Add a _CharSpan string literal operator

This allows constexpr CharSpans to be created.

src/lib/core/Unchecked.h

@@ -0,0 +1,36 @@
+/*
+ *
+ *    Copyright (c) 2023 Project CHIP Authors
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+/**
+ *    @file
+ *      This file defines the chip::Optional class to handle values which may
+ *      or may not be present.
+ *
+ */
+#pragma once
+
+namespace chip {
+
+/// Unchecked is disambiguation tag that can be used to provide and select a variant of a
+/// constructor or other method that omits the runtime checks performed by the default variant.
+struct UncheckedType
+{
+    explicit UncheckedType() = default;
+};
+inline constexpr UncheckedType Unchecked{};
+
+} // namespace chip

src/lib/support/Span.h

@@ -23,6 +23,7 @@
 #include <string.h>
 #include <type_traits>
 
+#include <lib/core/Unchecked.h>
 #include <lib/support/CodeUtils.h>
 
 namespace chip {
@@ -149,7 +150,10 @@ class Span
         return Span(reinterpret_cast<T *>(&bytes[1]), length);
     }
 
-    // Allow creating CharSpans from a character string.
+    // Creates a CharSpans from a null-terminated C character string.
+    //
+    // Note that for string literals, the user-defined `_CharSpan` string
+    // literal operator should be used instead, e.g. `"Hello"_CharSpan`.
     template <class U, typename = std::enable_if_t<std::is_same<T, const U>::value && std::is_same<const char, T>::value>>
     static Span fromCharString(U * chars)
     {
@@ -165,8 +169,18 @@ class Span
 private:
     pointer mDataBuf;
     size_t mDataLen;
+
+    constexpr Span(UncheckedType tag, pointer databuf, size_t datalen) : mDataBuf(databuf), mDataLen(datalen) {}
+
+    friend constexpr Span<char const> operator"" _CharSpan(char const *, size_t);
 };
 
+// Creates a CharSpan from a string literal.
+constexpr Span<char const> operator"" _CharSpan(char const * literal, size_t size)
+{
+    return Span<char const>(Unchecked, literal, size);
+}
+
 namespace detail {
 
 // To make FixedSpan (specifically various FixedByteSpan types) default constructible

src/lib/support/tests/TestSpan.cpp

@@ -291,6 +291,14 @@ static void TestFromCharString(nlTestSuite * inSuite, void * inContext)
     NL_TEST_ASSERT(inSuite, s1.data_equal(CharSpan(str, 3)));
 }
 
+static void TestLiteral(nlTestSuite * inSuite, void * inContext)
+{
+    constexpr CharSpan literal = "HI!"_CharSpan;
+    NL_TEST_ASSERT(inSuite, literal.size() == 3);
+    NL_TEST_ASSERT(inSuite, literal.data_equal(CharSpan::fromCharString("HI!")));
+    NL_TEST_ASSERT(inSuite, ""_CharSpan.size() == 0);
+}
+
 static void TestConversionConstructors(nlTestSuite * inSuite, void * inContext)
 {
     struct Foo
@@ -330,6 +338,7 @@ static const nlTest sTests[] = {
     NL_TEST_DEF_FN(TestSubSpan),
     NL_TEST_DEF_FN(TestFromZclString),
     NL_TEST_DEF_FN(TestFromCharString),
+    NL_TEST_DEF_FN(TestLiteral),
     NL_TEST_DEF_FN(TestConversionConstructors),
     NL_TEST_SENTINEL(),
 };