Rule Request: unowned_captures

[mdiep]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

Dunno if there's general consensus on this, but I have a strong preference to use [weak foo] captures over [unowned foo] captures. While weak can have performance implications and can require some additional code, it's safer than unowned. I've encountered quite a few mysterious crashes that were caused by unowned captures, even in cases where it seemed like the lifetimes were the same.

2. Provide several examples of what would and wouldn't trigger violations.

Triggering:

{ [unowned self] in _ }()

Non-triggering:

{ [weak self] in _ }()

3. Should the rule be configurable, if so what parameters should be configurable?

Nope.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a
   rule as opt-in.

I'm not sure if there's enough consensus about this to enable it by default.

[marcelofabri]

Swift 4.1 provides that information in SourceKit structure:

{
  "key.diagnostic_stage" : "source.diagnostic.stage.swift.parse",
  "key.length" : 31,
  "key.offset" : 0,
  "key.substructure" : [
    {
      "key.bodylength" : 0,
      "key.bodyoffset" : 30,
      "key.kind" : "source.lang.swift.expr.call",
      "key.length" : 31,
      "key.name" : "foo() { [unowned self] in _ }",
      "key.namelength" : 29,
      "key.nameoffset" : 0,
      "key.offset" : 0,
      "key.substructure" : [
        {
          "key.bodylength" : 24,
          "key.bodyoffset" : 4,
          "key.kind" : "source.lang.swift.expr.call",
          "key.length" : 29,
          "key.name" : "foo",
          "key.namelength" : 3,
          "key.nameoffset" : 0,
          "key.offset" : 0,
          "key.substructure" : [
            {
              "key.attributes" : [
                {
                  "key.attribute" : "source.decl.attribute.weak",
                  "key.length" : 0,
                  "key.offset" : 0
                }
              ],
              "key.kind" : "source.lang.swift.decl.var.local",
              "key.length" : 4,
              "key.name" : "self",
              "key.namelength" : 4,
              "key.nameoffset" : 17,
              "key.offset" : 17
            },
            {
              "key.attributes" : [
                {
                  "key.attribute" : "source.decl.attribute.weak",
                  "key.length" : 0,
                  "key.offset" : 0
                }
              ],
              "key.kind" : "source.lang.swift.decl.var.local",
              "key.length" : 4,
              "key.name" : "self",
              "key.namelength" : 4,
              "key.nameoffset" : 17,
              "key.offset" : 17
            }
          ]
        }
      ]
    }
  ]
}

However, this will probably not be enough because the attribute range for unowned is wrong 😭

I've filled https://bugs.swift.org/browse/SR-7219 to track this.