Adapt `fsyspath` for C++20 conventions.

In C++20, `std::filesystem::u8path()` (that accepts a UTF-8 encoded
`std::string` and returns a `std::filesystem::path`) is deprecated. Instead,
to engage UTF-8 coding conversions, we're supposed to pass the `path`
constructor a `std::u8string`, i.e. a `std::basic_string<char8_t>`. Since
`char8_t` is a type distinct from both `char` and `unsigned char`, we must Do
Something to pass a UTF-8 encoded `std::string` into `std::filesystem::path`.

To avoid copying characters from a `std::string` into a temporary
`std::u8string` and from there into the `std::filesystem::path`, make a
`boost::transform_iterator` that accepts a `std::string_view::iterator` and
adapts it to dereference `char8_t` characters. Make `fsyspath(std::string_view)`
engage the base-class constructor accepting (iterator, iterator), adapting
`string_view::begin()` and `end()` to deliver `char8_t` characters.

Use the same tactic for `fsyspath::operator=(std::string_view)`, explicitly
calling `std::filesystem::path::assign()` with the adapted iterators.

To resolve ambiguities, provide both constructors and assignment operators
accepting `(const std::string&)` and `(const char*)`, explicitly converting
each to `std::string_view`.

At the same time, `std::filesystem::path::u8string()` now returns
`std::u8string` rather than `std::string`. Since `std::filesystem::path`
delivers only that `std::u8string` rather than iterators into its internal
representation, we can't avoid capturing it and copying to the returned
`std::string`.

Remove explicit `.u8string()` calls from a few existing `fsyspath` instances,
now that `fsyspath` supports implicit conversion to `std::string`.

1 files changed, 4 insertions, 4 deletions

diff --git a/indra/llcommon/lua_function.cpp b/indra/llcommon/lua_function.cpp
index a9f88f3170..e28c0caa27 100644
--- a/indra/llcommon/lua_function.cpp
+++ b/indra/llcommon/lua_function.cpp
@@ -170,7 +170,7 @@ fsyspath source_path(lua_State* L)
     {
         lua_getinfo(L, i, "s", &ar);
     }
-    return ar.source;
+    return { ar.source };
 }
 
 } // namespace lluau
@@ -1108,7 +1108,7 @@ lua_function(source_path, "source_path(): return the source path of the running
 {
     lua_checkdelta(L, 1);
     lluau_checkstack(L, 1);
-    lua_pushstdstring(L, lluau::source_path(L).u8string());
+    lua_pushstdstring(L, lluau::source_path(L));
     return 1;
 }
 
@@ -1119,7 +1119,7 @@ lua_function(source_dir, "source_dir(): return the source directory of the runni
 {
     lua_checkdelta(L, 1);
     lluau_checkstack(L, 1);
-    lua_pushstdstring(L, lluau::source_path(L).parent_path().u8string());
+    lua_pushstdstring(L, lluau::source_path(L).parent_path());
     return 1;
 }
 
@@ -1132,7 +1132,7 @@ lua_function(abspath, "abspath(path): "
     lua_checkdelta(L);
     auto path{ lua_tostdstring(L, 1) };
     lua_pop(L, 1);
-    lua_pushstdstring(L, (lluau::source_path(L).parent_path() / path).u8string());
+    lua_pushstdstring(L, (lluau::source_path(L).parent_path() / path));
     return 1;
 }