From d23a5b1d88df513aff1dc1ba09730221567f2857 Mon Sep 17 00:00:00 2001
From: Ankit Meel <ankitjmeel@gmail.com>
Date: Wed, 16 Dec 2020 13:02:28 +0530
Subject: BLI: constexpr Span, IndexRange, StringRef(Null/Base)

Motivated by `std::string_view` being usable in
const (compile-time) context.
One functional change was needed for StringRef:
`std::char_traits<char>::length(str)` instead of `strlen`.

Reviewed By: JacquesLucke, LazyDodo
Differential Revision: https://developer.blender.org/D9788
---
 source/blender/blenlib/BLI_index_range.hh          |  45 ++++----
 source/blender/blenlib/BLI_span.hh                 | 124 ++++++++++----------
 source/blender/blenlib/BLI_string_ref.hh           | 126 +++++++++++----------
 .../blender/blenlib/tests/BLI_index_range_test.cc  |   7 ++
 source/blender/blenlib/tests/BLI_span_test.cc      |  15 +++
 .../blender/blenlib/tests/BLI_string_ref_test.cc   |   8 ++
 6 files changed, 180 insertions(+), 145 deletions(-)

diff --git a/source/blender/blenlib/BLI_index_range.hh b/source/blender/blenlib/BLI_index_range.hh
index 2b060c986cd..4121542c887 100644
--- a/source/blender/blenlib/BLI_index_range.hh
+++ b/source/blender/blenlib/BLI_index_range.hh
@@ -73,21 +73,22 @@ class IndexRange {
   int64_t size_ = 0;
 
  public:
-  IndexRange() = default;
+  constexpr IndexRange() = default;
 
-  explicit IndexRange(int64_t size) : start_(0), size_(size)
+  constexpr explicit IndexRange(int64_t size) : start_(0), size_(size)
   {
     BLI_assert(size >= 0);
   }
 
-  IndexRange(int64_t start, int64_t size) : start_(start), size_(size)
+  constexpr IndexRange(int64_t start, int64_t size) : start_(start), size_(size)
   {
     BLI_assert(start >= 0);
     BLI_assert(size >= 0);
   }
 
   template<typename T>
-  IndexRange(const tbb::blocked_range<T> &range) : start_(range.begin()), size_(range.size())
+  constexpr IndexRange(const tbb::blocked_range<T> &range)
+      : start_(range.begin()), size_(range.size())
   {
   }
 
@@ -96,33 +97,33 @@ class IndexRange {
     int64_t current_;
 
    public:
-    Iterator(int64_t current) : current_(current)
+    constexpr Iterator(int64_t current) : current_(current)
     {
     }
 
-    Iterator &operator++()
+    constexpr Iterator &operator++()
     {
       current_++;
       return *this;
     }
 
-    bool operator!=(const Iterator &iterator) const
+    constexpr bool operator!=(const Iterator &iterator) const
     {
       return current_ != iterator.current_;
     }
 
-    int64_t operator*() const
+    constexpr int64_t operator*() const
     {
       return current_;
     }
   };
 
-  Iterator begin() const
+  constexpr Iterator begin() const
   {
     return Iterator(start_);
   }
 
-  Iterator end() const
+  constexpr Iterator end() const
   {
     return Iterator(start_ + size_);
   }
@@ -130,7 +131,7 @@ class IndexRange {
   /**
    * Access an element in the range.
    */
-  int64_t operator[](int64_t index) const
+  constexpr int64_t operator[](int64_t index) const
   {
     BLI_assert(index >= 0);
     BLI_assert(index < this->size());
@@ -140,7 +141,7 @@ class IndexRange {
   /**
    * Two ranges compare equal when they contain the same numbers.
    */
-  friend bool operator==(IndexRange a, IndexRange b)
+  constexpr friend bool operator==(IndexRange a, IndexRange b)
   {
     return (a.size_ == b.size_) && (a.start_ == b.start_ || a.size_ == 0);
   }
@@ -148,7 +149,7 @@ class IndexRange {
   /**
    * Get the amount of numbers in the range.
    */
-  int64_t size() const
+  constexpr int64_t size() const
   {
     return size_;
   }
@@ -156,7 +157,7 @@ class IndexRange {
   /**
    * Create a new range starting at the end of the current one.
    */
-  IndexRange after(int64_t n) const
+  constexpr IndexRange after(int64_t n) const
   {
     BLI_assert(n >= 0);
     return IndexRange(start_ + size_, n);
@@ -165,7 +166,7 @@ class IndexRange {
   /**
    * Create a new range that ends at the start of the current one.
    */
-  IndexRange before(int64_t n) const
+  constexpr IndexRange before(int64_t n) const
   {
     BLI_assert(n >= 0);
     return IndexRange(start_ - n, n);
@@ -175,7 +176,7 @@ class IndexRange {
    * Get the first element in the range.
    * Asserts when the range is empty.
    */
-  int64_t first() const
+  constexpr int64_t first() const
   {
     BLI_assert(this->size() > 0);
     return start_;
@@ -185,7 +186,7 @@ class IndexRange {
    * Get the last element in the range.
    * Asserts when the range is empty.
    */
-  int64_t last() const
+  constexpr int64_t last() const
   {
     BLI_assert(this->size() > 0);
     return start_ + size_ - 1;
@@ -194,7 +195,7 @@ class IndexRange {
   /**
    * Get the element one after the end. The returned value is undefined when the range is empty.
    */
-  int64_t one_after_last() const
+  constexpr int64_t one_after_last() const
   {
     return start_ + size_;
   }
@@ -202,7 +203,7 @@ class IndexRange {
   /**
    * Get the first element in the range. The returned value is undefined when the range is empty.
    */
-  int64_t start() const
+  constexpr int64_t start() const
   {
     return start_;
   }
@@ -210,7 +211,7 @@ class IndexRange {
   /**
    * Returns true when the range contains a certain number, otherwise false.
    */
-  bool contains(int64_t value) const
+  constexpr bool contains(int64_t value) const
   {
     return value >= start_ && value < start_ + size_;
   }
@@ -218,7 +219,7 @@ class IndexRange {
   /**
    * Returns a new range, that contains a sub-interval of the current one.
    */
-  IndexRange slice(int64_t start, int64_t size) const
+  constexpr IndexRange slice(int64_t start, int64_t size) const
   {
     BLI_assert(start >= 0);
     BLI_assert(size >= 0);
@@ -226,7 +227,7 @@ class IndexRange {
     BLI_assert(new_start + size <= start_ + size_ || size == 0);
     return IndexRange(new_start, size);
   }
-  IndexRange slice(IndexRange range) const
+  constexpr IndexRange slice(IndexRange range) const
   {
     return this->slice(range.start(), range.size());
   }
diff --git a/source/blender/blenlib/BLI_span.hh b/source/blender/blenlib/BLI_span.hh
index 5b4d2769f57..3f410efe908 100644
--- a/source/blender/blenlib/BLI_span.hh
+++ b/source/blender/blenlib/BLI_span.hh
@@ -93,15 +93,15 @@ template<typename T> class Span {
   /**
    * Create a reference to an empty array.
    */
-  Span() = default;
+  constexpr Span() = default;
 
