Rollup merge of rust-lang#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.

src/libcore/str/mod.rs

@@ -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")]