stabilize duration_consts_float

[RalfJung]

Waiting for FCP in #72440 to pass.

as_millis_f32 and as_millis_f64 are not stable at all yet, so I moved their const-stability together with their regular stability (tracked at #122451).

Fixes #72440

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[bors]

☔ The latest upstream changes (presumably #131288) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

FCP passed, so @cuviper this should be ready to land. :)

I'll resolve the conflicts.

[tgross35]

This PR const stabilizes the following functions, which are already stable:

impl Duration {
    pub const fn as_secs_f64(&self) -> f64;
    pub const fn as_secs_f32(&self) -> f32;
    pub const fn div_duration_f64(self, rhs: Duration) -> f64;
    pub const fn div_duration_f32(self, rhs: Duration) -> f32;
}

These just rely on const_fn_floating_point_arithmetic which was merged recently. FCP completed at #72440 (comment).

The remaining functions are still not stable so the const gate was changed to duration_consts_float:

impl Duration {
    pub const fn as_millis_f64(&self) -> f64;
    pub const fn as_millis_f32(&self) -> f32;
}

@bors r+

[bors]

📌 Commit 181e667 has been approved by tgross35

It is now in the queue for this repository.

[RalfJung]

The remaining functions are still not stable so the const gate was changed to duration_consts_float:

Correction, it was changed to duration_millis_float -- the gate that also governs calling them from non-const code.