-  Span(const T *start, int64_t size) : data_(start), size_(size)
+  constexpr Span(const T *start, int64_t size) : data_(start), size_(size)
   {
     BLI_assert(size >= 0);
   }
 
   template<typename U, typename std::enable_if_t<is_convertible_pointer_v<U, T>> * = nullptr>
-  Span(const U *start, int64_t size) : data_(static_cast<const T *>(start)), size_(size)
+  constexpr Span(const U *start, int64_t size) : data_(static_cast<const T *>(start)), size_(size)
   {
     BLI_assert(size >= 0);
   }
@@ -117,16 +117,17 @@ template<typename T> class Span {
    *  Span<int> span = {1, 2, 3, 4};
    *  call_function_with_array(span);
    */
-  Span(const std::initializer_list<T> &list)
+  constexpr Span(const std::initializer_list<T> &list)
       : Span(list.begin(), static_cast<int64_t>(list.size()))
   {
   }
 
-  Span(const std::vector<T> &vector) : Span(vector.data(), static_cast<int64_t>(vector.size()))
+  constexpr Span(const std::vector<T> &vector)
+      : Span(vector.data(), static_cast<int64_t>(vector.size()))
   {
   }
 
-  template<std::size_t N> Span(const std::array<T, N> &array) : Span(array.data(), N)
+  template<std::size_t N> constexpr Span(const std::array<T, N> &array) : Span(array.data(), N)
   {
   }
 
@@ -135,7 +136,7 @@ template<typename T> class Span {
    *   Span<T *> -> Span<const T *>
    */
   template<typename U, typename std::enable_if_t<is_convertible_pointer_v<U, T>> * = nullptr>
-  Span(Span<U> array) : data_(static_cast<const T *>(array.data())), size_(array.size())
+  constexpr Span(Span<U> array) : data_(static_cast<const T *>(array.data())), size_(array.size())
   {
   }
 
@@ -143,7 +144,7 @@ template<typename T> class Span {
    * Returns a contiguous part of the array. This invokes undefined behavior when the slice does
    * not stay within the bounds of the array.
    */
-  Span slice(int64_t start, int64_t size) const
+  constexpr Span slice(int64_t start, int64_t size) const
   {
     BLI_assert(start >= 0);
     BLI_assert(size >= 0);
@@ -151,7 +152,7 @@ template<typename T> class Span {
     return Span(data_ + start, size);
   }
 
-  Span slice(IndexRange range) const
+  constexpr Span slice(IndexRange range) const
   {
     return this->slice(range.start(), range.size());
   }
@@ -160,7 +161,7 @@ template<typename T> class Span {
    * Returns a new Span with n elements removed from the beginning. This invokes undefined
    * behavior when the array is too small.
    */
-  Span drop_front(int64_t n) const
+  constexpr Span drop_front(int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= this->size());
@@ -171,7 +172,7 @@ template<typename T> class Span {
    * Returns a new Span with n elements removed from the beginning. This invokes undefined
    * behavior when the array is too small.
    */
-  Span drop_back(int64_t n) const
+  constexpr Span drop_back(int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= this->size());
@@ -182,7 +183,7 @@ template<typename T> class Span {
    * Returns a new Span that only contains the first n elements. This invokes undefined
    * behavior when the array is too small.
    */
-  Span take_front(int64_t n) const
+  constexpr Span take_front(int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= this->size());
@@ -193,7 +194,7 @@ template<typename T> class Span {
    * Returns a new Span that only contains the last n elements. This invokes undefined
    * behavior when the array is too small.
    */
-  Span take_back(int64_t n) const
+  constexpr Span take_back(int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= this->size());
@@ -204,25 +205,25 @@ template<typename T> class Span {
    * Returns the pointer to the beginning of the referenced array. This may be nullptr when the
    * size is zero.
    */
-  const T *data() const
+  constexpr const T *data() const
   {
     return data_;
   }
 
