Self-describing numbers

You are encouraged to solve this task according to the task description, using any language you may know.
There are several so-called "self-describing" or "self-descriptive" integers.

An integer is said to be "self-describing" if it has the property that, when digit positions are labeled 0 to N-1, the digit in each position is equal to the number of times that that digit appears in the number.

For example,   2020   is a four-digit self describing number:

   position   0   has value   2   and there are two 0s in the number;
   position   1   has value   0   and there are no 1s in the number;
   position   2   has value   2   and there are two 2s;
   position   3   has value   0   and there are zero 3s.


Self-describing numbers < 100.000.000  are:     1210,   2020,   21200,   3211000,   42101000.



Task Description
 Write a function/routine/method/... that will check whether a given positive integer is self-describing.
 As an optional stretch goal - generate and display the set of self-describing numbers.


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import std.stdio, std.algorithm, std.range, std.conv, std.string;
 
bool isSelfDescribing(in long n) pure nothrow @safe {
    auto nu = n.text.representation.map!q{ a - '0' };
    return nu.length.iota.map!(a => nu.count(a)).equal(nu);
}
 
void main() {
    4_000_000.iota.filter!isSelfDescribing.writeln;
}