On Tuesday, 3 March 2015 at 15:03:41 UTC, Andrei Alexandrescu
wrote:
On 3/3/15 5:45 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?=
<schue...@gmx.net>" wrote:
On Tuesday, 3 March 2015 at 09:05:46 UTC, Walter Bright wrote:
On 3/2/2015 9:58 PM, weaselcat wrote:
Borrowing 'a' from a struct would make the parent struct
immutable
during the
borrow scope of 'a', I believe.
Right, now consider that struct is a leaf in a complex graph
of data
structures.
Then you still cannot have more than one mutable reference to
the entire
graph. Because that is impractical, Rust uses unsafe (i.e.
@trusted in D
speak) accessors that "cast away" the ownership, but do so in
a way that
doesn't violate the guarantees.
For example, the type system doesn't allow you to get mutable
references
to the left and right children of a binary tree node. But
there can be
an accessor method that internally does some unsafe magic to
return a
tuple with mutable references to them, annotated with the
information
that they are mutably borrowed from the node. Both child refs
are
mutable, and the parent node is inaccessible as long as they
exist.
Well... the bigger problem is that it's relying on a
convention. The accessor method needs to be constructed in a
particular way that's easy to get wrong and that the compiler
has no way to check for us.
:o)
To avoid misunderstandings: It is in reply to a sub-thread where
Walter asked about how Rust's type system works. This is an
example for Rust, not for D.
Therefore, your reply isn't really valid. In Rust, it is an
escape hatch from a fundamentally safe type system, whereas in D
it would be a necessary convention to make usage of RC safe.