-  const T *begin() const
+  constexpr const T *begin() const
   {
     return data_;
   }
-  const T *end() const
+  constexpr const T *end() const
   {
     return data_ + size_;
   }
 
-  std::reverse_iterator<const T *> rbegin() const
+  constexpr std::reverse_iterator<const T *> rbegin() const
   {
     return std::reverse_iterator<const T *>(this->end());
   }
-  std::reverse_iterator<const T *> rend() const
+  constexpr std::reverse_iterator<const T *> rend() const
   {
     return std::reverse_iterator<const T *>(this->begin());
   }
@@ -231,7 +232,7 @@ template<typename T> class Span {
    * Access an element in the array. This invokes undefined behavior when the index is out of
    * bounds.
    */
-  const T &operator[](int64_t index) const
+  constexpr const T &operator[](int64_t index) const
   {
     BLI_assert(index >= 0);
     BLI_assert(index < size_);
@@ -241,7 +242,7 @@ template<typename T> class Span {
   /**
    * Returns the number of elements in the referenced array.
    */
-  int64_t size() const
+  constexpr int64_t size() const
   {
     return size_;
   }
@@ -249,7 +250,7 @@ template<typename T> class Span {
   /**
    * Returns true if the size is zero.
    */
-  bool is_empty() const
+  constexpr bool is_empty() const
   {
     return size_ == 0;
   }
@@ -257,7 +258,7 @@ template<typename T> class Span {
   /**
    * Returns the number of bytes referenced by this Span.
    */
-  int64_t size_in_bytes() const
+  constexpr int64_t size_in_bytes() const
   {
     return sizeof(T) * size_;
   }
@@ -266,7 +267,7 @@ template<typename T> class Span {
    * Does a linear search to see of the value is in the array.
    * Returns true if it is, otherwise false.
    */
-  bool contains(const T &value) const
+  constexpr bool contains(const T &value) const
   {
     for (const T &element : *this) {
       if (element == value) {
@@ -280,7 +281,7 @@ template<typename T> class Span {
    * Does a constant time check to see if the pointer points to a value in the referenced array.
    * Return true if it is, otherwise false.
    */
-  bool contains_ptr(const T *ptr) const
+  constexpr bool contains_ptr(const T *ptr) const
   {
     return (this->begin() <= ptr) && (ptr < this->end());
   }
@@ -289,7 +290,7 @@ template<typename T> class Span {
    * Does a linear search to count how often the value is in the array.
    * Returns the number of occurrences.
    */
-  int64_t count(const T &value) const
+  constexpr int64_t count(const T &value) const
   {
     int64_t counter = 0;
     for (const T &element : *this) {
@@ -304,7 +305,7 @@ template<typename T> class Span {
    * Return a reference to the first element in the array. This invokes undefined behavior when the
    * array is empty.
    */
-  const T &first() const
+  constexpr const T &first() const
   {
     BLI_assert(size_ > 0);
     return data_[0];
@@ -314,7 +315,7 @@ template<typename T> class Span {
    * Returns a reference to the last element in the array. This invokes undefined behavior when the
    * array is empty.
    */
-  const T &last() const
+  constexpr const T &last() const
   {
     BLI_assert(size_ > 0);
     return data_[size_ - 1];
@@ -324,7 +325,7 @@ template<typename T> class Span {
    * Returns the element at the given index. If the index is out of range, return the fallback
    * value.
    */
-  T get(int64_t index, const T &fallback) const
+  constexpr T get(int64_t index, const T &fallback) const
   {
     if (index < size_ && index >= 0) {
       return data_[index];
@@ -336,7 +337,7 @@ template<typename T> class Span {
    * Check if the array contains duplicates. Does a linear search for every element. So the total
    * running time is O(n^2). Only use this for small arrays.
    */
-  bool has_duplicates__linear_search() const
+  constexpr bool has_duplicates__linear_search() const
   {
     /* The size should really be smaller than that. If it is not, the calling code should be
      * changed. */
@@ -358,7 +359,7 @@ template<typename T> class Span {
    * called on small arrays, because it has a running time of O(n*m) where n and m are the sizes of
    * the arrays.
    */
-  bool intersects__linear_search(Span other) const
+  constexpr bool intersects__linear_search(Span other) const
   {
     /* The size should really be smaller than that. If it is not, the calling code should be
      * changed. */
@@ -377,7 +378,7 @@ template<typename T> class Span {
    * Returns the index of the first occurrence of the given value. This invokes undefined behavior
    * when the value is not in the array.
    */
-  int64_t first_index(const T &search_value) const
+  constexpr int64_t first_index(const T &search_value) const
   {
     const int64_t index = this->first_index_try(search_value);
     BLI_assert(index >= 0);
@@ -387,7 +388,7 @@ template<typename T> class Span {
   /**
    * Returns the index of the first occurrence of the given value or -1 if it does not exist.
    */
-  int64_t first_index_try(const T &search_value) const
+  constexpr int64_t first_index_try(const T &search_value) const
   {
     for (int64_t i = 0; i < size_; i++) {
       if (data_[i] == search_value) {
@@ -401,7 +402,7 @@ template<typename T> class Span {
    * Utility to make it more convenient to iterate over all indices that can be used with this
    * array.
    */
-  IndexRange index_range() const
+  constexpr IndexRange index_range() const
   {
     return IndexRange(size_);
   }
@@ -409,7 +410,7 @@ template<typename T> class Span {
   /**
    * Returns a new Span to the same underlying memory buffer. No conversions are done.
    */
-  template<typename NewT> Span<NewT> cast() const
+  template<typename NewT> Span<NewT> constexpr cast() const
   {
     BLI_assert((size_ * sizeof(T)) % sizeof(NewT) == 0);
     int64_t new_size = size_ * sizeof(T) / sizeof(NewT);
@@ -450,21 +451,22 @@ template<typename T> class MutableSpan {
   int64_t size_;
 
