[wpilib] Make Color::HexString() constexpr (#5985)

Related improvements to wpi::ct_string:
* Implicitly convert to std::string
* Add operator== for std::string, std::string_view, and const Char*

wpiutil/src/main/native/include/wpi/ct_string.h

@@ -31,7 +31,7 @@ struct ct_string {
 
   template <size_t M>
     requires(M <= (N + 1))
-  consteval ct_string(Char const (&s)[M]) {  // NOLINT
+  constexpr ct_string(Char const (&s)[M]) {  // NOLINT
     if constexpr (M == (N + 1)) {
       if (s[N] != Char{}) {
         throw std::logic_error{"char array not null terminated"};
@@ -50,7 +50,7 @@ struct ct_string {
     }
   }
 
-  explicit consteval ct_string(std::basic_string_view<Char, Traits> s) {
+  explicit constexpr ct_string(std::basic_string_view<Char, Traits> s) {
     // avoid dependency on <algorithm>
     // auto p = std::ranges::copy(s, chars.begin()).out;
     auto p = chars.begin();
@@ -63,6 +63,64 @@ struct ct_string {
     }
   }
 
+  constexpr bool operator==(const ct_string<Char, Traits, N>&) const = default;
+
+  constexpr bool operator==(const std::basic_string<Char, Traits>& rhs) const {
+    if (size() != rhs.size()) {
+      return false;
+    }
+
+    for (size_t i = 0; i < size(); ++i) {
+      if (chars[i] != rhs[i]) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  constexpr bool operator==(std::basic_string_view<Char, Traits> rhs) const {
+    if (size() != rhs.size()) {
+      return false;
+    }
+
+    for (size_t i = 0; i < size(); ++i) {
+      if (chars[i] != rhs[i]) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  template <size_t M>
+    requires(N + 1 == M)
+  constexpr bool operator==(Char const (&rhs)[M]) const {
+    for (size_t i = 0; i < M; ++i) {
+      if (chars[i] != rhs[i]) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  constexpr bool operator==(const Char* rhs) const {
+    for (size_t i = 0; i < N + 1; ++i) {
+      if (chars[i] != rhs[i]) {
+        return false;
+      }
+
+      // If index of rhs's null terminator is less than lhs's size - 1, rhs is
+      // shorter than lhs
+      if (rhs[i] == '\0' && i < N) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
   constexpr auto size() const noexcept { return N; }
 
   constexpr auto begin() const noexcept { return chars.begin(); }
@@ -71,6 +129,11 @@ struct ct_string {
   constexpr auto data() const noexcept { return chars.data(); }
   constexpr auto c_str() const noexcept { return chars.data(); }
 
+  constexpr operator std::basic_string<Char, Traits>()  // NOLINT
+      const noexcept {
+    return std::basic_string<Char, Traits>{chars.data(), N};
+  }
+
   constexpr operator std::basic_string_view<Char, Traits>()  // NOLINT
       const noexcept {
     return std::basic_string_view<Char, Traits>{chars.data(), N};
@@ -82,13 +145,13 @@ ct_string(Char const (&s)[M]) -> ct_string<Char, std::char_traits<Char>, M - 1>;
 
 inline namespace literals {
 template <ct_string S>
-consteval auto operator""_ct_string() {
+constexpr auto operator""_ct_string() {
   return S;
 }
 }  // namespace literals
 
 template <typename Char, typename Traits, size_t N1, size_t N2>
-consteval auto operator+(ct_string<Char, Traits, N1> const& s1,
+constexpr auto operator+(ct_string<Char, Traits, N1> const& s1,
                          ct_string<Char, Traits, N2> const& s2) noexcept {
   return Concat(s1, s2);
 }
@@ -102,7 +165,7 @@ consteval auto operator+(ct_string<Char, Traits, N1> const& s1,
  * @return concatenated string
  */
 template <typename Char, typename Traits, size_t N1, size_t... N>
-consteval auto Concat(ct_string<Char, Traits, N1> const& s1,
+constexpr auto Concat(ct_string<Char, Traits, N1> const& s1,
                       ct_string<Char, Traits, N> const&... s) {
   // Need a dummy array to instantiate a ct_string.
   constexpr Char dummy[1] = {};
@@ -136,7 +199,7 @@ consteval auto Concat(ct_string<Char, Traits, N1> const& s1,
 template <intmax_t N, int Base = 10, typename Char = char,
           typename Traits = std::char_traits<Char>>
   requires(Base >= 2 && Base <= 36)
-consteval auto NumToCtString() {
+constexpr auto NumToCtString() {
   constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 
   auto buflen = [] {