is a tracking issue for adding helpers to core::fmt that can use closures for formatting values.

ACP: https://github.com/rust-lang/libs-team/issues/288

Public API

// core::fmt

impl DebugList<'_, '_> {
    fn entry_with<F>(&mut self, entry_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
}

impl DebugMap<'_, '_> {
    fn key_with<F>(&mut self, key_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
    
    fn value_with<F>(&mut self, value_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
}

impl DebugSet<'_, '_> {
    fn entry_with<F>(&mut self, entry_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
}

impl DebugStruct<'_, '_> {
    fn field_with<F>(&mut self, name: &str, field_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
}

impl DebugTuple<'_, '_> { // also DebugSet
    fn field_with<F>(&mut self, field_fmt: F) -> &mut Self
    where
        F: FnOnce(&mut Formatter) -> fmt::Result;
}

Steps / History

• Implementation: #117730
• Moved FormatterFn / FromFn / from_fn to separate tracking issue: #146705
• Final comment period (FCP)^1
• Stabilization PR: #146099

Unresolved Questions

• Naming: Do any strong preferences exist regarding the new method names?
  • For now I've named them {orig_method}_with, for example DebugStruct::field_with() is like DebugStruct::field().
  • Resolved: The FormatterFn type was renamed to FromFn (and wrapped in from_fn(), the helper methods kept their original names.
• The DebugMap struct got key_with() and value_with(), but not entry_with() -- is it worth adding that?
  • Resolved to not add entry_with; see https://github.com/rust-lang/libs-team/issues/563#issuecomment-2760089572
• Should FormatterFn<F> place a trait bound on F so that call sites can be tidier?

Read the public API and the completed implementation referenced as #117730, then review stabilization PR #146099 and the resolved questions in this issue. Check the stabilization criteria and final comment period status; done means the API is approved and the stabilization work is merged.