  public:
-  MutableSpan() = default;
+  constexpr MutableSpan() = default;
 
-  MutableSpan(T *start, const int64_t size) : data_(start), size_(size)
+  constexpr MutableSpan(T *start, const int64_t size) : data_(start), size_(size)
   {
   }
 
-  MutableSpan(std::vector<T> &vector) : MutableSpan(vector.data(), vector.size())
+  constexpr MutableSpan(std::vector<T> &vector) : MutableSpan(vector.data(), vector.size())
   {
   }
 
-  template<std::size_t N> MutableSpan(std::array<T, N> &array) : MutableSpan(array.data(), N)
+  template<std::size_t N>
+  constexpr MutableSpan(std::array<T, N> &array) : MutableSpan(array.data(), N)
   {
   }
 
-  operator Span<T>() const
+  constexpr operator Span<T>() const
   {
     return Span<T>(data_, size_);
   }
@@ -472,7 +474,7 @@ template<typename T> class MutableSpan {
   /**
    * Returns the number of elements in the array.
    */
-  int64_t size() const
+  constexpr int64_t size() const
   {
     return size_;
   }
@@ -480,7 +482,7 @@ template<typename T> class MutableSpan {
   /**
    * Replace all elements in the referenced array with the given value.
    */
-  void fill(const T &value)
+  constexpr void fill(const T &value)
   {
     initialized_fill_n(data_, size_, value);
   }
@@ -489,7 +491,7 @@ template<typename T> class MutableSpan {
    * Replace a subset of all elements with the given value. This invokes undefined behavior when
    * one of the indices is out of bounds.
    */
-  void fill_indices(Span<int64_t> indices, const T &value)
+  constexpr void fill_indices(Span<int64_t> indices, const T &value)
   {
     for (int64_t i : indices) {
       BLI_assert(i < size_);
@@ -501,30 +503,30 @@ template<typename T> class MutableSpan {
    * Returns a pointer to the beginning of the referenced array. This may be nullptr, when the size
    * is zero.
    */
-  T *data() const
+  constexpr T *data() const
   {
     return data_;
   }
 
-  T *begin() const
+  constexpr T *begin() const
   {
     return data_;
   }
-  T *end() const
+  constexpr T *end() const
   {
     return data_ + size_;
   }
 
-  std::reverse_iterator<T *> rbegin() const
+  constexpr std::reverse_iterator<T *> rbegin() const
   {
     return std::reverse_iterator<T *>(this->end());
   }
-  std::reverse_iterator<T *> rend() const
+  constexpr std::reverse_iterator<T *> rend() const
   {
     return std::reverse_iterator<T *>(this->begin());
   }
 
