Ampersand instead of colon for bind variables

I’ve troubleshooted one query today and I was very surprised that bind variables in this query were specified with &ampersand instead of :colon! I have never seen this before and I couldn’t find anything about this in documentation…
Unfortunately SQL*Plus doesn’t support ampersand yet, even if you disable define (“set define off”),
so I’ve tested such behaviour with this code:

set def off serverout on
exec declare s varchar2(1); begin execute immediate 'select 1 from dual where dummy=&var' into s using 'X'; dbms_output.put_line(s); end;

And it really works! //at least on 11.2.0.2 and 12.2.0.1

SQL> set def off serverout on
SQL> exec declare s varchar2(1); begin execute immediate 'select 1 from dual where dummy=&var' into s using 'X'; dbms_output.put_line(s); end;
1

PL/SQL procedure successfully completed.

SQL> select substr(sql_text,1,40) stext,sql_id,executions,rows_processed from v$sqlarea a where sql_text like '%dual%&var';

STEXT                                 SQL_ID        EXECUTIONS ROWS_PROCESSED
------------------------------------- ------------- ---------- --------------
select 1 from dual where dummy=&var   ckkw4u3atxz02          3              3

SQL> select * from table(dbms_xplan.display_cursor('ckkw4u3atxz02'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
SQL_ID  ckkw4u3atxz02, child number 0
-------------------------------------
select 1 from dual where dummy=&var

Plan hash value: 272002086

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |     2 (100)|          |
|*  1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("DUMMY"=:VAR)


18 rows selected.

Update: Btw, it works for SQL only, not for PL/SQL:

SQL> var v varchar2(1);
SQL> begin &v = 'Z'; end;
  2  /
begin &v = 'Z'; end;
      *
ERROR at line 1:
ORA-06550: line 1, column 7:
PLS-00103: Encountered the symbol "&" when expecting one of the following:

SQL> exec &v := 'X';
BEGIN &v := 'X'; END;

      *
ERROR at line 1:
ORA-06550: line 1, column 7:
PLS-00103: Encountered the symbol "&" when expecting one of the following:
The symbol "&" was ignored.

SQL> exec :v := 'X';

PL/SQL procedure successfully completed.

SQL> select * from dual where dummy=&v
  2  ;

D
-
X

And we can can use mixed placeholders:

SQL> select * from dual where dummy=&v and &v=:v;

D
-
X