AK+Userland: Extend the compiletime format string check to other functions

Thanks to @trflynn89 for the neat implicit consteval ctor trick!
This allows us to basically slap `CheckedFormatString` on any
formatting function, and have its format argument checked at compiletime.

Note that there is a validator bug where it doesn't parse inner replaced
fields like `{:~>{}}` correctly (what should be 'left align with next
argument as size' is parsed as `{:~>{` following a literal closing
brace), so the compiletime checks are disabled on these temporarily by
forcing them to be StringViews.

This commit also removes the now unused `AK::StringLiteral` type (which
was introduced for use with NTTP strings).

AK/CheckedFormatString.h

@@ -0,0 +1,242 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/AllOf.h>
+#include <AK/AnyOf.h>
+#include <AK/StdLibExtras.h>
+#include <AK/StringView.h>
+
+#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
+// FIXME: Seems like clang doesn't like calling 'consteval' functions inside 'consteval' functions quite the same way as GCC does,
+//        it seems to entirely forget that it accepted that parameters to a 'consteval' function to begin with.
+#    ifdef __clang__
+#        define DBGLN_NO_COMPILETIME_FORMAT_CHECK
+#    endif
+#endif
+
+#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
+namespace AK::Format::Detail {
+
+// We have to define a local "purely constexpr" Array that doesn't lead back to us (via e.g. ASSERT)
+template<typename T, size_t Size>
+struct Array {
+    constexpr static size_t size() { return Size; }
+    constexpr const T& operator[](size_t index) const { return __data[index]; }
+    constexpr T& operator[](size_t index) { return __data[index]; }
+    using ConstIterator = SimpleIterator<const Array, const T>;
+    using Iterator = SimpleIterator<Array, T>;
+
+    constexpr ConstIterator begin() const { return ConstIterator::begin(*this); }
+    constexpr Iterator begin() { return Iterator::begin(*this); }
+
+    constexpr ConstIterator end() const { return ConstIterator::end(*this); }
+    constexpr Iterator end() { return Iterator::end(*this); }
+
+    T __data[Size];
+};
+
+template<typename... Args>
+void compiletime_fail(Args...);
+
+template<size_t N>
+consteval auto extract_used_argument_index(const char (&fmt)[N], size_t specifier_start_index, size_t specifier_end_index, size_t& next_implicit_argument_index)
+{
+    struct {
+        size_t index_value { 0 };
+        bool saw_explicit_index { false };
+    } state;
+    for (size_t i = specifier_start_index; i < specifier_end_index; ++i) {
+        auto c = fmt[i];
+        if (c > '9' || c < '0')
+            break;
+
+        state.index_value *= 10;
+        state.index_value += c - '0';
+        state.saw_explicit_index = true;
+    }
+
+    if (!state.saw_explicit_index)
+        return next_implicit_argument_index++;
+
+    return state.index_value;
+}
+
+// FIXME: We should rather parse these format strings at compile-time if possible.
+template<size_t N>
+consteval auto count_fmt_params(const char (&fmt)[N])
+{
+    struct {
+        // FIXME: Switch to variable-sized storage whenever we can come up with one :)
+        Array<size_t, 128> used_arguments { 0 };
+        size_t total_used_argument_count { 0 };
+        size_t next_implicit_argument_index { 0 };
+        bool has_explicit_argument_references { false };
+
+        size_t unclosed_braces { 0 };
+        size_t extra_closed_braces { 0 };
+
+        Array<size_t, 4> last_format_specifier_start { 0 };
+        size_t total_used_last_format_specifier_start_count { 0 };
+    } result;
+
+    for (size_t i = 0; i < N; ++i) {
+        auto ch = fmt[i];
+        switch (ch) {
+        case '{':
+            if (i + 1 < N && fmt[i + 1] == '{') {
+                ++i;
+                continue;
+            }
+
+            // Note: There's no compile-time throw, so we have to abuse a compile-time string to store errors.
+            if (result.total_used_last_format_specifier_start_count >= result.last_format_specifier_start.size() - 1)
+                compiletime_fail("Format-String Checker internal error: Format specifier nested too deep");
+
+            result.last_format_specifier_start[result.total_used_last_format_specifier_start_count++] = i + 1;
+
+            ++result.unclosed_braces;
+            break;
+        case '}':
+            if (i + 1 < N && fmt[i + 1] == '}') {
+                ++i;
+                continue;
+            }