-  T &operator[](const int64_t index) const
+  constexpr T &operator[](const int64_t index) const
   {
     BLI_assert(index < this->size());
     return data_[index];
@@ -534,7 +536,7 @@ template<typename T> class MutableSpan {
    * Returns a contiguous part of the array. This invokes undefined behavior when the slice would
    * go out of bounds.
    */
-  MutableSpan slice(const int64_t start, const int64_t length) const
+  constexpr MutableSpan slice(const int64_t start, const int64_t length) const
   {
     BLI_assert(start + length <= this->size());
     return MutableSpan(data_ + start, length);
@@ -544,7 +546,7 @@ template<typename T> class MutableSpan {
    * Returns a new MutableSpan with n elements removed from the beginning. This invokes
    * undefined behavior when the array is too small.
    */
-  MutableSpan drop_front(const int64_t n) const
+  constexpr MutableSpan drop_front(const int64_t n) const
   {
     BLI_assert(n <= this->size());
     return this->slice(n, this->size() - n);
@@ -554,7 +556,7 @@ template<typename T> class MutableSpan {
    * Returns a new MutableSpan with n elements removed from the end. This invokes undefined
    * behavior when the array is too small.
    */
-  MutableSpan drop_back(const int64_t n) const
+  constexpr MutableSpan drop_back(const int64_t n) const
   {
     BLI_assert(n <= this->size());
     return this->slice(0, this->size() - n);
@@ -564,7 +566,7 @@ template<typename T> class MutableSpan {
    * Returns a new MutableSpan that only contains the first n elements. This invokes undefined
    * behavior when the array is too small.
    */
-  MutableSpan take_front(const int64_t n) const
+  constexpr MutableSpan take_front(const int64_t n) const
   {
     BLI_assert(n <= this->size());
     return this->slice(0, n);
@@ -574,7 +576,7 @@ template<typename T> class MutableSpan {
    * Return a new MutableSpan that only contains the last n elements. This invokes undefined
    * behavior when the array is too small.
    */
-  MutableSpan take_back(const int64_t n) const
+  constexpr MutableSpan take_back(const int64_t n) const
   {
     BLI_assert(n <= this->size());
     return this->slice(this->size() - n, n);
@@ -584,7 +586,7 @@ template<typename T> class MutableSpan {
    * Returns an (immutable) Span that references the same array. This is usually not needed,
    * due to implicit conversions. However, sometimes automatic type deduction needs some help.
    */
-  Span<T> as_span() const
+  constexpr Span<T> as_span() const
   {
     return Span<T>(data_, size_);
   }
@@ -593,7 +595,7 @@ template<typename T> class MutableSpan {
    * Utility to make it more convenient to iterate over all indices that can be used with this
    * array.
    */
-  IndexRange index_range() const
+  constexpr IndexRange index_range() const
   {
     return IndexRange(size_);
   }
@@ -602,7 +604,7 @@ template<typename T> class MutableSpan {
    * Returns a reference to the last element. This invokes undefined behavior when the array is
    * empty.
    */
-  T &last() const
+  constexpr T &last() const
   {
     BLI_assert(size_ > 0);
     return data_[size_ - 1];
@@ -612,7 +614,7 @@ template<typename T> class MutableSpan {
    * Does a linear search to count how often the value is in the array.
    * Returns the number of occurrences.
    */
-  int64_t count(const T &value) const
+  constexpr int64_t count(const T &value) const
   {
     int64_t counter = 0;
     for (const T &element : *this) {
@@ -628,7 +630,7 @@ template<typename T> class MutableSpan {
    * destination contains uninitialized data and T is not trivially copy constructible.
    * The size of both spans is expected to be the same.
    */
-  void copy_from(Span<T> values)
+  constexpr void copy_from(Span<T> values)
   {
     BLI_assert(size_ == values.size());
     initialized_copy_n(values.data(), size_, data_);
@@ -637,7 +639,7 @@ template<typename T> class MutableSpan {
   /**
    * Returns a new span to the same underlying memory buffer. No conversions are done.
    */
-  template<typename NewT> MutableSpan<NewT> cast() const
+  template<typename NewT> constexpr MutableSpan<NewT> cast() const
   {
     BLI_assert((size_ * sizeof(T)) % sizeof(NewT) == 0);
     int64_t new_size = size_ * sizeof(T) / sizeof(NewT);
@@ -648,7 +650,7 @@ template<typename T> class MutableSpan {
 /**
  * Utilities to check that arrays have the same size in debug builds.
  */
-template<typename T1, typename T2> void assert_same_size(const T1 &v1, const T2 &v2)
+template<typename T1, typename T2> constexpr void assert_same_size(const T1 &v1, const T2 &v2)
 {
   UNUSED_VARS_NDEBUG(v1, v2);
 #ifdef DEBUG
@@ -659,7 +661,7 @@ template<typename T1, typename T2> void assert_same_size(const T1 &v1, const T2
 }
 
 template<typename T1, typename T2, typename T3>
-void assert_same_size(const T1 &v1, const T2 &v2, const T3 &v3)
+constexpr void assert_same_size(const T1 &v1, const T2 &v2, const T3 &v3)
 {
   UNUSED_VARS_NDEBUG(v1, v2, v3);
 #ifdef DEBUG
diff --git a/source/blender/blenlib/BLI_string_ref.hh b/source/blender/blenlib/BLI_string_ref.hh
index 8597e54d03b..a2562c6100a 100644
--- a/source/blender/blenlib/BLI_string_ref.hh
+++ b/source/blender/blenlib/BLI_string_ref.hh
@@ -64,7 +64,7 @@ class StringRefBase {
   const char *data_;
   int64_t size_;
 
-  StringRefBase(const char *data, const int64_t size) : data_(data), size_(size)
+  constexpr StringRefBase(const char *data, const int64_t size) : data_(data), size_(size)
   {
   }
 
@@ -75,12 +75,12 @@ class StringRefBase {
   /**
    * Return the (byte-)length of the referenced string, without any null-terminator.
    */
-  int64_t size() const
+  constexpr int64_t size() const
   {
     return size_;
   }
 
-  bool is_empty() const
+  constexpr bool is_empty() const
   {
     return size_ == 0;
   }
@@ -88,12 +88,12 @@ class StringRefBase {
   /**
    * Return a pointer to the start of the string.
    */
-  const char *data() const
+  constexpr const char *data() const
   {
     return data_;
   }
 
-  operator Span<char>() const
+  constexpr operator Span<char>() const
   {
     return Span<char>(data_, size_);
   }
@@ -107,22 +107,22 @@ class StringRefBase {
     return std::string(data_, static_cast<size_t>(size_));
   }
 
-  operator std::string_view() const
+  constexpr operator std::string_view() const
   {
     return std::string_view(data_, static_cast<size_t>(size_));
   }
 
-  const char *begin() const
+  constexpr const char *begin() const
   {
     return data_;
   }
 
-  const char *end() const
+  constexpr const char *end() const
   {
     return data_ + size_;
   }
 
-  IndexRange index_range() const
+  constexpr IndexRange index_range() const
   {
     return IndexRange(size_);
   }
@@ -165,19 +165,19 @@ class StringRefBase {
   /**
    * Returns true when the string begins with the given prefix. Otherwise false.
    */
-  bool startswith(StringRef prefix) const;
+  constexpr bool startswith(StringRef prefix) const;
 
