const by default.

[Walter Bright]

BCS wrote:
> Walter Bright wrote:
> [...]
>  > class C { int m; }
>  > void bar(C c) { c.m = 3; } // ok
> [...]
>>
>> C foo(in C c) { return c; } // ok
>>
>> void foo(in C c) { bar(c); } // ok
>>
>> void foo(in C c)
>> {   C d = c;
>>     d.m = 3;  // ok
>> }
>>
> 
> How would the last three work?

The compiler would accept them, though I'd say it would be a bad 
programming practice.

> IIRC, const implies that data is not 
> accessible for writes. In the first of those three, the return value has 
> no indication of const'ness so something like this could be done

That's right.

> What I want is a way to get an assurance by the compiler that a 
> referenced value will not be changed. This would require that const 
> references (or a non mutable ones) can never be made into a mutable 
> references.
> 
> If I'm missing something, please point it out.

You haven't missed something. The 'in' thing proposed above is not an 
airtight guarantee, you could call it more of a 'sieve'. It's more of a 
documentation aid. The compiler checking would be minimal and easily 
subverted.