Enable FixedString construction from a string_view

Summary: `FixedString` should be constructable from a `std::string_view`. This allows passing `std::string_view` as key to `.emplace` in heterogeneous maps where the key is a `FixedString`.

Reviewed By: yfeldblum

Differential Revision: D29456948

fbshipit-source-id: 4d2e428c7de05c1dc34b50727f8107aec915b632

folly/FixedString.h

@@ -669,6 +669,21 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
       : BasicFixedString{
             that, detail::fixedstring::checkOverflow(count, N), Indices{}} {}
 
+#if FOLLY_HAS_STRING_VIEW
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct from a `std::basic_string_view<Char>`
+   * \param that The source basic_string_view
+   * \pre `that.size() <= N`
+   * \post `size() == that.size()`
+   * \post `0 == strncmp(data(), that.begin(), size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when that.size() > N
+   */
+  constexpr /* implicit */ BasicFixedString(
+      std::basic_string_view<Char> that) noexcept(false)
+      : BasicFixedString{that.data(), that.size()} {}
+#endif
+
   /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
    * Construct an BasicFixedString that contains `count` characters, all
    *   of which are `ch`.

folly/test/FixedStringTest.cpp

@@ -109,6 +109,16 @@ TEST(FixedStringCtorTest, FromStringOffsetAndCount) {
   // constexpr folly::FixedString<5> s4{s, 6, 6};
 }
 
+#if FOLLY_HAS_STRING_VIEW
+TEST(FixedStringCtorTest, FromStringView) {
+  constexpr folly::FixedString<11> s{
+      std::string_view{"hello world"},
+  };
+  static_assert(s == "hello world", "");
+  static_assert(s.size() == 11u, "");
+}
+#endif
+
 TEST(FixedStringCtorTest, FromInitializerList) {
   constexpr folly::FixedString<11> s{
       'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'};