swiftlang/swift

[SR-3316] "Script mode" globals have internal visibility and can be accessed before initialization from other files

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#45,904 opened on Dec 2, 2016

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access controlcompilergood first issuescript modeswift evolution proposal needed

Description
Previous ID SR-3316
Radar None
Original Reporter @jckarter
Type Bug
Votes 0
Component/s Compiler
Labels Bug, StarterProposal
Assignee None
Priority Medium

md5: e092a51eda538e02ab484391d0b1b1a5

Issue Description:

Variables declared in a script default to internal visibility, like other global declarations, but this is problematic, because semantically they behave more like local variables. Another file can access the global before its initializer has even run:

// main.swift
class C {}
var x: C
foo()
x = C()

// foo.swift
func foo() {
  print(x) // loads garbage from `x` from main.swift
}

In a future version of Swift, script globals should be changed to be private by default. We could possibly also allow a script global to be declared with explicit access control, which would make it visible to other files but would also have to make the variable behave more like a library global, with a dispatch_once-d initializer, e.g.:

// main.swift
var x = 42 // private, eagerly initialized
private var y = 17 // private, initialized on demand
internal var z = 38 // internal, initialized on demand

Contributor guide

Tech stack
swift
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Swift access controlCompiler initializationScript mode semantics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Examine the Swift compiler source for how script level variables are currently declared with internal visibility. The proposed change is to default them to private, with explicit access control triggering on demand initialization. Review the AST and semantic analysis code (e.g., in lib/AST/ or lib/Sema/) to identify the appropriate location for this change. The issue includes examples of desired behavior. Check if there are any related discussions or linked PRs in the comments.
[SR-3316] "Script mode" globals have internal visibility and can be accessed before initialization from other files · swiftlang/swift#45904 | Good First Issue