Rollup merge of #69385 - ridiculousfish:relax_get_unchecked, r=nagisa

Relax str::get_unchecked precondition to permit empty slicing

Prior to this commit, `str` documented that `get_unchecked` had
the precondition that "`begin` must come before `end`". This would appear
to prohibit empty slices (i.e. begin == end).

In practice, get_unchecked is called often with empty slices. Let's relax
the precondition so as to allow them.
This commit is contained in:
Pietro Albini 2020-02-24 11:15:31 +01:00 committed by GitHub
commit d73aa67208
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@ -2486,7 +2486,7 @@ impl str {
/// Callers of this function are responsible that these preconditions are
/// satisfied:
///
/// * The starting index must come before the ending index;
/// * The starting index must not exceed the ending index;
/// * Indexes must be within bounds of the original slice;
/// * Indexes must lie on UTF-8 sequence boundaries.
///
@ -2518,7 +2518,7 @@ impl str {
/// Callers of this function are responsible that these preconditions are
/// satisfied:
///
/// * The starting index must come before the ending index;
/// * The starting index must not exceed the ending index;
/// * Indexes must be within bounds of the original slice;
/// * Indexes must lie on UTF-8 sequence boundaries.
///
@ -2563,7 +2563,7 @@ impl str {
/// Callers of this function are responsible that three preconditions are
/// satisfied:
///
/// * `begin` must come before `end`.
/// * `begin` must not exceed `end`.
/// * `begin` and `end` must be byte positions within the string slice.
/// * `begin` and `end` must lie on UTF-8 sequence boundaries.
///
@ -2612,7 +2612,7 @@ impl str {
/// Callers of this function are responsible that three preconditions are
/// satisfied:
///
/// * `begin` must come before `end`.
/// * `begin` must not exceed `end`.
/// * `begin` and `end` must be byte positions within the string slice.
/// * `begin` and `end` must lie on UTF-8 sequence boundaries.
#[stable(feature = "str_slice_mut", since = "1.5.0")]