AK: Add SourceLocation support

C++20 added std::source_location, which lets you capture the
callers __FILE__ / __LINE__ / __FUNCTION__ etc as a default
argument to functions.
See: https://en.cppreference.com/w/cpp/utility/source_location

During a bug investigation @ADKaster suggested we could use this
to make the LOCK_DEBUG feature of the kernel more user friendly
and allow it to automatically instrument all call sites.

We then implemented / tested it over discord. :^)

Co-Authored-by: Andrew Kaster <andrewdkaster@gmail.com>

AK/SourceLocation.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2021, Andrew Kaster <andrewdkaster@gmail.com>
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+class SourceLocation {
+public:
+    [[nodiscard]] constexpr StringView function_name() const { return StringView(m_function); }
+    [[nodiscard]] constexpr StringView file_name() const { return StringView(m_file); }
+    [[nodiscard]] constexpr u32 line_number() const { return m_line; }
+
+    [[nodiscard]] static constexpr SourceLocation current(const char* const file = __builtin_FILE(), u32 line = __builtin_LINE(), const char* const function = __builtin_FUNCTION())
+    {
+        return SourceLocation(file, line, function);
+    }
+
+private:
+    constexpr SourceLocation(const char* const file, u32 line, const char* const function)
+        : m_function(function)
+        , m_file(file)
+        , m_line(line)
+    {
+    }
+
+    const char* const m_function { nullptr };
+    const char* const m_file { nullptr };
+    const u32 m_line { 0 };
+};
+
+}
+
+using AK::SourceLocation;

AK/Tests/CMakeLists.txt

@@ -45,6 +45,7 @@ set(AK_TEST_SOURCES
     TestRefPtr.cpp
     TestSinglyLinkedList.cpp
     TestSourceGenerator.cpp
+    TestSourceLocation.cpp
     TestSpan.cpp
     TestString.cpp
     TestStringUtils.cpp

AK/Tests/TestSourceLocation.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Andrew Kaster <andrewdkaster@gmail.com>
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/SourceLocation.h>
+#include <AK/StringView.h>
+#include <AK/TestSuite.h>
+
+TEST_CASE(basic_scenario)
+{
+    auto location = SourceLocation::current();
+    EXPECT_EQ(StringView(__FILE__), location.file_name());
+    EXPECT_EQ(StringView(__FUNCTION__), location.function_name());
+    EXPECT_EQ(__LINE__ - 3u, location.line_number());
+}
+
+static StringView test_default_arg(const SourceLocation& loc = SourceLocation::current())
+{
+    return loc.function_name();
+}
+
+TEST_CASE(default_arg_scenario)
+{
+    auto actual_calling_function = test_default_arg();
+    auto expected_calling_function = StringView(__FUNCTION__);
+
+    EXPECT_EQ(expected_calling_function, actual_calling_function);
+}
+
+TEST_MAIN(SourceLocation)