runtime: add NoCopy documentation struct type?

[bradfitz]

From a conversation with Alan Donovan about how we might detect accidental copies of
types that shouldn't be copyable, I proposed:

package runtime  // or wherever

// NoCopy is an annotation that the containing type shouldn't be copied
// by value. It has zero size so may be embedded freely.
type NoCopy struct{}

Then we could do govet checks.

Or even compiler checks (more controversial probably).

Then sync.Cond and sync.Mutex could use it. The latter has no checks, because
sync.Cond's runtime copyChecker takes up space.

Only tagging this Go1.4 for eventual discussion.

[adonovan]

Comment 1:

For the record, I like this idea.  I recently came across the need for it
while working on
https://code.google.com/p/go/source/browse/container/intsets/sparse.go?repo=tools.
 One downside of govet-time checking is that you only run it when
you've
found most of the bugs, so there's still a strong motivation to add a
dynamic check (as sync.Cond and intsets.Sparse do).

[ianlancetaylor]

Comment 2:

For vet purposes, we could also consider using (yet another) magic comment.
I'm curious as to how often this arises.  At first thought it seems that it can only
arise on a small struct that we expect people to embed directly rather than via a
pointer.  I'm fine with adding something to vet, but I wonder whether this is really
common enough to merit a language change.

Labels changed: added repo-main.

[rsc]

Comment 3:

Too late.

Labels changed: added release-go1.5, removed release-go1.4.

[rsc]

Too late (again) for this cycle. If this is going to happen, it needs an owner who will push on getting it discussed and done during the main cycle.

I do think this might be worth doing, with the vet check. (Separately, I want to make vet a bit more central to the dev process for 1.6 somehow. Rob filed a bug for that, but I don't have the number handy. If that happened, the main objection to vet goes away.)

Not sure about the name. For what it's worth, DISALLOW_EVIL_CONSTRUCTORS seems to be available again. :-)

[minux]

The vet issue is probably #11041.

[OneOfOne]

Since the cycle just started.

I like the type NoCopy struct{} idea or a yucky-yet-helpful //go:nocopy magic comment.

Go is a safe language, so having that check at compile time would rather be nice and will protect people from losing hair during late night coding sessions.