src: use C++20 consteval for FastStringKey

This makes it easier to avoid unintentional mis-usage of the API.

PR-URL: #59148
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Ilyas Shabi <ilyasshabi94@gmail.com>

Author: addaleax

src/util-inl.h

@@ -598,7 +598,14 @@ constexpr bool FastStringKey::operator==(const FastStringKey& other) const {
   return name_ == other.name_;
 }
 
-constexpr FastStringKey::FastStringKey(std::string_view name)
+consteval FastStringKey::FastStringKey(std::string_view name)
+    : FastStringKey(name, 0) {}
+
+constexpr FastStringKey FastStringKey::AllowDynamic(std::string_view name) {
+  return FastStringKey(name, 0);
+}
+
+constexpr FastStringKey::FastStringKey(std::string_view name, int dummy)
     : name_(name), cached_hash_(HashImpl(name)) {}
 
 constexpr std::string_view FastStringKey::as_string_view() const {

src/util.h

@@ -817,7 +817,11 @@ class PersistentToLocal {
 // computations.
 class FastStringKey {
  public:
-  constexpr explicit FastStringKey(std::string_view name);
+  // consteval ensures that the argument is a compile-time constant.
+  consteval explicit FastStringKey(std::string_view name);
+  // passing something that is not a compile-time constant needs explicit
+  // opt-in via this helper, as it defeats the purpose of FastStringKey.
+  static constexpr FastStringKey AllowDynamic(std::string_view name);
 
   struct Hash {
     constexpr size_t operator()(const FastStringKey& key) const;
@@ -827,6 +831,8 @@ class FastStringKey {
   constexpr std::string_view as_string_view() const;
 
  private:
+  constexpr explicit FastStringKey(std::string_view name, int dummy);
+
   static constexpr size_t HashImpl(std::string_view str);
 
   const std::string_view name_;