Auto merge of #74010 - pierwill:pierwill-o-notation, r=GuillaumeGomez

Use italics for O notation

In documentation, I think it makes sense to italicize O notation (*O(n)*) as opposed to using back-ticks (`O(n)`). Visually, back-ticks focus the reader on the literal characters being used, making them ideal for representing code. Using italics, as far I can tell, more closely follows typographic conventions in mathematics and computer science.

Just a suggestion, of course! 😇

src/liballoc/collections/binary_heap.rs

@@ -1,9 +1,9 @@
 //! A priority queue implemented with a binary heap.
 //!
-//! Insertion and popping the largest element have `O(log(n))` time complexity.
-//! Checking the largest element is `O(1)`. Converting a vector to a binary heap
-//! can be done in-place, and has `O(n)` complexity. A binary heap can also be
-//! converted to a sorted vector in-place, allowing it to be used for an `O(n * log(n))`
+//! Insertion and popping the largest element have *O*(log(*n*)) time complexity.
+//! Checking the largest element is *O*(1). Converting a vector to a binary heap
+//! can be done in-place, and has *O*(*n*) complexity. A binary heap can also be
+//! converted to a sorted vector in-place, allowing it to be used for an *O*(*n* \* log(*n*))
 //! in-place heapsort.
 //!
 //! # Examples
@@ -235,7 +235,7 @@ use super::SpecExtend;
 ///
 /// | [push] | [pop]     | [peek]/[peek\_mut] |
 /// |--------|-----------|--------------------|
-/// | O(1)~  | O(log(n)) | O(1)               |
+/// | O(1)~  | *O*(log(*n*)) | *O*(1)               |
 ///
 /// The value for `push` is an expected cost; the method documentation gives a
 /// more detailed analysis.
@@ -398,7 +398,7 @@ impl<T: Ord> BinaryHeap<T> {
     ///
     /// # Time complexity
     ///
-    /// Cost is `O(1)` in the worst case.
+    /// Cost is *O*(1) in the worst case.
     #[stable(feature = "binary_heap_peek_mut", since = "1.12.0")]
     pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
         if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: true }) }
@@ -422,7 +422,7 @@ impl<T: Ord> BinaryHeap<T> {
     ///
     /// # Time complexity
     ///
-    /// The worst case cost of `pop` on a heap containing *n* elements is `O(log(n))`.
+    /// The worst case cost of `pop` on a heap containing *n* elements is *O*(log(*n*)).
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn pop(&mut self) -> Option<T> {
         self.data.pop().map(|mut item| {
@@ -455,15 +455,15 @@ impl<T: Ord> BinaryHeap<T> {
     ///
     /// The expected cost of `push`, averaged over every possible ordering of
     /// the elements being pushed, and over a sufficiently large number of
-    /// pushes, is `O(1)`. This is the most meaningful cost metric when pushing
+    /// pushes, is *O*(1). This is the most meaningful cost metric when pushing
     /// elements that are *not* already in any sorted pattern.
     ///
     /// The time complexity degrades if elements are pushed in predominantly
     /// ascending order. In the worst case, elements are pushed in ascending
-    /// sorted order and the amortized cost per push is `O(log(n))` against a heap
+    /// sorted order and the amortized cost per push is *O*(log(*n*)) against a heap
     /// containing *n* elements.
     ///
-    /// The worst case cost of a *single* call to `push` is `O(n)`. The worst case
+    /// The worst case cost of a *single* call to `push` is *O*(*n*). The worst case
     /// occurs when capacity is exhausted and needs a resize. The resize cost
     /// has been amortized in the previous figures.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -643,7 +643,7 @@ impl<T: Ord> BinaryHeap<T> {
     /// The remaining elements will be removed on drop in heap order.
     ///
     /// Note:
-    /// * `.drain_sorted()` is `O(n * log(n))`; much slower than `.drain()`.
+    /// * `.drain_sorted()` is *O*(*n* \* log(*n*)); much slower than `.drain()`.
     ///   You should use the latter for most cases.
     ///
     /// # Examples
@@ -756,7 +756,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// # Time complexity
     ///
-    /// Cost is `O(1)` in the worst case.
+    /// Cost is *O*(1) in the worst case.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn peek(&self) -> Option<&T> {
         self.data.get(0)
@@ -1312,7 +1312,7 @@ unsafe impl<T: Ord> TrustedLen for DrainSorted<'_, T> {}
 impl<T: Ord> From<Vec<T>> for BinaryHeap<T> {
     /// Converts a `Vec<T>` into a `BinaryHeap<T>`.
     ///
-    /// This conversion happens in-place, and has `O(n)` time complexity.
+    /// This conversion happens in-place, and has *O*(*n*) time complexity.
     fn from(vec: Vec<T>) -> BinaryHeap<T> {
         let mut heap = BinaryHeap { data: vec };
         heap.rebuild();