   /**
    * Returns true when the string ends with the given suffix. Otherwise false.
    */
-  bool endswith(StringRef suffix) const;
+  constexpr bool endswith(StringRef suffix) const;
 
-  StringRef substr(int64_t start, const int64_t size) const;
+  constexpr StringRef substr(int64_t start, const int64_t size) const;
 
   /**
    * Get the first char in the string. This invokes undefined behavior when the string is empty.
    */
-  const char &front() const
+  constexpr const char &front() const
   {
     BLI_assert(size_ >= 1);
     return data_[0];
@@ -186,7 +186,7 @@ class StringRefBase {
   /**
    * Get the last char in the string. This invokes undefined behavior when the string is empty.
    */
-  const char &back() const
+  constexpr const char &back() const
   {
     BLI_assert(size_ >= 1);
     return data_[size_ - 1];
@@ -196,18 +196,18 @@ class StringRefBase {
    * The behavior of those functions matches the standard library implementation of
    * std::string_view.
    */
-  int64_t find(char c, int64_t pos = 0) const;
-  int64_t find(StringRef str, int64_t pos = 0) const;
-  int64_t rfind(char c, int64_t pos = INT64_MAX) const;
-  int64_t rfind(StringRef str, int64_t pos = INT64_MAX) const;
-  int64_t find_first_of(StringRef chars, int64_t pos = 0) const;
-  int64_t find_first_of(char c, int64_t pos = 0) const;
-  int64_t find_last_of(StringRef chars, int64_t pos = INT64_MAX) const;
-  int64_t find_last_of(char c, int64_t pos = INT64_MAX) const;
-  int64_t find_first_not_of(StringRef chars, int64_t pos = 0) const;
-  int64_t find_first_not_of(char c, int64_t pos = 0) const;
-  int64_t find_last_not_of(StringRef chars, int64_t pos = INT64_MAX) const;
-  int64_t find_last_not_of(char c, int64_t pos = INT64_MAX) const;
+  constexpr int64_t find(char c, int64_t pos = 0) const;
+  constexpr int64_t find(StringRef str, int64_t pos = 0) const;
+  constexpr int64_t rfind(char c, int64_t pos = INT64_MAX) const;
+  constexpr int64_t rfind(StringRef str, int64_t pos = INT64_MAX) const;
+  constexpr int64_t find_first_of(StringRef chars, int64_t pos = 0) const;
+  constexpr int64_t find_first_of(char c, int64_t pos = 0) const;
+  constexpr int64_t find_last_of(StringRef chars, int64_t pos = INT64_MAX) const;
+  constexpr int64_t find_last_of(char c, int64_t pos = INT64_MAX) const;
+  constexpr int64_t find_first_not_of(StringRef chars, int64_t pos = 0) const;
+  constexpr int64_t find_first_not_of(char c, int64_t pos = 0) const;
+  constexpr int64_t find_last_not_of(StringRef chars, int64_t pos = INT64_MAX) const;
+  constexpr int64_t find_last_not_of(char c, int64_t pos = INT64_MAX) const;
 };
 
 /**
@@ -216,7 +216,7 @@ class StringRefBase {
 class StringRefNull : public StringRefBase {
 
  public:
-  StringRefNull() : StringRefBase("", 0)
+  constexpr StringRefNull() : StringRefBase("", 0)
   {
   }
 
@@ -226,7 +226,7 @@ class StringRefNull : public StringRefBase {
    */
   StringRefNull(const char *str) : StringRefBase(str, static_cast<int64_t>(strlen(str)))
   {
-    BLI_assert(str != NULL);
+    BLI_assert(str != nullptr);
     BLI_assert(data_[size_] == '\0');
   }
 
@@ -234,7 +234,7 @@ class StringRefNull : public StringRefBase {
    * Construct a StringRefNull from a null terminated c-string. This invokes undefined behavior
    * when the given size is not the correct size of the string.
    */
-  StringRefNull(const char *str, const int64_t size) : StringRefBase(str, size)
+  constexpr StringRefNull(const char *str, const int64_t size) : StringRefBase(str, size)
   {
     BLI_assert(static_cast<int64_t>(strlen(str)) == size);
   }
@@ -250,7 +250,7 @@ class StringRefNull : public StringRefBase {
   /**
    * Get the char at the given index.
    */
-  char operator[](const int64_t index) const
+  constexpr char operator[](const int64_t index) const
   {
     BLI_assert(index >= 0);
     /* Use '<=' instead of just '<', so that the null character can be accessed as well. */
@@ -263,7 +263,7 @@ class StringRefNull : public StringRefBase {
    *
    * This is like ->data(), but can only be called on a StringRefNull.
    */
-  const char *c_str() const
+  constexpr const char *c_str() const
   {
     return data_;
   }
@@ -274,25 +274,26 @@ class StringRefNull : public StringRefBase {
  */
 class StringRef : public StringRefBase {
  public:
-  StringRef() : StringRefBase(nullptr, 0)
+  constexpr StringRef() : StringRefBase(nullptr, 0)
   {
   }
 
   /**
    * StringRefNull can be converted into StringRef, but not the other way around.
    */
-  StringRef(StringRefNull other) : StringRefBase(other.data(), other.size())
+  constexpr StringRef(StringRefNull other) : StringRefBase(other.data(), other.size())
   {
   }
 
