Function push_vfstring 

pub fn push_vfstring<'a>(
    state: &mut LuaState,
    fmt: &[u8],
    args: &[FmtArg<'a>],
) -> Result<GcRef<LuaString>, LuaError>

Builds a formatted Lua string from a format byte string and structured arguments, pushes it onto the stack, and returns the top-of-stack value.

Supported format specifiers (same subset as C’s luaO_pushvfstring): %s, %c, %d, %I, %f, %U, %%. %p is not supported; see FmtArg documentation.

PORT NOTE: va_list replaced by &[FmtArg]. Call sites that previously passed variadic arguments must be updated to build a &[FmtArg] slice.