std: add Duration::as_millis

[seanmonstar]

Following "do whatever is easiest", this was about the same amount of work to implement vs write up an RFC.

Motivation: there already exists a Duration::from_millis function, but no mirror Duration::as_millis. Milliseconds are commonly desired in programming, and the lack of as_millis feels unbalanced.

[rust-highfive]

r? @aturon

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[sfackler]

This can overflow, so should probably return an Option instead.

[seanmonstar]

@sfackler updated to use Option

[alexcrichton]

The RFC for Duration explicitly avoided this method:

This proposal does not include a method to get a number of milliseconds from a Duration, because the number of milliseconds could exceed u64, and we would have to decide whether to return an Option, panic, or wait for a standard bignum. In the interest of limiting this proposal to APIs with a straight-forward design, this proposal defers such a method.

As a result, I think that accessor methods like this may wish to go through an RFC. For example I would have questions along the lines of:

• What about other units of time? Should we have as_foo for minutes, hours, microseconds, nanoseconds, etc?
• Should there also be subsec_foo for microseconds and milliseconds?

[seanmonstar]

@alexcrichton This is motivated by balancing the accessors with the constructors: there exists from_millis but no as_millis. As there aren't any from_mins, from_hours, etc, they seem out of scope.

But I definitely can see how the color of this shed could be rainbow, and I can write a function to do the same thing for my needs.

[alexcrichton]

I understand the motivation, but I don't personally think that it's strong enough to bypass an RFC when the original RFC explicitly avoided this.