   /**
    * Create a StringRef from a null-terminated c-string.
    */
-  StringRef(const char *str) : StringRefBase(str, str ? static_cast<int64_t>(strlen(str)) : 0)
+  constexpr StringRef(const char *str)
+      : StringRefBase(str, str ? static_cast<int64_t>(std::char_traits<char>::length(str)) : 0)
   {
   }
 
-  StringRef(const char *str, const int64_t length) : StringRefBase(str, length)
+  constexpr StringRef(const char *str, const int64_t length) : StringRefBase(str, length)
   {
   }
 
@@ -300,7 +301,7 @@ class StringRef : public StringRefBase {
    * Create a StringRef from a start and end pointer. This invokes undefined behavior when the
    * second point points to a smaller address than the first one.
    */
-  StringRef(const char *begin, const char *one_after_end)
+  constexpr StringRef(const char *begin, const char *one_after_end)
       : StringRefBase(begin, static_cast<int64_t>(one_after_end - begin))
   {
     BLI_assert(begin <= one_after_end);
@@ -314,7 +315,8 @@ class StringRef : public StringRefBase {
   {
   }
 
-  StringRef(std::string_view view) : StringRefBase(view.data(), static_cast<int64_t>(view.size()))
+  constexpr StringRef(std::string_view view)
+      : StringRefBase(view.data(), static_cast<int64_t>(view.size()))
   {
   }
 
@@ -323,7 +325,7 @@ class StringRef : public StringRefBase {
    *
    * This is similar to std::string_view::remove_prefix.
    */
-  StringRef drop_prefix(const int64_t n) const
+  constexpr StringRef drop_prefix(const int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= size_);
@@ -334,7 +336,7 @@ class StringRef : public StringRefBase {
    * Return a new StringRef with the given prefix being skipped. This invokes undefined behavior if
    * the string does not begin with the given prefix.
    */
-  StringRef drop_prefix(StringRef prefix) const
+  constexpr StringRef drop_prefix(StringRef prefix) const
   {
     BLI_assert(this->startswith(prefix));
     return this->drop_prefix(prefix.size());
@@ -345,7 +347,7 @@ class StringRef : public StringRefBase {
    *
    * This is similar to std::string_view::remove_suffix.
    */
-  StringRef drop_suffix(const int64_t n) const
+  constexpr StringRef drop_suffix(const int64_t n) const
   {
     BLI_assert(n >= 0);
     BLI_assert(n <= size_);
@@ -355,7 +357,7 @@ class StringRef : public StringRefBase {
   /**
    * Get the char at the given index.
    */
-  char operator[](int64_t index) const
+  constexpr char operator[](int64_t index) const
   {
     BLI_assert(index >= 0);
     BLI_assert(index < size_);
@@ -391,7 +393,7 @@ inline std::string operator+(StringRef a, StringRef b)
  * not a problem when std::string_view is only used at api boundaries. To compare a StringRef and a
  * std::string_view, one should convert the std::string_view to StringRef (which is very cheap).
  * Ideally, we only use StringRef in our code to avoid this problem altogether. */
-inline bool operator==(StringRef a, StringRef b)
+constexpr inline bool operator==(StringRef a, StringRef b)
 {
   if (a.size() != b.size()) {
     return false;
@@ -399,27 +401,27 @@ inline bool operator==(StringRef a, StringRef b)
   return STREQLEN(a.data(), b.data(), (size_t)a.size());
 }
 
-inline bool operator!=(StringRef a, StringRef b)
+constexpr inline bool operator!=(StringRef a, StringRef b)
 {
   return !(a == b);
 }
 
-inline bool operator<(StringRef a, StringRef b)
+constexpr inline bool operator<(StringRef a, StringRef b)
 {
   return std::string_view(a) < std::string_view(b);
 }
 
-inline bool operator>(StringRef a, StringRef b)
+constexpr inline bool operator>(StringRef a, StringRef b)
 {
   return std::string_view(a) > std::string_view(b);
 }
 
-inline bool operator<=(StringRef a, StringRef b)
+constexpr inline bool operator<=(StringRef a, StringRef b)
 {
   return std::string_view(a) <= std::string_view(b);
 }
 
-inline bool operator>=(StringRef a, StringRef b)
+constexpr inline bool operator>=(StringRef a, StringRef b)
 {
   return std::string_view(a) >= std::string_view(b);
 }
@@ -427,7 +429,7 @@ inline bool operator>=(StringRef a, StringRef b)
 /**
  * Return true when the string starts with the given prefix.
  */
-inline bool StringRefBase::startswith(StringRef prefix) const
+constexpr inline bool StringRefBase::startswith(StringRef prefix) const
 {
   if (size_ < prefix.size_) {
     return false;
@@ -443,7 +445,7 @@ inline bool StringRefBase::startswith(StringRef prefix) const
 /**
  * Return true when the string ends with the given suffix.
  */
-inline bool StringRefBase::endswith(StringRef suffix) const
+constexpr inline bool StringRefBase::endswith(StringRef suffix) const
 {
   if (size_ < suffix.size_) {
     return false;
@@ -460,8 +462,8 @@ inline bool StringRefBase::endswith(StringRef suffix) const
 /**
  * Return a new #StringRef containing only a sub-string of the original string.
  */
-inline StringRef StringRefBase::substr(const int64_t start,
-                                       const int64_t max_size = INT64_MAX) const
+constexpr inline StringRef StringRefBase::substr(const int64_t start,
+                                                 const int64_t max_size = INT64_MAX) const
 {
   BLI_assert(max_size >= 0);
   BLI_assert(start >= 0);
@@ -469,7 +471,7 @@ inline StringRef StringRefBase::substr(const int64_t start,
   return StringRef(data_ + start, substr_size);
 }
 
