Head Const

[Marc Schütz via Digitalmars-d]

On Friday, 19 February 2016 at 06:39:53 UTC, Walter Bright wrote:
> Allow @mutable, and no more mechanical checking in D. Recall 
> that D supports opaque types, meaning types are not fully known 
> to the compiler.

Allow @trusted, an no more mechanical checking in D. Recall that 
D supports extern functions, meaning function bodies are not 
fully known to the compiler.

Seriously, yes, @mutable is not mechanically checkable, which is 
why it is @system. That's intended, and not a flaw. It is 
supposed to make it possible to express things that don't 
otherwise fit the type system, and still be reasonably safe.

Jonathan's idea of attaching @mutable to the declaration handles 
the problem of opaque data structures really well. We can debate 
whether we require that annotation every time a struct contains 
@mutable members (even indirectly) to make it more obvious (if 
so, it still needs to be inferred for templated types), or if it 
should only be required for opaque types.