Allowing users to pass in dynamic closures for mocks

Functions/Mock.Tests.ps1

@@ -1516,3 +1516,20 @@ Describe 'Mocking Get-Command' {
         { Mock Get-Command } | Should Not Throw
     }
 }
+
+Describe 'Mocks with closures' {
+    $closureVariable = 'from closure'
+    $scriptBlock = { "Variable resolved $closureVariable" }
+    $closure = $scriptBlock.GetNewClosure()
+    $closureVariable = 'from script'
+
+    function TestClosure([switch] $Closure) { 'Not mocked' }
+
+    Mock TestClosure $closure -ParameterFilter { $Closure }
+    Mock TestClosure $scriptBlock
+
+    It 'Resolves variables in the closure rather than Pester''s current scope' {
+        TestClosure | Should Be 'Variable resolved from script'
+        TestClosure -Closure | Should Be 'Variable resolved from closure'
+    }
+}

Functions/Mock.ps1

@@ -177,8 +177,17 @@ about_Mocking
         $ModuleName = ''
     }
 
-    $mockWithCopy = [scriptblock]::Create($MockWith.ToString())
-    Set-ScriptBlockScope -ScriptBlock $mockWithCopy -SessionState $contextInfo.Session
+    if (Test-IsClosure -ScriptBlock $MockWith)
+    {
+        # If the user went out of their way to call GetNewClosure(), go ahead and leave the block bound to that
+        # dynamic module's scope.
+        $mockWithCopy = $MockWith
+    }
+    else
+    {
+        $mockWithCopy = [scriptblock]::Create($MockWith.ToString())
+        Set-ScriptBlockScope -ScriptBlock $mockWithCopy -SessionState $contextInfo.Session
+    }
 
     $block = @{
         Mock       = $mockWithCopy
@@ -1350,3 +1359,24 @@ function Get-DynamicParametersForMockedFunction
         return & $mock.DynamicParamScriptBlock @Parameters @splat
     }
 }
+
+function Test-IsClosure
+{
+    [CmdletBinding()]
+    param (
+        [Parameter(Mandatory = $true)]
+        [scriptblock]
+        $ScriptBlock
+    )
+
+    $sessionStateInternal = Get-ScriptBlockScope -ScriptBlock $ScriptBlock
+
+    $flags = [System.Reflection.BindingFlags]'Instance,NonPublic'
+    $module = $sessionStateInternal.GetType().GetProperty('Module', $flags).GetValue($sessionStateInternal, $null)
+
+    return (
+        $null -ne $module -and
+        $module.Name -match '^__DynamicModule_([a-f\d-]+)$' -and
+        $null -ne ($matches[1] -as [guid])
+    )
+}