-inline int64_t index_or_npos_to_int64(size_t index)
+constexpr inline int64_t index_or_npos_to_int64(size_t index)
 {
   /* The compiler will probably optimize this check away. */
   if (index == std::string_view::npos) {
@@ -478,62 +480,62 @@ inline int64_t index_or_npos_to_int64(size_t index)
   return static_cast<int64_t>(index);
 }
 
-inline int64_t StringRefBase::find(char c, int64_t pos) const
+constexpr inline int64_t StringRefBase::find(char c, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(std::string_view(*this).find(c, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find(StringRef str, int64_t pos) const
+constexpr inline int64_t StringRefBase::find(StringRef str, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(std::string_view(*this).find(str, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find_first_of(StringRef chars, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_first_of(StringRef chars, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(
       std::string_view(*this).find_first_of(chars, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find_first_of(char c, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_first_of(char c, int64_t pos) const
 {
   return this->find_first_of(StringRef(&c, 1), pos);
 }
 
-inline int64_t StringRefBase::find_last_of(StringRef chars, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_last_of(StringRef chars, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(
       std::string_view(*this).find_last_of(chars, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find_last_of(char c, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_last_of(char c, int64_t pos) const
 {
   return this->find_last_of(StringRef(&c, 1), pos);
 }
 
-inline int64_t StringRefBase::find_first_not_of(StringRef chars, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_first_not_of(StringRef chars, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(
       std::string_view(*this).find_first_not_of(chars, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find_first_not_of(char c, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_first_not_of(char c, int64_t pos) const
 {
   return this->find_first_not_of(StringRef(&c, 1), pos);
 }
 
-inline int64_t StringRefBase::find_last_not_of(StringRef chars, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_last_not_of(StringRef chars, int64_t pos) const
 {
   BLI_assert(pos >= 0);
   return index_or_npos_to_int64(
       std::string_view(*this).find_last_not_of(chars, static_cast<size_t>(pos)));
 }
 
-inline int64_t StringRefBase::find_last_not_of(char c, int64_t pos) const
+constexpr inline int64_t StringRefBase::find_last_not_of(char c, int64_t pos) const
 {
   return this->find_last_not_of(StringRef(&c, 1), pos);
 }
diff --git a/source/blender/blenlib/tests/BLI_index_range_test.cc b/source/blender/blenlib/tests/BLI_index_range_test.cc
index d472ded0f18..ddaa067f50e 100644
--- a/source/blender/blenlib/tests/BLI_index_range_test.cc
+++ b/source/blender/blenlib/tests/BLI_index_range_test.cc
@@ -140,4 +140,11 @@ TEST(index_range, AsSpan)
   EXPECT_EQ(span[3], 7);
 }
 
+TEST(index_range, constexpr_)
+{
+  constexpr IndexRange range = IndexRange(1, 1);
+  std::array<int, range[0]> compiles = {1};
+  BLI_STATIC_ASSERT(range.size() == 1, "");
+  EXPECT_EQ(compiles[0], 1);
+}
 }  // namespace blender::tests
diff --git a/source/blender/blenlib/tests/BLI_span_test.cc b/source/blender/blenlib/tests/BLI_span_test.cc
index 9a8d9df7873..d1c9f312b97 100644
--- a/source/blender/blenlib/tests/BLI_span_test.cc
+++ b/source/blender/blenlib/tests/BLI_span_test.cc
@@ -337,4 +337,19 @@ TEST(span, MutableReverseIterator)
   EXPECT_EQ_ARRAY(src.data(), Span({14, 15, 16, 17}).data(), 4);
 }
 
+TEST(span, constexpr_)
+{
+  static constexpr std::array<int, 3> src = {3, 2, 1};
+  constexpr Span<int> span(src);
+  BLI_STATIC_ASSERT(span[2] == 1, "");
+  BLI_STATIC_ASSERT(span.size() == 3, "");
+  BLI_STATIC_ASSERT(span.slice(1, 2).size() == 2, "");
+  BLI_STATIC_ASSERT(span.has_duplicates__linear_search() == false, "");
+
+  std::integral_constant<bool, span.first_index(1) == 2> ic;
+  BLI_STATIC_ASSERT(ic.value, "");
+
+  EXPECT_EQ(span.slice(1, 2).size(), 2);
+}
+
 }  // namespace blender::tests
diff --git a/source/blender/blenlib/tests/BLI_string_ref_test.cc b/source/blender/blenlib/tests/BLI_string_ref_test.cc
index 2d488feff71..401a7bc1118 100644
--- a/source/blender/blenlib/tests/BLI_string_ref_test.cc
+++ b/source/blender/blenlib/tests/BLI_string_ref_test.cc
@@ -298,4 +298,12 @@ TEST(string_ref, ToStringView)
   EXPECT_EQ(view, "hello");
 }
 
+TEST(string_ref, constexpr_)
+{
+  constexpr StringRef sref("World");
+  BLI_STATIC_ASSERT(sref[2] == 'r', "");
+  BLI_STATIC_ASSERT(sref.size() == 5, "");
+  std::array<int, static_cast<std::size_t>(sref.find_first_of('o'))> compiles = {1};
+  EXPECT_EQ(compiles[0], 1);
+}
 }  // namespace blender::tests
-- 
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