+            if (result.unclosed_braces) {
+                --result.unclosed_braces;
+
+                if (result.total_used_last_format_specifier_start_count == 0)
+                    compiletime_fail("Format-String Checker internal error: Expected location information");
+
+                const auto specifier_start_index = result.last_format_specifier_start[--result.total_used_last_format_specifier_start_count];
+
+                if (result.total_used_argument_count >= result.used_arguments.size())
+                    compiletime_fail("Format-String Checker internal error: Too many format arguments in format string");
+
+                auto used_argument_index = extract_used_argument_index<N>(fmt, specifier_start_index, i, result.next_implicit_argument_index);
+                if (used_argument_index + 1 != result.next_implicit_argument_index)
+                    result.has_explicit_argument_references = true;
+                result.used_arguments[result.total_used_argument_count++] = used_argument_index;
+
+            } else {
+                ++result.extra_closed_braces;
+            }
+            break;
+        default:
+            continue;
+        }
+    }
+    return result;
+}
+}
+
+#endif
+
+namespace AK::Format::Detail {
+template<typename... Args>
+struct CheckedFormatString {
+    template<size_t N>
+    consteval CheckedFormatString(const char (&fmt)[N])
+        : m_string { fmt }
+    {
+#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
+        check_format_parameter_consistency<N, sizeof...(Args)>(fmt);
+#endif
+    }
+
+    template<typename T>
+    CheckedFormatString(const T& unchecked_fmt) requires(requires(T t) { StringView { t }; })
+        : m_string(unchecked_fmt)
+    {
+    }
+
+    auto view() const { return m_string; }
+
+private:
+#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
+    template<size_t N, size_t param_count>
+    consteval static bool check_format_parameter_consistency(const char (&fmt)[N])
+    {
+        auto check = count_fmt_params<N>(fmt);
+        if (check.unclosed_braces != 0)
+            compiletime_fail("Extra unclosed braces in format string");
+        if (check.extra_closed_braces != 0)
+            compiletime_fail("Extra closing braces in format string");
+
+        {
+            auto begin = check.used_arguments.begin();
+            auto end = check.used_arguments.begin() + check.total_used_argument_count;
+            auto has_all_referenced_arguments = !AK::any_of(begin, end, [](auto& entry) { return entry >= param_count; });
+            if (!has_all_referenced_arguments)
+                compiletime_fail("Format string references nonexistent parameter");
+        }
+
+        if (!check.has_explicit_argument_references && check.total_used_argument_count != param_count)
+            compiletime_fail("Format string does not reference all passed parameters");
+
+        // Ensure that no passed parameter is ignored or otherwise not referenced in the format
+        // As this check is generally pretty expensive, try to avoid it where it cannot fail.
+        // We will only do this check if the format string has explicit argument refs
+        // otherwise, the check above covers this check too, as implicit refs
+        // monotonically increase, and cannot have 'gaps'.
+        if (check.has_explicit_argument_references) {
+            auto all_parameters = iota_array<size_t, param_count>(0);
+            constexpr auto contains = [](auto begin, auto end, auto entry) {
+                for (; begin != end; begin++) {
+                    if (*begin == entry)
+                        return true;
+                }
+
+                return false;
+            };
+            auto references_all_arguments = AK::all_of(
+                all_parameters.begin(),
+                all_parameters.end(),
+                [&](auto& entry) {
+                    return contains(
+                        check.used_arguments.begin(),
+                        check.used_arguments.begin() + check.total_used_argument_count,
+                        entry);
+                });
+            if (!references_all_arguments)
+                compiletime_fail("Format string does not reference all passed parameters");
+        }
+
+        return true;
+    }
+#endif
+
+    StringView m_string;
+};
+}
+
+namespace AK {
+
+template<typename... Args>
+using CheckedFormatString = Format::Detail::CheckedFormatString<typename IdentityType<Args>::Type...>;
+
+}

