Move the scanned string into the value instead of copying it (#5458)

* Move the scanned string into the value instead of copying it

The SAX interface documents that the string handed to json_sax::string() may
be moved from, and the DOM handlers already move the one handed to binary().
string() did not, so every string value was copy-constructed out of the
lexer's token buffer, which then kept the buffer alive at its high-water mark
until the next token overwrote it.

Moving hands that buffer to the new value instead. The allocation count is
unchanged - the value needed one either way - but the copy is gone.

  jeopardy      247.3 ms -> 240.9 ms  (-2.6%)
  citm_catalog    4.61 ms ->   4.48 ms (-2.8%)
  40k 30-char strings 7.80 ms -> 7.64 ms (-2.1%)

Note this deliberately does not extend to the object key. Moving the key
hands the lexer's buffer - sized for the largest token seen so far - to a key
that is usually short, so the next value has to grow a fresh buffer. Measured,
that costs 11.9% on a document of many small keys with longer values.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Spell out the move rationale at every handle_value(std::move) site

The comment explaining why the value is moved sat only on
json_sax_dom_parser::string(), and the callback parser's string() pointed
at it with "see json_sax_dom_parser::string()". That reference cannot be
searched for - the function is declared as `bool string(string_t& val)`
inside the class, so the qualified name appears nowhere - and the two
binary() overloads, which have always moved, carried no explanation at all.

Put the same comment on all four sites and name json_sax, which is
greppable, instead of a member that is not.

Comment-only; no generated code changes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -10935,12 +10935,16 @@ class json_sax_dom_parser
 
     bool string(string_t& val)
     {
-        handle_value(val);
+        // json_sax documents that the passed value may be moved from,
+        // so hand the buffer over instead of copying it
+        handle_value(std::move(val));
         return true;
     }
 
     bool binary(binary_t& val)
     {
+        // json_sax documents that the passed value may be moved from,
+        // so hand the buffer over instead of copying it
         handle_value(std::move(val));
         return true;
     }
@@ -11245,12 +11249,16 @@ class json_sax_dom_callback_parser
 
     bool string(string_t& val)
     {
-        handle_value(val);
+        // json_sax documents that the passed value may be moved from,
+        // so hand the buffer over instead of copying it
+        handle_value(std::move(val));
         return true;
     }
 
     bool binary(binary_t& val)
     {
+        // json_sax documents that the passed value may be moved from,
+        // so hand the buffer over instead of copying it
         handle_value(std::move(val));
         return true;
     }