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is required
Combined from primary sources listed below.

In Attribute§

See primary documentation in context for trait is required. 

multi trait_mod:<is> (Attribute $attr, :$required!)

The trait is required will mark the attribute as to be filled with a value when the object is instantiated. Failing to do so will result in a runtime error.

class C {
    has $.a is required
}
my $c = C.new;
CATCH{ default { say .^name, ': ', .Str } }
# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required, but you did not provide a value for it.␤»

This trait also allows attributes to be typed with types that have a :D smiley without giving them a default value:

class Power {
    has Numeric:D $.base     is required;
    has Numeric:D $.exponent is required;
    multi method Numeric(::?CLASS:D: --> Numeric:D) {
        $!base ** $!exponent
    }
}

Available as of 6.d language version (early implementation exists in Rakudo compiler 2018.08+): You can specify a reason why the attribute is required:

class D {
    has $.a is required("it is a good idea");
}
my $d = D.new;
CATCH{ default { say .^name, ': ', .Str } }
# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required because it is a good idea,␤but you did not provide a value for it.␤»

is required doesn't just affect the default constructor, it checks for the attribute at a lower level, so it will work for custom constructors written using bless.