Change 'unsafeDowncast' to 'as!' (#186)

glbrntt · web-flow · commit 24c800fb494f · 2024-07-24T06:47:26.000-07:00
Motivation:

The 'unsafeDowncast' can cause a miscompile leading to unexpected
runtime behaviour.

Modifications:

- Use 'as!' instead

Result:

No miscompiles on 5.10
diff --git a/Sources/ConcurrencyHelpers/Lock.swift b/Sources/ConcurrencyHelpers/Lock.swift
@@ -136,7 +136,8 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {
         let buffer = Self.create(minimumCapacity: 1) { _ in
             return value
         }
-        let storage = unsafeDowncast(buffer, to: Self.self)
+        // Avoid 'unsafeDowncast' as there is a miscompilation on 5.10.
+        let storage = buffer as! Self
 
         storage.withUnsafeMutablePointers { _, lockPtr in
             LockOperations.create(lockPtr)

Original file line number	Diff line number	Diff line change
`@@ -136,7 +136,8 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {`
`136`	`136`	`let buffer = Self.create(minimumCapacity: 1) { _ in`
`137`	`137`	`return value`
`138`	`138`	`}`
`139`		`- let storage = unsafeDowncast(buffer, to: Self.self)`
	`139`	`+ // Avoid 'unsafeDowncast' as there is a miscompilation on 5.10.`
	`140`	`+ let storage = buffer as! Self`
`140`	`141`
`141`	`142`	`storage.withUnsafeMutablePointers { _, lockPtr in`
`142`	`143`	`LockOperations.create(lockPtr)`