core::iter

Trait Step

Source 
pub trait Step:
    Clone
    + PartialOrd
    + Sized {
    // Required methods
    fn steps_between(start: &Self, end: &Self) -> Option<usize>;
    fn forward_checked(start: Self, count: usize) -> Option<Self>;
    fn backward_checked(start: Self, count: usize) -> Option<Self>;

    // Provided methods
    fn forward(start: Self, count: usize) -> Self { ... }
    unsafe fn forward_unchecked(start: Self, count: usize) -> Self { ... }
    fn backward(start: Self, count: usize) -> Self { ... }
    unsafe fn backward_unchecked(start: Self, count: usize) -> Self { ... }
}
🔬This is a nightly-only experimental API. (step_trait #42168)
Expand description

Objects that have a notion of successor and predecessor operations.

The successor operation moves towards values that compare greater. The predecessor operation moves towards values that compare lesser.

Required Methods§

Source

fn steps_between(start: &Self, end: &Self) -> Option<usize>

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the number of successor steps required to get from start to end.

Returns None if the number of steps would overflow usize (or is infinite, or if end would never be reached).

§Invariants

For any a, b, and n:

  • steps_between(&a, &b) == Some(n) if and only if Step::forward_checked(&a, n) == Some(b)
  • steps_between(&a, &b) == Some(n) if and only if Step::backward_checked(&b, n) == Some(a)
  • steps_between(&a, &b) == Some(n) only if a <= b
    • Corollary: steps_between(&a, &b) == Some(0) if and only if a == b
    • Note that a <= b does not imply steps_between(&a, &b) != None; this is the case when it would require more than usize::MAX steps to get to b
  • steps_between(&a, &b) == None if a > b
Source

fn forward_checked(start: Self, count: usize) -> Option<Self>

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the successor of self count times.

If this would overflow the range of values supported by Self, returns None.

§Invariants

For any a, n, and m:

  • Step::forward_checked(a, n).and_then(|x| Step::forward_checked(x, m)) == Step::forward_checked(a, m).and_then(|x| Step::forward_checked(x, n))
  • Step::forward_checked(a, n).and_then(|x| Step::forward_checked(x, m)) == try { Step::forward_checked(a, n.checked_add(m)) }

For any a and n:

  • Step::forward_checked(a, n) == (0..n).try_fold(a, |x, _| Step::forward_checked(&x, 1))
    • Corollary: Step::forward_checked(a, 0) == Some(a)
Source

fn backward_checked(start: Self, count: usize) -> Option<Self>

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the predecessor of self count times.

If this would overflow the range of values supported by Self, returns None.

§Invariants

For any a, n, and m:

  • Step::backward_checked(a, n).and_then(|x| Step::backward_checked(x, m)) == n.checked_add(m).and_then(|x| Step::backward_checked(a, x))
  • Step::backward_checked(a, n).and_then(|x| Step::backward_checked(x, m)) == try { Step::backward_checked(a, n.checked_add(m)?) }

For any a and n:

  • Step::backward_checked(a, n) == (0..n).try_fold(a, |x, _| Step::backward_checked(x, 1))
    • Corollary: Step::backward_checked(a, 0) == Some(a)

Provided Methods§

Source

fn forward(start: Self, count: usize) -> Self

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the successor of self count times.

If this would overflow the range of values supported by Self, this function is allowed to panic, wrap, or saturate. The suggested behavior is to panic when debug assertions are enabled, and to wrap or saturate otherwise.

Unsafe code should not rely on the correctness of behavior after overflow.

§Invariants

For any a, n, and m, where no overflow occurs:

  • Step::forward(Step::forward(a, n), m) == Step::forward(a, n + m)

For any a and n, where no overflow occurs:

  • Step::forward_checked(a, n) == Some(Step::forward(a, n))
  • Step::forward(a, n) == (0..n).fold(a, |x, _| Step::forward(x, 1))
    • Corollary: Step::forward(a, 0) == a
  • Step::forward(a, n) >= a
  • Step::backward(Step::forward(a, n), n) == a
Source

unsafe fn forward_unchecked(start: Self, count: usize) -> Self

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the successor of self count times.

§Safety

It is undefined behavior for this operation to overflow the range of values supported by Self. If you cannot guarantee that this will not overflow, use forward or forward_checked instead.

§Invariants

For any a:

  • if there exists b such that b > a, it is safe to call Step::forward_unchecked(a, 1)
  • if there exists b, n such that steps_between(&a, &b) == Some(n), it is safe to call Step::forward_unchecked(a, m) for any m <= n.
    • Corollary: Step::forward_unchecked(a, 0) is always safe.

For any a and n, where no overflow occurs:

  • Step::forward_unchecked(a, n) is equivalent to Step::forward(a, n)
Source

fn backward(start: Self, count: usize) -> Self

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the predecessor of self count times.

If this would overflow the range of values supported by Self, this function is allowed to panic, wrap, or saturate. The suggested behavior is to panic when debug assertions are enabled, and to wrap or saturate otherwise.

Unsafe code should not rely on the correctness of behavior after overflow.

§Invariants

For any a, n, and m, where no overflow occurs:

  • Step::backward(Step::backward(a, n), m) == Step::backward(a, n + m)

For any a and n, where no overflow occurs:

  • Step::backward_checked(a, n) == Some(Step::backward(a, n))
  • Step::backward(a, n) == (0..n).fold(a, |x, _| Step::backward(x, 1))
    • Corollary: Step::backward(a, 0) == a
  • Step::backward(a, n) <= a
  • Step::forward(Step::backward(a, n), n) == a
Source

unsafe fn backward_unchecked(start: Self, count: usize) -> Self

🔬This is a nightly-only experimental API. (step_trait #42168)

Returns the value that would be obtained by taking the predecessor of self count times.

§Safety

It is undefined behavior for this operation to overflow the range of values supported by Self. If you cannot guarantee that this will not overflow, use backward or backward_checked instead.

§Invariants

For any a:

  • if there exists b such that b < a, it is safe to call Step::backward_unchecked(a, 1)
  • if there exists b, n such that steps_between(&b, &a) == Some(n), it is safe to call Step::backward_unchecked(a, m) for any m <= n.
    • Corollary: Step::backward_unchecked(a, 0) is always safe.

For any a and n, where no overflow occurs:

  • Step::backward_unchecked(a, n) is equivalent to Step::backward(a, n)

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors§