Retain initialized constant properties flagged as assign-only

[danwood]

Symptom

A let stored property that is assigned in an explicit initializer but never read is reported as an assign-only property, even though removing it is not safe.

Real-world example

public struct NavigationItem: Identifiable, Hashable {
    public let id = UUID()
    public let title: String
    public let color: Color?

    public init(title: String, color: Color? = nil) {
        self.title = title
        self.color = color
    }
}

// Used elsewhere:
NavigationItem(title: "Designs", color: .accentColor)

Here color is read only through SwiftUI's synthesized body / Hashable / Identifiable machinery, which the index does not attribute as a read. It is therefore reported as assign-only. Scanning a real SwiftUI app surfaced hundreds of such properties.

Why it's a false positive

Acting on the report by removing the declaration breaks the build:

• the self.color = color assignment in the initializer body is left without a target, and
• the color: initializer parameter is dropped, breaking every call site that passes it.

A let can only be assigned once, in an initializer body, so when that initializer is written explicitly and is actually used, the property is a genuine stored value that simply isn't read — not an assign-only property.

Fix

When a property would otherwise be reported as assign-only, it is instead retained if it is a let constant initialized by an explicit initializer that is itself referenced.

The exclusion is deliberately narrow so existing detection is unchanged:

• a let initialized only by a compiler-synthesized memberwise initializer is still reported (removing it is safe — the synthesizer simply stops generating the parameter);
• a let whose initializer is never called is still reported;
• var assign-only detection is untouched.

To distinguish a let from a var, the declaration visitor now records whether each variable declaration is a let binding, plumbed through the indexer onto the declaration.

A regression test (testRetainsInitializedConstantProperties) covers both the retained case (used initializer) and the still-reported case (unused initializer).