src/liballoc/collections/linked_list.rs

@@ -404,7 +404,7 @@ impl<T> LinkedList<T> {
     /// This reuses all the nodes from `other` and moves them into `self`. After
     /// this operation, `other` becomes empty.
     ///
-    /// This operation should compute in `O(1)` time and `O(1)` memory.
+    /// This operation should compute in *O*(1) time and *O*(1) memory.
     ///
     /// # Examples
     ///
@@ -561,7 +561,7 @@ impl<T> LinkedList<T> {
 
     /// Returns `true` if the `LinkedList` is empty.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -582,7 +582,7 @@ impl<T> LinkedList<T> {
 
     /// Returns the length of the `LinkedList`.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -608,7 +608,7 @@ impl<T> LinkedList<T> {
 
     /// Removes all elements from the `LinkedList`.
     ///
-    /// This operation should compute in `O(n)` time.
+    /// This operation should compute in *O*(*n*) time.
     ///
     /// # Examples
     ///
@@ -751,7 +751,7 @@ impl<T> LinkedList<T> {
 
     /// Adds an element first in the list.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -774,7 +774,7 @@ impl<T> LinkedList<T> {
     /// Removes the first element and returns it, or `None` if the list is
     /// empty.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -797,7 +797,7 @@ impl<T> LinkedList<T> {
 
     /// Appends an element to the back of a list.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -817,7 +817,7 @@ impl<T> LinkedList<T> {
     /// Removes the last element from a list and returns it, or `None` if
     /// it is empty.
     ///
-    /// This operation should compute in `O(1)` time.
+    /// This operation should compute in *O*(1) time.
     ///
     /// # Examples
     ///
@@ -838,7 +838,7 @@ impl<T> LinkedList<T> {
     /// Splits the list into two at the given index. Returns everything after the given index,
     /// including the index.
     ///
-    /// This operation should compute in `O(n)` time.
+    /// This operation should compute in *O*(*n*) time.
     ///
     /// # Panics
     ///
@@ -894,7 +894,7 @@ impl<T> LinkedList<T> {
 
     /// Removes the element at the given index and returns it.
     ///
-    /// This operation should compute in `O(n)` time.
+    /// This operation should compute in *O*(*n*) time.
     ///
     /// # Panics
     /// Panics if at >= len

src/liballoc/collections/vec_deque.rs

@@ -1,7 +1,7 @@
 //! A double-ended queue implemented with a growable ring buffer.
 //!
-//! This queue has `O(1)` amortized inserts and removals from both ends of the
-//! container. It also has `O(1)` indexing like a vector. The contained elements
+//! This queue has *O*(1) amortized inserts and removals from both ends of the
+//! container. It also has *O*(1) indexing like a vector. The contained elements
 //! are not required to be copyable, and the queue will be sendable if the
 //! contained type is sendable.
 