AK/Format.h

@@ -26,6 +26,8 @@
 
 #pragma once
 
+#include <AK/CheckedFormatString.h>
+
 #include <AK/AllOf.h>
 #include <AK/AnyOf.h>
 #include <AK/Array.h>
@@ -37,161 +39,6 @@
 #    include <stdio.h>
 #endif
 
-#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
-// Note: Clang 12 adds support for CTAD, but still fails to build the dbgln() checks, so they're disabled altogether for now.
-// See https://oss-fuzz-build-logs.storage.googleapis.com/log-79750138-f41e-4f39-8812-7c536f1d2e35.txt, for example.
-#    if defined(__clang__)
-#        define DBGLN_NO_COMPILETIME_FORMAT_CHECK
-#    endif
-#endif
-
-#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
-namespace {
-
-template<size_t N>
-consteval auto extract_used_argument_index(const char (&fmt)[N], size_t specifier_start_index, size_t specifier_end_index, size_t& next_implicit_argument_index)
-{
-    struct {
-        size_t index_value { 0 };
-        bool saw_explicit_index { false };
-    } state;
-    for (size_t i = specifier_start_index; i < specifier_end_index; ++i) {
-        auto c = fmt[i];
-        if (c > '9' || c < '0')
-            break;
-
-        state.index_value *= 10;
-        state.index_value += c - '0';
-        state.saw_explicit_index = true;
-    }
-
-    if (!state.saw_explicit_index)
-        return next_implicit_argument_index++;
-
-    return state.index_value;
-}
-
-// FIXME: We should rather parse these format strings at compile-time if possible.
-template<size_t N>
-consteval auto count_fmt_params(const char (&fmt)[N])
-{
-    struct {
-        // FIXME: Switch to variable-sized storage whenever we can come up with one :)
-        Array<size_t, 128> used_arguments { 0 };
-        size_t total_used_argument_count { 0 };
-        size_t next_implicit_argument_index { 0 };
-        bool has_explicit_argument_references { false };
-
-        size_t unclosed_braces { 0 };
-        size_t extra_closed_braces { 0 };
-
-        Array<size_t, 4> last_format_specifier_start { 0 };
-        size_t total_used_last_format_specifier_start_count { 0 };
-
-        StringLiteral<128> internal_error { { 0 } };
-    } result;
-
-    for (size_t i = 0; i < N; ++i) {
-        auto ch = fmt[i];
-        switch (ch) {
-        case '{':
-            if (i + 1 < N && fmt[i + 1] == '{') {
-                ++i;
-                continue;
-            }
-
-            // Note: There's no compile-time throw, so we have to abuse a compile-time string to store errors.
-            if (result.total_used_last_format_specifier_start_count >= result.last_format_specifier_start.size() - 1)
-                result.internal_error = "Format-String Checker internal error: Format specifier nested too deep";
-
-            result.last_format_specifier_start[result.total_used_last_format_specifier_start_count++] = i + 1;
-
-            ++result.unclosed_braces;
-            break;
-        case '}':
-            if (i + 1 < N && fmt[i + 1] == '}') {
-                ++i;
-                continue;
-            }
-            if (result.unclosed_braces) {
-                --result.unclosed_braces;
-
-                if (result.total_used_last_format_specifier_start_count == 0)
-                    result.internal_error = "Format-String Checker internal error: Expected location information";
-
-                const auto specifier_start_index = result.last_format_specifier_start[--result.total_used_last_format_specifier_start_count];
