Merge pull request #1042 from fabulous-dev/fix-combinemut-error

When combining 2 arrays, array B should be reused but array A was passed instead

CHANGELOG.md

@@ -7,7 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-_No unreleased changes_
+### Fixed
+- Fix an issue in `MutStackArray1.combineMut` that could result in a crash by @TimLariviere (https://github.com/fabulous-dev/Fabulous/pull/1042)
 
 ## [2.3.0] - 2023-03-17
 

src/Fabulous.Tests/ArrayTests.fs

@@ -0,0 +1,24 @@
+namespace Fabulous.Tests
+
+open System
+open Fabulous.StackAllocatedCollections
+open NUnit.Framework
+open Fabulous
+
+[<TestFixture>]
+type ``Array tests``() =
+    [<Test>]
+    member _.``MutStackArray1.combineMut reuses array B if can fit all data``() =
+        let arrB = Array.zeroCreate 7
+
+        let a = MutStackArray1.Many((2us, Array.zeroCreate 4))
+        let b = MutStackArray1.Many((5us, arrB))
+        let c = MutStackArray1.combineMut(&a, b)
+        let cOpt = MutStackArray1.toArraySlice &c
+        let struct (usedC, arrC) = cOpt.Value
+
+        // We should have the same number of used items
+        Assert.AreEqual(7us, usedC)
+
+        // Reference should be equal to arrB since the array was reused
+        Assert.True(Object.ReferenceEquals(arrC, arrB))

src/Fabulous.Tests/Fabulous.Tests.fsproj

@@ -14,6 +14,7 @@
     <Compile Include="Generators.fs" />
     <Compile Include="AttributesTests.fs" />
     <Compile Include="ViewTests.fs" />
+    <Compile Include="ArrayTests.fs" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="BenchmarkDotNet" />

src/Fabulous/Array.fs

@@ -498,7 +498,7 @@ module StackAllocatedCollections =
                         let arr = ArraySlice.shiftByMut &sliceB (uint16 usedA)
 
                         Array.blit arrA 0 arr 0 usedA
-                        Many(newSize, arrA)
+                        Many(newSize, arr)
                     else
                         // None of them can fit the result
                         // thus allocate a new one