Version 2.6 of Free Pascal added the constref parameter qualifier.
var x : longint ;
procedure test_cr(constref cr: longint);
writeln('Parameter 5 has been passed by reference: ', cr );
It is like the const parameter qualifier. This qualifier informs the compiler that within the entire program there is no code that will change the value of the parameter while the procedure/function is executing.
This means that not only the parameter, but also the variable (in above example: x) passed by the caller (e.g. a global var) must not be changed until the call with the constref parameter has returned.
In addition to being like const parameter qualifier, the constref qualifier enforces that the parameter is passed by reference.
This differs from the const parameters, which may be passed as reference or value depending on what the compiler thinks is best.
The new feature notes for version 2.6 suggest that this can be used for interfacing with external routines in other languages, where this type of parameter passing is required. Other uses of constref may hinder the compiler from optimizing code.