On 3/3/15 5:45 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <[email protected]>" 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)
Andrei
