AK: Allow constructing a UTF-16 view from a UTF-16 string literal

UTF-16 string literals are a language-level feature. It is convenient to
be able to construct a Utf16View from these strings.

AK/Utf16View.h

@@ -71,6 +71,14 @@ public:
     {
     }
 
+    template<size_t Size>
+    Utf16View(char16_t const (&code_units)[Size])
+        : m_code_units(
+            reinterpret_cast<u16 const*>(&code_units[0]),
+            code_units[Size - 1] == u'\0' ? Size - 1 : Size)
+    {
+    }
+
     bool operator==(Utf16View const& other) const { return m_code_units == other.m_code_units; }
 
     enum class AllowInvalidCodeUnits {

Tests/AK/TestUtf16.cpp

@@ -89,6 +89,36 @@ TEST_CASE(decode_utf16)
     EXPECT_EQ(i, expected.size());
 }
 
+TEST_CASE(utf16_literal)
+{
+    {
+        Utf16View view { u"" };
+        EXPECT(view.validate());
+        EXPECT_EQ(view.length_in_code_units(), 0u);
+    }
+    {
+        Utf16View view { u"a" };
+        EXPECT(view.validate());
+        EXPECT_EQ(view.length_in_code_units(), 1u);
+        EXPECT_EQ(view.code_unit_at(0), 0x61u);
+    }
+    {
+        Utf16View view { u"abc" };
+        EXPECT(view.validate());
+        EXPECT_EQ(view.length_in_code_units(), 3u);
+        EXPECT_EQ(view.code_unit_at(0), 0x61u);
+        EXPECT_EQ(view.code_unit_at(1), 0x62u);
+        EXPECT_EQ(view.code_unit_at(2), 0x63u);
+    }
+    {
+        Utf16View view { u"🙃" };
+        EXPECT(view.validate());
+        EXPECT_EQ(view.length_in_code_units(), 2u);
+        EXPECT_EQ(view.code_unit_at(0), 0xd83du);
+        EXPECT_EQ(view.code_unit_at(1), 0xde43u);
+    }
+}
+
 TEST_CASE(iterate_utf16)
 {
     auto string = MUST(AK::utf8_to_utf16("Привет 😀"sv));