This is a tracking issue for constify-ing non-trait Duration methods

The feature gates for the issueis #![feature(duration_consts_float)]. The methods covered by that are:

• Duration::as_secs_f64
• Duration::as_secs_f32
• Duration::as_millis_f64
• Duration::as_millis_f32
• Duration::div_duration_f64
• Duration::div_duration_f32

Steps

• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history

• Constify most non-trait Duration methods as described: Constify most non-trait Duration methods as described in #72440 #72481
• Make all methods of Duration unstably const: Make all methods of Duration unstably const #76335
• Partially stabilize duration_consts_2: Partially stabilize duration_consts_2 #89542