-
-                if (result.total_used_argument_count >= result.used_arguments.size())
-                    result.internal_error = "Format-String Checker internal error: Too many format arguments in format string";
-
-                auto used_argument_index = extract_used_argument_index<N>(fmt, specifier_start_index, i, result.next_implicit_argument_index);
-                if (used_argument_index + 1 != result.next_implicit_argument_index)
-                    result.has_explicit_argument_references = true;
-                result.used_arguments[result.total_used_argument_count++] = used_argument_index;
-
-            } else {
-                ++result.extra_closed_braces;
-            }
-            break;
-        default:
-            continue;
-        }
-    }
-    return result;
-}
-}
-
-template<size_t N, StringLiteral<N> fmt, size_t param_count, auto check = count_fmt_params<N>(fmt.data)>
-constexpr bool check_format_parameter_consistency()
-{
-    static_assert(check.internal_error.data[0] == 0, "Some internal error occured, try looking at the check function type for the error");
-    static_assert(check.unclosed_braces == 0, "Extra unclosed braces in format string");
-    static_assert(check.extra_closed_braces == 0, "Extra closing braces in format string");
-
-    {
-        constexpr auto begin = check.used_arguments.begin();
-        constexpr auto end = check.used_arguments.begin() + check.total_used_argument_count;
-        constexpr auto has_all_referenced_arguments = !AK::any_of(begin, end, [](auto& entry) { return entry >= param_count; });
-        static_assert(has_all_referenced_arguments, "Format string references nonexistent parameter");
-    }
-
-    if constexpr (!check.has_explicit_argument_references)
-        static_assert(check.total_used_argument_count == param_count, "Format string does not reference all passed parameters");
-
-    // Ensure that no passed parameter is ignored or otherwise not referenced in the format
-    // As this check is generally pretty expensive, try to avoid it where it cannot fail.
-    // We will only do this check if the format string has explicit argument refs
-    // otherwise, the check above covers this check too, as implicit refs
-    // monotonically increase, and cannot have 'gaps'.
-    if constexpr (check.has_explicit_argument_references) {
-        constexpr auto all_parameters = iota_array<size_t, param_count>(0);
-        auto contains = [](auto begin, auto end, auto entry) {
-            for (; begin != end; begin++) {
-                if (*begin == entry)
-                    return true;
-            }
-
-            return false;
-        };
-        constexpr auto references_all_arguments = AK::all_of(
-            all_parameters.begin(),
-            all_parameters.end(),
-            [&](auto& entry) {
-                return contains(
-                    check.used_arguments.begin(),
-                    check.used_arguments.begin() + check.total_used_argument_count,
-                    entry);
-            });
-        static_assert(references_all_arguments, "Format string does not reference all passed parameters");
-    }
-
-    return true;
-}
-
-template<auto fmt, auto param_count>
-concept ConsistentFormatParameters = check_format_parameter_consistency<fmt.size, fmt, param_count>();
-#endif
-
 namespace AK {
 
