ICU-23192 Add a FixedString(std::string_view) constructor.

Author: roubert

icu4c/source/common/fixedstring.h

@@ -30,6 +30,14 @@ class FixedString : public UMemory {
 
     FixedString(const FixedString&) = delete;
 
+    FixedString(std::string_view init) {
+        size_t size = init.size();
+        if (reserve(size + 1)) {
+            uprv_memcpy(ptr, init.data(), size);
+            ptr[size] = '\0';
+        }
+    }
+
     FixedString& operator=(const FixedString& other) {
         *this = other.data();
         return *this;

icu4c/source/test/intltest/strtest.cpp

@@ -888,6 +888,7 @@ void
 StringTest::TestFixedString() {
     static_assert(std::is_default_constructible_v<FixedString>);
     static_assert(!std::is_copy_constructible_v<FixedString>);
+    static_assert(std::is_constructible_v<FixedString, std::string_view>);
     static_assert(std::is_copy_assignable_v<FixedString>);
     static_assert(std::is_assignable_v<FixedString, std::string_view>);
     static_assert(!std::is_move_constructible_v<FixedString>);
@@ -910,6 +911,11 @@ StringTest::TestFixedString() {
 
     static constexpr char text[] = "foo";
 
+    FixedString init(text);
+    assertFalse("initialized is empty", init.isEmpty());
+    assertFalse("initialized alias is nullptr", init.getAlias() == nullptr);
+    assertEquals("initialized data is text", text, init.data());
+
     s = text;
     assertFalse("assigned is empty", s.isEmpty());
     assertFalse("assigned alias is nullptr", s.getAlias() == nullptr);