Struct `Instant`

struct Instant(_)

A measurement of a monotonically nondecreasing clock. Opaque and useful only with Duration.

Instants are always guaranteed, barring platform bugs, to be no less than any previously measured instant when created, and are often useful for tasks such as measuring benchmarks or timing how long an operation takes.

Note, however, that instants are not guaranteed to be steady. In other words, each tick of the underlying clock might not be the same length (e.g. some seconds may be longer than others). An instant may jump forwards or experience time dilation (slow down or speed up), but it will never go backwards. As part of this non-guarantee it is also not specified whether system suspends count as elapsed time or not. The behavior varies across platforms and Rust versions.

Instants are opaque types that can only be compared to one another. There is no method to get "the number of seconds" from an instant. Instead, it only allows measuring the duration between two instants (or comparing two instants).

The size of an Instant struct may vary depending on the target operating system.

Example:

use std::time::{Duration, Instant};
use std::thread::sleep;

fn main() {
   let now = Instant::now();

   // we sleep for 2 seconds
   sleep(Duration::new(2, 0));
   // it prints '2'
   println!("{}", now.elapsed().as_secs());
}

OS-specific behaviors

An Instant is a wrapper around system-specific types and it may behave differently depending on the underlying operating system. For example, the following snippet is fine on Linux but panics on macOS:

use std::time::{Instant, Duration};

let now = Instant::now();
let days_per_10_millennia = 365_2425;
let solar_seconds_per_day = 60 * 60 * 24;
let millennium_in_solar_seconds = 31_556_952_000;
assert_eq!(millennium_in_solar_seconds, days_per_10_millennia * solar_seconds_per_day / 10);

let duration = Duration::new(millennium_in_solar_seconds, 0);
println!("{:?}", now + duration);

For cross-platform code, you can comfortably use durations of up to around one hundred years.

Underlying System calls

The following system calls are currently being used by now() to find out the current time:

Platform	System call
SGX	`insecure_time` usercall. More information on timekeeping in SGX
UNIX	clock_gettime with `CLOCK_MONOTONIC`
Darwin	clock_gettime with `CLOCK_UPTIME_RAW`
VXWorks	clock_gettime with `CLOCK_MONOTONIC`
SOLID	`get_tim`
WASI	__wasi_clock_time_get with `monotonic`
Windows	QueryPerformanceCounter

Disclaimer: These system calls might change over time.

Note: mathematical operations like add may panic if the underlying structure cannot represent the new point in time.

Monotonicity

On all platforms Instant will try to use an OS API that guarantees monotonic behavior if available, which is the case for all tier 1 platforms. In practice such guarantees are – under rare circumstances – broken by hardware, virtualization or operating system bugs. To work around these bugs and platforms not offering monotonic clocks duration_since, elapsed and sub saturate to zero. In older Rust versions this lead to a panic instead. checked_duration_since can be used to detect and handle situations where monotonicity is violated, or Instants are subtracted in the wrong order.

This workaround obscures programming errors where earlier and later instants are accidentally swapped. For this reason future Rust versions may reintroduce panics.

Implementations

`impl Instant`

fn now() -> Instant

Returns an instant corresponding to "now".

Examples

use std::time::Instant;

let now = Instant::now();

fn duration_since(self: &Self, earlier: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Panics

Previous Rust versions panicked when earlier was later than self. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.duration_since(now));
println!("{:?}", now.duration_since(new_now)); // 0ns

fn checked_duration_since(self: &Self, earlier: Instant) -> Option<Duration>

Returns the amount of time elapsed from another instant to this one, or None if that instant is later than this one.

Due to monotonicity bugs, even under correct logical ordering of the passed Instants, this method can return None.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.checked_duration_since(now));
println!("{:?}", now.checked_duration_since(new_now)); // None

fn saturating_duration_since(self: &Self, earlier: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.saturating_duration_since(now));
println!("{:?}", now.saturating_duration_since(new_now)); // 0ns

fn elapsed(self: &Self) -> Duration

Returns the amount of time elapsed since this instant.

Panics

Previous Rust versions panicked when the current time was earlier than self. Currently this method returns a Duration of zero in that case. Future versions may reintroduce the panic. See Monotonicity.

Examples

use std::thread::sleep;
use std::time::{Duration, Instant};

let instant = Instant::now();
let three_secs = Duration::from_secs(3);
sleep(three_secs);
assert!(instant.elapsed() >= three_secs);

fn checked_add(self: &Self, duration: Duration) -> Option<Instant>

Returns Some(t) where t is the time self + duration if t can be represented as Instant (which means it's inside the bounds of the underlying data structure), None otherwise.

fn checked_sub(self: &Self, duration: Duration) -> Option<Instant>

Returns Some(t) where t is the time self - duration if t can be represented as Instant (which means it's inside the bounds of the underlying data structure), None otherwise.

`impl Add for Instant`

fn add(self: Self, other: Duration) -> Instant: Panics

This function may panic if the resulting point in time cannot be represented by the underlying data structure. See Instant::checked_add for a version without panic.

`impl AddAssign for Instant`

fn add_assign(self: &mut Self, other: Duration)

`impl Clone for Instant`

fn clone(self: &Self) -> Instant

`impl Copy for Instant`

`impl Debug for Instant`

fn fmt(self: &Self, f: &mut fmt::Formatter<'_>) -> fmt::Result

`impl Eq for Instant`

`impl Freeze for Instant`

`impl Hash for Instant`

fn hash<__H: $crate::hash::Hasher>(self: &Self, state: &mut __H)

`impl Ord for Instant`

fn cmp(self: &Self, other: &Instant) -> $crate::cmp::Ordering

`impl PartialEq for Instant`

fn eq(self: &Self, other: &Instant) -> bool

`impl PartialOrd for Instant`

fn partial_cmp(self: &Self, other: &Instant) -> $crate::option::Option<$crate::cmp::Ordering>

`impl RefUnwindSafe for Instant`

`impl Send for Instant`

`impl StructuralPartialEq for Instant`

`impl Sub for Instant`

fn sub(self: Self, other: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Panics

Previous Rust versions panicked when other was later than self. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

`impl Sub for Instant`

fn sub(self: Self, other: Duration) -> Instant

`impl SubAssign for Instant`

fn sub_assign(self: &mut Self, other: Duration)

`impl Sync for Instant`

`impl Unpin for Instant`

`impl UnwindSafe for Instant`

`impl<T> Any for Instant`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for Instant`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for Instant`

fn borrow_mut(self: &mut Self) -> &mut T

`impl<T> CloneToUninit for Instant`

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

`impl<T> From for Instant`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> ToOwned for Instant`

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

`impl<T, U> Into for Instant`

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for Instant`

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

`impl<T, U> TryInto for Instant`

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>