Add guide for approachable concurrency in Swift 6.2

Updated SKILL.md to reference project concurrency settings and added a new reference guide for Swift 6.2 approachable concurrency mode. The new guide covers detection, expected behavior changes, recommended fixes, and common pitfalls for projects using main-actor-by-default and default actor isolation.

swift-concurrency-expert/SKILL.md

@@ -14,6 +14,7 @@ Review and fix Swift Concurrency issues in Swift 6.2+ codebases by applying acto
 ### 1. Triage the issue
 
 - Capture the exact compiler diagnostics and the offending symbol(s).
+- Check project concurrency settings: Swift language version (6.2+), strict concurrency level, and whether approachable concurrency (default actor isolation / main-actor-by-default) is enabled.
 - Identify the current actor context (`@MainActor`, `actor`, `nonisolated`) and whether a default actor isolation mode is enabled.
 - Confirm whether the code is UI-bound or intended to run off the main actor.
 
@@ -32,4 +33,5 @@ Common fixes:
 ## Reference material
 
 - See `references/swift-6-2-concurrency.md` for Swift 6.2 changes, patterns, and examples.
+- See `references/approachable-concurrency.md` when the project is opted into approachable concurrency mode.
 - See `references/swiftui-concurrency-tour-wwdc.md` for SwiftUI-specific concurrency guidance.

swift-concurrency-expert/references/approachable-concurrency.md

@@ -0,0 +1,63 @@
+## Approachable Concurrency (Swift 6.2) - project mode quick guide
+
+Use this reference when the project has opted into the Swift 6.2 approachable
+concurrency settings (default actor isolation / main-actor-by-default).
+
+## Detect the mode
+
+Check Xcode build settings under "Swift Compiler - Concurrency":
+- Swift language version (must be 6.2+).
+- Default actor isolation / Main Actor by default.
+- Strict concurrency checking level (Complete/Targeted/Minimal).
+
+For SwiftPM, inspect Package.swift swiftSettings for the same flags.
+
+## Behavior changes to expect
+
+- Async functions stay on the caller's actor by default; they don't hop to a
+  global concurrent executor unless the implementation chooses to.
+- Main-actor-by-default reduces data race errors for UI-bound code and global
+  state, because mutable state is implicitly protected.
+- Protocol conformances can be isolated (e.g., `extension Foo: @MainActor Bar`).
+
+## How to apply fixes in this mode
+
+- Prefer minimal annotations; let main-actor-by-default do the work when the
+  code is UI-bound.
+- Use isolated conformances instead of forcing `nonisolated` workarounds.
+- Keep global or shared mutable state on the main actor unless there is a clear
+  performance need to offload it.
+
+## When to opt out or offload work
+
+- Use `@concurrent` on async functions that must run on the concurrent pool.
+- Make types or members `nonisolated` only when they are truly thread-safe and
+  used off the main actor.
+- Continue to respect Sendable boundaries when values cross actors or tasks.
+
+## Common pitfalls
+
+- `Task.detached` ignores inherited actor context; avoid it unless you truly
+  need to break isolation.
+- Main-actor-by-default can hide performance issues if CPU-heavy work stays on
+  the main actor; move that work into `@concurrent` async functions.
+
+## Keywords (from source cheat sheet)
+
+| Keyword | What it does |
+| --- | --- |
+| `async` | Function can pause |
+| `await` | Pause here until done |
+| `Task { }` | Start async work, inherits context |
+| `Task.detached { }` | Start async work, no inherited context |
+| `@MainActor` | Runs on main thread |
+| `actor` | Type with isolated mutable state |
+| `nonisolated` | Opts out of actor isolation |
+| `Sendable` | Safe to pass between isolation domains |
+| `@concurrent` | Always run on background (Swift 6.2+) |
+| `async let` | Start parallel work |
+| `TaskGroup` | Dynamic parallel work |
+
+## Source
+
+https://fuckingapproachableswiftconcurrency.com/en/