Bug 1880008 - Add char8_t string overloads to MFBT string functions when compiling with -std=c++20. r=xpcom-reviewers,emilio

When compiled with -std=c++17, u8"" string literals have type `const char[]`. When compiled with -std=c++20, u8"" string literals have type `const char8_t[]`. This patch adds the minimum char8_t overloads needed to compile Firefox with -std=c++20. If we want to use char8_t in more code or replace `Utf8Unit` with char8_t, that will require an extensive redesign of MFBT's and xpcom's string classes after we update from -std=c++17 to c++20.

Differential Revision: https://phabricator.services.mozilla.com/D201672

mfbt/TextUtils.h

@@ -43,6 +43,13 @@ inline constexpr bool IsAscii(char aChar) {
   return IsAscii(static_cast<unsigned char>(aChar));
 }
 
+#ifdef __cpp_char8_t
+/** Returns true iff |aChar| is ASCII, i.e. in the range [0, 0x80). */
+inline constexpr bool IsAscii(char8_t aChar) {
+  return IsAscii(static_cast<unsigned char>(aChar));
+}
+#endif
+
 /** Returns true iff |aChar| is ASCII, i.e. in the range [0, 0x80). */
 inline constexpr bool IsAscii(char16_t aChar) { return aChar < 0x80; }
 

mfbt/Utf8.h

@@ -176,6 +176,11 @@ union Utf8Unit {
     // assume the conversion does what we want it to.
   }
 
+#ifdef __cpp_char8_t
+  explicit constexpr Utf8Unit(char8_t aUnit)
+      : mValue(static_cast<char>(aUnit)) {}
+#endif
+
   constexpr bool operator==(const Utf8Unit& aOther) const {
     return mValue == aOther.mValue;
   }

xpcom/string/nsTStringRepr.h

@@ -240,6 +240,18 @@ class nsTStringRepr {
    */
   bool EqualsIgnoreCase(const std::string_view& aString) const;
 
+#ifdef __cpp_char8_t
+  template <typename Q = T, typename EnableIfChar = mozilla::CharOnlyT<Q>>
+  bool NS_FASTCALL Equals(const char8_t* aData) const {
+    return Equals(reinterpret_cast<const char*>(aData));
+  }
+
+  template <typename Q = T, typename EnableIfChar = mozilla::CharOnlyT<Q>>
+  bool NS_FASTCALL Equals(const char8_t* aData, comparator_type aComp) const {
+    return Equals(reinterpret_cast<const char*>(aData), aComp);
+  }
+#endif
+
 #if defined(MOZ_USE_CHAR16_WRAPPER)
   template <typename Q = T, typename EnableIfChar16 = Char16OnlyT<Q>>
   bool NS_FASTCALL Equals(char16ptr_t aData) const {