 class TypeErasedFormatParams;
@@ -523,48 +370,38 @@ void vformat(const LogStream& stream, StringView fmtstr, TypeErasedFormatParams)
 void vout(FILE*, StringView fmtstr, TypeErasedFormatParams, bool newline = false);
 
 template<typename... Parameters>
-void out(FILE* file, StringView fmtstr, const Parameters&... parameters) { vout(file, fmtstr, VariadicFormatParams { parameters... }); }
-template<typename... Parameters>
-void outln(FILE* file, StringView fmtstr, const Parameters&... parameters) { vout(file, fmtstr, VariadicFormatParams { parameters... }, true); }
+void out(FILE* file, CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { vout(file, fmtstr.view(), VariadicFormatParams { parameters... }); }
+
 template<typename... Parameters>
-void outln(FILE* file, const char* fmtstr, const Parameters&... parameters) { vout(file, fmtstr, VariadicFormatParams { parameters... }, true); }
+void outln(FILE* file, CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { vout(file, fmtstr.view(), VariadicFormatParams { parameters... }, true); }
+
 inline void outln(FILE* file) { fputc('\n', file); }
 
 template<typename... Parameters>
-void out(StringView fmtstr, const Parameters&... parameters) { out(stdout, fmtstr, parameters...); }
-template<typename... Parameters>
-void outln(StringView fmtstr, const Parameters&... parameters) { outln(stdout, fmtstr, parameters...); }
+void out(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { out(stdout, move(fmtstr), parameters...); }
+
 template<typename... Parameters>
-void outln(const char* fmtstr, const Parameters&... parameters) { outln(stdout, fmtstr, parameters...); }
+void outln(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { outln(stdout, move(fmtstr), parameters...); }
+
 inline void outln() { outln(stdout); }
 
 template<typename... Parameters>
-void warn(StringView fmtstr, const Parameters&... parameters) { out(stderr, fmtstr, parameters...); }
-template<typename... Parameters>
-void warnln(StringView fmtstr, const Parameters&... parameters) { outln(stderr, fmtstr, parameters...); }
+void warn(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { out(stderr, move(fmtstr), parameters...); }
+
 template<typename... Parameters>
-void warnln(const char* fmtstr, const Parameters&... parameters) { outln(stderr, fmtstr, parameters...); }
+void warnln(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters) { outln(stderr, move(fmtstr), parameters...); }
+
 inline void warnln() { outln(stderr); }
 #endif
 
 void vdbgln(StringView fmtstr, TypeErasedFormatParams);
 
-#ifndef DBGLN_NO_COMPILETIME_FORMAT_CHECK
-template<StringLiteral fmt, bool enabled = true, typename... Parameters>
-void dbgln(const Parameters&... parameters) requires ConsistentFormatParameters<fmt, sizeof...(Parameters)>
-{
-    dbgln<enabled>(StringView { fmt.data }, parameters...);
-}
-#endif
-
 template<bool enabled = true, typename... Parameters>
-void dbgln(StringView fmtstr, const Parameters&... parameters)
+void dbgln(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&... parameters)
 {
     if constexpr (enabled)
-        vdbgln(fmtstr, VariadicFormatParams { parameters... });
+        vdbgln(fmtstr.view(), VariadicFormatParams { parameters... });
 }
-template<bool enabled = true, typename... Parameters>
-void dbgln(const char* fmtstr, const Parameters&... parameters) { dbgln<enabled>(StringView { fmtstr }, parameters...); }
 
 template<bool enabled = true>
 void dbgln() { dbgln<enabled>(""); }
@@ -575,10 +412,10 @@ void set_debug_enabled(bool);
 void vdmesgln(StringView fmtstr, TypeErasedFormatParams);
 
 template<typename... Parameters>
-void dmesgln(StringView fmtstr, const Parameters&... parameters) { vdmesgln(fmtstr, VariadicFormatParams { parameters... }); }
-template<typename... Parameters>
-void dmesgln(const char* fmtstr, const Parameters&... parameters) { vdmesgln(fmtstr, VariadicFormatParams { parameters... }); }
-inline void dmesgln() { dmesgln(""); }
+void dmesgln(CheckedFormatString<Parameters...>&& fmt, const Parameters&... parameters)
+{
+    vdmesgln(fmt.view(), VariadicFormatParams { parameters... });
+}
 #endif
 
 template<typename T, typename = void>
@@ -647,13 +484,8 @@ using AK::warnln;
 
 using AK::dbgln;
 
+using AK::CheckedFormatString;
 using AK::FormatIfSupported;
 using AK::FormatString;
 
-#ifdef DBGLN_NO_COMPILETIME_FORMAT_CHECK
-#    define dbgln(fmt, ...) dbgln(fmt, ##__VA_ARGS__)
-#    define dbgln_if(flag, fmt, ...) dbgln<flag>(fmt, ##__VA_ARGS__)
-#else
-#    define dbgln(fmt, ...) dbgln<fmt>(__VA_ARGS__)
-#    define dbgln_if(flag, fmt, ...) dbgln<fmt, flag>(__VA_ARGS__)
-#endif
+#define dbgln_if(flag, fmt, ...) dbgln<flag>(fmt, ##__VA_ARGS__)