@@ -1512,7 +1512,7 @@ impl<T> VecDeque<T> {
     /// Removes an element from anywhere in the `VecDeque` and returns it,
     /// replacing it with the first element.
     ///
-    /// This does not preserve ordering, but is `O(1)`.
+    /// This does not preserve ordering, but is *O*(1).
     ///
     /// Returns `None` if `index` is out of bounds.
     ///
@@ -1547,7 +1547,7 @@ impl<T> VecDeque<T> {
     /// Removes an element from anywhere in the `VecDeque` and returns it, replacing it with the
     /// last element.
     ///
-    /// This does not preserve ordering, but is `O(1)`.
+    /// This does not preserve ordering, but is *O*(1).
     ///
     /// Returns `None` if `index` is out of bounds.
     ///
@@ -2331,7 +2331,7 @@ impl<T> VecDeque<T> {
     ///
     /// # Complexity
     ///
-    /// Takes `O(min(mid, len() - mid))` time and no extra space.
+    /// Takes `*O*(min(mid, len() - mid))` time and no extra space.
     ///
     /// # Examples
     ///
@@ -2374,7 +2374,7 @@ impl<T> VecDeque<T> {
     ///
     /// # Complexity
     ///
-    /// Takes `O(min(k, len() - k))` time and no extra space.
+    /// Takes `*O*(min(k, len() - k))` time and no extra space.
     ///
     /// # Examples
     ///
@@ -3076,15 +3076,15 @@ impl<T> From<VecDeque<T>> for Vec<T> {
     /// [`Vec<T>`]: crate::vec::Vec
     /// [`VecDeque<T>`]: crate::collections::VecDeque
     ///
-    /// This never needs to re-allocate, but does need to do `O(n)` data movement if
+    /// This never needs to re-allocate, but does need to do *O*(*n*) data movement if
     /// the circular buffer doesn't happen to be at the beginning of the allocation.
     ///
     /// # Examples
     ///
     /// ```
     /// use std::collections::VecDeque;
     ///
-    /// // This one is O(1).
+    /// // This one is *O*(1).
     /// let deque: VecDeque<_> = (1..5).collect();
     /// let ptr = deque.as_slices().0.as_ptr();
     /// let vec = Vec::from(deque);

src/liballoc/string.rs

@@ -1162,7 +1162,7 @@ impl String {
 
     /// Removes a [`char`] from this `String` at a byte position and returns it.
     ///
-    /// This is an `O(n)` operation, as it requires copying every element in the
+    /// This is an *O*(*n*) operation, as it requires copying every element in the
     /// buffer.
     ///
     /// # Panics
@@ -1262,7 +1262,7 @@ impl String {
 
     /// Inserts a character into this `String` at a byte position.
     ///
-    /// This is an `O(n)` operation as it requires copying every element in the
+    /// This is an *O*(*n*) operation as it requires copying every element in the
     /// buffer.
     ///
     /// # Panics
@@ -1309,7 +1309,7 @@ impl String {
 
     /// Inserts a string slice into this `String` at a byte position.
     ///
-    /// This is an `O(n)` operation as it requires copying every element in the
+    /// This is an *O*(*n*) operation as it requires copying every element in the
     /// buffer.
     ///
     /// # Panics
@@ -1971,7 +1971,7 @@ impl hash::Hash for String {
 ///
 /// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
 /// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
-/// every operation, which would lead to `O(n^2)` running time when building an `n`-byte string by
+/// every operation, which would lead to *O*(*n*^2) running time when building an *n*-byte string by
 /// repeated concatenation.
 ///
 /// The string on the right-hand side is only borrowed; its contents are copied into the returned

src/libcore/slice/mod.rs

@@ -1672,7 +1672,7 @@ impl<T> [T] {
     /// Sorts the slice, but may not preserve the order of equal elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place
-    /// (i.e., does not allocate), and `O(n * log(n))` worst-case.
+    /// (i.e., does not allocate), and *O*(*n* \* log(*n*)) worst-case.
     ///
     /// # Current implementation
     ///
@@ -1708,7 +1708,7 @@ impl<T> [T] {
     /// elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place
-    /// (i.e., does not allocate), and `O(n * log(n))` worst-case.
+    /// (i.e., does not allocate), and *O*(*n* \* log(*n*)) worst-case.
     ///
     /// The comparator function must define a total ordering for the elements in the slice. If
     /// the ordering is not total, the order of the elements is unspecified. An order is a
@@ -1763,8 +1763,8 @@ impl<T> [T] {
     /// elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place
-    /// (i.e., does not allocate), and `O(m * n * log(n))` worst-case, where the key function is
-    /// `O(m)`.
+    /// (i.e., does not allocate), and *O*(m \* *n* \* log(*n*)) worst-case, where the key function is
+    /// *O*(*m*).
     ///
     /// # Current implementation
     ///
@@ -1803,7 +1803,7 @@ impl<T> [T] {
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index`. Additionally, this reordering is
     /// unstable (i.e. any number of equal elements may end up at position `index`), in-place
-    /// (i.e. does not allocate), and `O(n)` worst-case. This function is also/ known as "kth
+    /// (i.e. does not allocate), and *O*(*n*) worst-case. This function is also/ known as "kth
     /// element" in other libraries. It returns a triplet of the following values: all elements less
     /// than the one at the given index, the value at the given index, and all elements greater than
     /// the one at the given index.
@@ -1852,7 +1852,7 @@ impl<T> [T] {
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index` using the comparator function.
     /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
-    /// position `index`), in-place (i.e. does not allocate), and `O(n)` worst-case. This function
+    /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) worst-case. This function
     /// is also known as "kth element" in other libraries. It returns a triplet of the following
     /// values: all elements less than the one at the given index, the value at the given index,
     /// and all elements greater than the one at the given index, using the provided comparator
@@ -1906,7 +1906,7 @@ impl<T> [T] {
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index` using the key extraction function.
     /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
-    /// position `index`), in-place (i.e. does not allocate), and `O(n)` worst-case. This function
+    /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) worst-case. This function
     /// is also known as "kth element" in other libraries. It returns a triplet of the following
     /// values: all elements less than the one at the given index, the value at the given index, and
     /// all elements greater than the one at the given index, using the provided key extraction

