Linter: prevent orphaning of dynamic fields

[awelc]

Description

Why: The values bound to these dynamic fields will get orphaned when the struct value goes out of scope

Severity: High. Orphaning objects loses $$ because the storage rebate associated with the orphaned object can never be recovered

[awelc]

Another variant of this.

A struct may have multiple IDs:

struct X { 
  id: UID
  id2: UID
  id3: UID
}

or

struct Y {
  ids: vector<vector<UID>>
}

or

struct Z {
  ids: Table<String, UID>
}

We could try to figure out which UIDs have dynamic fields attached to them (step 1) and yell if you object::deleted on on of these UIDs.

[awelc]

Possible implementation strategy:

• Basically implement a version of god’s borrow checker, that is a simple, set-based reference analysis that tracks references by their created path.
  • We can be a bit lossy here so we could literally take the version of god’s borrow checker from the paper, and do not need to worry about the missing expressivity
  • When you see a call to a dynamic (object) field API, record any fields that have dynamic fields added.
• With the recorded map of what fields have had dynamic fields added, we proceed to the next analysis…
• Implement a second version of something that looks like god’s borrow tracker, but instead tracking references it tracks UID values. Either newly created or struct fields
  • When you see object::delete, check the source of that UID value (potentially more than 1 possible source). If the field of that source was in the recorded map generated in the first analysis (or if the UID was newly created in this scope), we give a lint warning.