[SR-2989] Multi-line function chains evaluated before breakpoint hits

[swift-ci]

Previous ID	SR-2989
Radar	rdar://28883410
Original Reporter	rosslebeau (JIRA User)
Type	Bug
Status	Resolved
Resolution	Won't Do

Environment

Swift 3.0
macOS 10.12 16A323

Additional Detail from JIRA

Votes	1
Component/s	LLDB for Swift
Labels	Bug
Assignee	@vedantk
Priority	Medium

md5: b5bc35bd9978fec0d64c651b37689c3f

Issue Description:

In certain cases, if you have a multi-line function chain, placing a breakpoint on the first line will actually result in the function chain being evaluated before the breakpoint hits.

For example:

func printMarker(_ x: Int) -> Int {
    print("marker")
    return x
}

struct User {
    let name: String
    let age: Int
}

struct Message {
    func mentionedUsersF() -> [User] {
        return [User(name: "Achilles", age: 32), User(name: "Tortoise", age: 105)]
    }
}

let message = Message()

var ages: [Int]

ages = [1, 2, 3]

ages = message
    .mentionedUsersF()
    .map { $0.age }
    .map(printMarker)

print(ages)

Place a breakpoint on the `ages = message` line, and you will see that "marker" has been printed at the time of breaking. The `ages` variable is still equal to `[1, 2, 3]`, however, so it hasn't been set yet.

This also occurs if you use a computed property instead of a function.

This does not happen if:

• Any of the function calls are moved up to the first line

• The struct contains a non-computed property (it can be unused, as long as there is one)

• A class is used instead of struct

• Functions that are not members of a struct are used instead

[belkadan]

@adrian-prantl, granataenrico (JIRA User), what do you think?

[adrian-prantl]

From the description this definitely sounds like a compiler bug.

[vedantk]

I looked into this recently and found that there isn't a good way to address this problem.

Any fix that enables stepping on the line "ages = message" prior to a call to map would result in inconsistent stepping behavior between Swift and C-based languages. Further, it might result in spurious extra stepping onto lines which just contain a reference to a decl.