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[Observation] Add type annotation on cached keypaths #78842

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[Observation] Add type annotation on cached keypaths #78842

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16 changes: 12 additions & 4 deletions lib/Macros/Sources/ObservationMacros/ObservableMacro.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -468,10 +468,18 @@ extension ObservationTrackedMacro: PeerMacro {

let storage = DeclSyntax(property.privatePrefixed("_", addingAttribute: ObservableMacro.ignoredAttribute))
if ObservableMacro.canCacheKeyPaths(context.lexicalContext) {
let cachedKeypath: DeclSyntax =
"""
private static let _cachedKeypath_\(identifier) = \\\(container.name).\(identifier)
"""
let cachedKeypath: DeclSyntax
if let type = property.type {
cachedKeypath =
"""
private static let _cachedKeypath_\(identifier): ReferenceWritableKeyPath<\(container.name), \(type)> = \\\(container.name).\(identifier)
"""
} else {
cachedKeypath =
"""
private static let _cachedKeypath_\(identifier) = \\\(container.name).\(identifier)
"""
}
return [storage, cachedKeypath]
} else {
return [storage]
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34 changes: 34 additions & 0 deletions test/stdlib/Observation/Observable.swift
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Expand Up @@ -218,6 +218,40 @@ class GenericClassParent<T> {
}
}

protocol MemberProtocol {}
struct MemberType: MemberProtocol {}

@Observable
class HasUniversalMember {
var member: any MemberProtocol = MemberType()
}

@Observable
class HasExistentialMember {
var member: some MemberProtocol = MemberType()
}

protocol HasAssociatedType {
associatedtype Associated
static var associatedMember: Associated { get }
}

@Observable
class HasHiddenExistential: HasAssociatedType {
var member: Associated = HasHiddenExistential.associatedMember
static var associatedMember: some MemberProtocol { MemberType() }
}

/*
// FIXME: This case is not handled yet. Properties that have no type annotation
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Question for reviewers: I don't know how best to handle this case.

The simplest thing to do is just disable the optimization when there's no explicit type, but that's an annoying performance hazard when it's idiomatic to use an inferred type for most default-initialized properties.

It's very simple to fix the rare case where this fails (just add a type annotation) but that's still leaving in a source break.

// that are inferred as containing a `some` type fail to compile.
@Observable
class HasMoreHiddenExistential: HasAssociatedType {
var member = HasMoreHiddenExistential.associatedMember
static var associatedMember: some MemberProtocol { MemberType() }
}
*/

@Observable
class RecursiveOuter {
var inner = RecursiveInner()
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