Track the start of the parsing position for each format specifier

The starting position is one of:

    1. The position immediately after the colon, if there is one.
    2. Otherwise, the position immediately after the format parameter
       index, if there is one.
    3. Otherwise, the position immeidately after the opening brace.

This will allow resetting the position of the format string lexer to
this starting position before formatting user-defined types at
runtime.

Author: trflynn89

fly/types/string/detail/format_specifier.hpp

@@ -267,6 +267,7 @@ struct BasicFormatSpecifier
     Type m_type {Type::None};
     Case m_case {Case::Lower};
 
+    std::size_t m_parse_index {0};
     std::size_t m_size {0};
 
 private:

fly/types/string/detail/format_string.hpp

@@ -159,6 +159,7 @@ constexpr auto BasicFormatString<CharType, ParameterTypes...>::parse_specifier()
     const auto starting_position = m_context.lexer().position() - 1;
 
     FormatSpecifier specifier(m_context);
+    specifier.m_parse_index = m_context.lexer().position();
 
     if (m_context.parameter_type(specifier.m_position) == ParameterType::UserDefined)
     {
@@ -191,6 +192,7 @@ BasicFormatString<CharType, ParameterTypes...>::parse_standard_specifier(FormatS
 {
     if (m_context.lexer().consume_if(s_colon))
     {
+        specifier.m_parse_index = m_context.lexer().position();
         specifier.parse(m_context);
     }
 

test/types/string/format_string.cpp

@@ -667,6 +667,46 @@ CATCH_TEMPLATE_TEST_CASE(
 
         CATCH_CHECK_FALSE(format.next_specifier());
     }
+
+    CATCH_SECTION("Specifiers track their parsing index in the format string")
+    {
+        {
+            auto format = make_format(FMT("ab {0} cd {1:d} ef {002:#0x}"), 1, 2, 3);
+            CATCH_CHECK_FALSE(format.context().has_error());
+
+            auto specifier1 = format.next_specifier();
+            CATCH_REQUIRE(specifier1);
+            CATCH_CHECK(specifier1->m_parse_index == 5);
+
+            auto specifier2 = format.next_specifier();
+            CATCH_REQUIRE(specifier2);
+            CATCH_CHECK(specifier2->m_parse_index == 13);
+
+            auto specifier3 = format.next_specifier();
+            CATCH_REQUIRE(specifier3);
+            CATCH_CHECK(specifier3->m_parse_index == 24);
+
+            CATCH_CHECK_FALSE(format.next_specifier());
+        }
+        {
+            auto format = make_format(FMT("ab {} cd {:d} ef {:#0x}"), 1, 2, 3);
+            CATCH_CHECK_FALSE(format.context().has_error());
+
+            auto specifier1 = format.next_specifier();
+            CATCH_REQUIRE(specifier1);
+            CATCH_CHECK(specifier1->m_parse_index == 4);
+
+            auto specifier2 = format.next_specifier();
+            CATCH_REQUIRE(specifier2);
+            CATCH_CHECK(specifier2->m_parse_index == 11);
+
+            auto specifier3 = format.next_specifier();
+            CATCH_REQUIRE(specifier3);
+            CATCH_CHECK(specifier3->m_parse_index == 19);
+
+            CATCH_CHECK_FALSE(format.next_specifier());
+        }
+    }
 }
 
 #if defined(FLY_COMPILER_DISABLE_CONSTEVAL)