A Modest Proposal: Final class instances

[dsimcha]

Several people have griped in the past that D class methods are virtual by
default.  I've pointed out to them that you can get around this by making the
methods final.  However, this is a bit of a blunt instrument, because some use
cases for a single class may call for polymorphism and other use cases for the
same class may call for fast performance and no polymorphism.  A possible
solution is, given a class:

class Foo {
    // Actual implementation.
}

final class FooFinal : Foo{
    // Dummy that just makes Foo final.
}

And then, when you need performance and not polymorphism, you would invoke
FooFinal instead of Foo.  However, this requires manual forwarding of
constructors and bloats the name space.  A simple syntactic sugar solution to
this dilemma that would add very little complexity to the language would be to
allow final to describe a class instance, as well as a class.  The following
would apply to a final instance:

1.  Method calls don't need to be virtual.
2.  An instance of a subclass cannot be converted to a final instance of the
base class.
3.  A final instance can be implicitly converted to a non-final instance, but
the opposite would not work.

Using final as an instance attribute like this would also allow another useful
feature:  Storing class instances inline in arrays, structs, or other classes.
 Basically, by marking a class instance as final, you'd be telling the
compiler that you do not need and are not using polymorphism in this case,
even if the class hierarchy uses it for other use cases, and therefore, all
relevant optimizations can be made.