AK/StdLibExtras.h

@@ -565,36 +565,9 @@ using MakeIntegerSequence = decltype(make_integer_sequence_impl<T, N>());
 template<unsigned N>
 using MakeIndexSequence = MakeIntegerSequence<unsigned, N>;
 
-template<unsigned long N>
-struct StringLiteral {
-    constexpr StringLiteral(const char (&in)[N])
-        : data {}
-        , size { N }
-    {
-        for (unsigned long i = 0; i < N; ++i)
-            data[i] = in[i];
-    }
-
-    template<unsigned long Nx>
-    constexpr StringLiteral& operator=(const StringLiteral<Nx>& other)
-    {
-        static_assert(Nx <= N, "Storing a string literal in a smaller one");
-        for (unsigned long i = 0; i < Nx; ++i)
-            data[i] = other[i];
-        return *this;
-    }
-
-    template<unsigned long Nx>
-    constexpr StringLiteral& operator=(const char (&other)[Nx])
-    {
-        static_assert(Nx <= N, "Storing a string literal in a smaller one");
-        for (unsigned long i = 0; i < Nx; ++i)
-            data[i] = other[i];
-        return *this;
-    }
-
-    char data[N];
-    unsigned long size;
+template<typename T>
+struct IdentityType {
+    using Type = T;
 };
 
 }
@@ -608,6 +581,7 @@ using AK::declval;
 using AK::DependentFalse;
 using AK::exchange;
 using AK::forward;
+using AK::IdentityType;
 using AK::IndexSequence;
 using AK::IntegerSequence;
 using AK::is_trivial;
@@ -630,6 +604,5 @@ using AK::max;
 using AK::min;
 using AK::move;
 using AK::RemoveConst;
-using AK::StringLiteral;
 using AK::swap;
 using AK::Void;

AK/TestSuite.h

@@ -27,11 +27,12 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/CheckedFormatString.h>
 
 namespace AK {
 
 template<typename... Parameters>
-void warnln(const char* fmtstr, const Parameters&...);
+void warnln(CheckedFormatString<Parameters...>&& fmtstr, const Parameters&...);
 
 }
 

Userland/Shell/Shell.cpp

@@ -1894,16 +1894,16 @@ void Shell::possibly_print_error() const
                 warn("\x1b[31m");
                 size_t length_written_so_far = 0;
                 if (line == (i64)source_position.position->start_line.line_number) {
-                    warn("{:~>{}}", "", 5 + source_position.position->start_line.line_column);
+                    warn(StringView { "{:~>{}}" }, "", 5 + source_position.position->start_line.line_column);
                     length_written_so_far += source_position.position->start_line.line_column;
                 } else {
-                    warn("{:~>{}}", "", 5);
+                    warn(StringView { "{:~>{}}" }, "", 5);
                 }
                 if (line == (i64)source_position.position->end_line.line_number) {
-                    warn("{:^>{}}", "", source_position.position->end_line.line_column - length_written_so_far);
+                    warn(StringView { "{:^>{}}" }, "", source_position.position->end_line.line_column - length_written_so_far);
                     length_written_so_far += source_position.position->start_line.line_column;
                 } else {
-                    warn("{:^>{}}", "", current_line.length() - length_written_so_far);
+                    warn(StringView { "{:^>{}}" }, "", current_line.length() - length_written_so_far);
                 }
                 warnln("\x1b[0m");
             }

Userland/Utilities/expr.cpp

@@ -45,11 +45,11 @@ Print the value of EXPRESSION to standard output.)");
     exit(0);
 }
 
-template<typename... Args>
-[[noreturn]] void fail(Args&&... args)
+template<typename Fmt, typename... Args>
+[[noreturn]] void fail(Fmt&& fmt, Args&&... args)
 {
     warn("ERROR: \e[31m");
-    warnln(args...);
+    warnln(StringView { fmt }, args...);
     warn("\e[0m");
     exit(1);
 }