Migrating a Real App to Swift 6: Data Races, a Dependency I Had to Evict, and the Compiler That Wouldn't Let Me Lie

Let me start with a confession: I have been writing concurrent code since the only tool in the box was a mutex and a prayer. After a decade of Swift I feel suspicion of any code that touches two threads and claims to be fine. So when Swift 6 showed up promising to prove my concurrency correct at compile time, I had two reactions at once. The grizzled half of me said "sure, kid." The other half — the half that has spent actual weekends chasing a heisenbug that only reproduced on a customer's M1 under sync load — said "...please. Please be real." This is the story of moving Ditto Edge Studio — a SwiftUI debug-and-query tool for the Ditto edge database — to Swift 6's strict concurrency mode. It's a real app: SQLCipher persistence, an embedded MCP server, a SpriteKit presence graph, live sync over Bluetooth and WebSocket. Not a to-do list. The kind of app where concurrency bugs hide in the cracks and wait for a demo. Spoiler: it was worth it. It was also more work than the WWDC talk implied, and the most valuable thing the compiler did happened in the one place I told it to stop looking. Let me show you. First, the Wall: A Dependency That Wasn't Coming to Swift 6 Here's the thing nobody warns you about. Swift 6 language mode isn't really a per-file setting. Your code can be immaculate — every actor isolated, every Sendable accounted for — and you'll still be stuck, because one dependency that isn't Swift 6-ready can hold your entire module hostage. Mine was a code editor. I'd been using a popular SwiftUI editor package for the DQL query editor, and it transitively pulled in a syntax-highlighting library. Both were lovely. Both were also written for a more innocent time, and neither was going to compile under Swift 6 strict concurrency without upstream changes that weren't happening on my timeline. I had the usual three options, and I want to be honest about how tempting the cowardly ones were: Pin the dependency and leave the whole app at Swift 5. Free today, expensive