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Rollup merge of #89621 - digama0:patch-2, r=yaahc
doc: guarantee call order for sort_by_cached_key `slice::sort_by_cached_key` takes a caching function `f: impl FnMut(&T) -> K`, which means that the order that calls to the caching function are made is user-visible. This adds a clause to the documentation to promise the current behavior, which is that `f` is called on all elements of the slice from left to right, unless the slice has len < 2 in which case `f` is not called. For example, this can be used to ensure that the following code is a correct way to involve the index of the element in the sort key: ```rust let mut index = 0; slice.sort_by_cached_key(|x| (my_key(index, x), index += 1).0); ```
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@ -375,7 +375,10 @@ pub fn sort_by_key<K, F>(&mut self, mut f: F)
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/// Sorts the slice with a key extraction function.
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///
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/// During sorting, the key function is called only once per element.
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/// During sorting, the key function is called at most once per element, by using
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/// temporary storage to remember the results of key evaluation.
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/// The order of calls to the key function is unspecified and may change in future versions
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/// of the standard library.
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///
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/// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* + *n* \* log(*n*))
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/// worst-case, where the key function is *O*(*m*).
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