special comment to indicate work that is pending a snapshot

[nikomatsakis]

We should add some sort of special comment like // PENDING SNAPSHOT abcdef. Here abcdef is the hashcode of the current snapshot. The build system can then issue warnings if such a comment exists and abcdefis not still the current snapshot.

[bstrie]

@nikomatsakis, do you still feel that this would be useful?

[nikomatsakis]

Still seems useful to me.

[emberian]

Still very useful.

[catamorphism]

(bug triage) This would be nice, but is not a blocker.