src/libcore/slice/sort.rs

@@ -121,7 +121,7 @@ where
 
 /// Partially sorts a slice by shifting several out-of-order elements around.
 ///
-/// Returns `true` if the slice is sorted at the end. This function is `O(n)` worst-case.
+/// Returns `true` if the slice is sorted at the end. This function is *O*(*n*) worst-case.
 #[cold]
 fn partial_insertion_sort<T, F>(v: &mut [T], is_less: &mut F) -> bool
 where
@@ -168,7 +168,7 @@ where
     false
 }
 
-/// Sorts a slice using insertion sort, which is `O(n^2)` worst-case.
+/// Sorts a slice using insertion sort, which is *O*(*n*^2) worst-case.
 fn insertion_sort<T, F>(v: &mut [T], is_less: &mut F)
 where
     F: FnMut(&T, &T) -> bool,
@@ -178,7 +178,7 @@ where
     }
 }
 
-/// Sorts `v` using heapsort, which guarantees `O(n * log(n))` worst-case.
+/// Sorts `v` using heapsort, which guarantees *O*(*n* \* log(*n*)) worst-case.
 #[cold]
 pub fn heapsort<T, F>(v: &mut [T], is_less: &mut F)
 where
@@ -751,7 +751,7 @@ where
     }
 }
 
-/// Sorts `v` using pattern-defeating quicksort, which is `O(n * log(n))` worst-case.
+/// Sorts `v` using pattern-defeating quicksort, which is *O*(*n* \* log(*n*)) worst-case.
 pub fn quicksort<T, F>(v: &mut [T], mut is_less: F)
 where
     F: FnMut(&T, &T) -> bool,

src/librustc_data_structures/sorted_map/index_map.rs

@@ -7,8 +7,8 @@ use std::iter::FromIterator;
 use crate::stable_hasher::{HashStable, StableHasher};
 use rustc_index::vec::{Idx, IndexVec};
 
-/// An indexed multi-map that preserves insertion order while permitting both `O(log n)` lookup of
-/// an item by key and `O(1)` lookup by index.
+/// An indexed multi-map that preserves insertion order while permitting both *O*(log *n*) lookup of
+/// an item by key and *O*(1) lookup by index.
 ///
 /// This data structure is a hybrid of an [`IndexVec`] and a [`SortedMap`]. Like `IndexVec`,
 /// `SortedIndexMultiMap` assigns a typed index to each item while preserving insertion order.
@@ -20,7 +20,7 @@ use rustc_index::vec::{Idx, IndexVec};
 /// items will be yielded in insertion order.
 ///
 /// Unlike a general-purpose map like `BTreeSet` or `HashSet`, `SortedMap` and
-/// `SortedIndexMultiMap` require `O(n)` time to insert a single item. This is because we may need
+/// `SortedIndexMultiMap` require *O*(*n*) time to insert a single item. This is because we may need
 /// to insert into the middle of the sorted array. Users should avoid mutating this data structure
 /// in-place.
 ///

src/librustc_index/bit_set.rs

@@ -772,7 +772,7 @@ impl<R: Idx, C: Idx> BitMatrix<R, C> {
     }
 
     /// Returns those indices that are true in rows `a` and `b`. This
-    /// is an O(n) operation where `n` is the number of elements
+    /// is an *O*(*n*) operation where *n* is the number of elements
     /// (somewhat independent from the actual size of the
     /// intersection, in particular).
     pub fn intersect_rows(&self, row1: R, row2: R) -> Vec<C> {