Add tab completion test for PowerShell

appveyor.yml

@@ -47,6 +47,8 @@ install:
       } else {
         Write-Host "MSYS2 not required" -ForegroundColor Green
       }
+  - pwsh: |
+      Install-Module Pester -Force
 
   # Install rust, x86_64-pc-windows-msvc host
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
@@ -81,6 +83,9 @@ test_script:
   - if defined BUILD_MSI pushd src\rustup-win-installer && cargo build --release --target %TARGET% & popd
   - if defined BUILD_MSI pushd src\rustup-win-installer\msi && powershell .\build.ps1 -Target %TARGET% & popd
   - cargo fmt --all -- --check
+  - pwsh: |
+      $pesterOutputFile = 'src\rustup-win-installer\msi\PesterTestsResults.xml'
+      Invoke-Pester -OutputFormat NUnitXml -OutputFile $pesterOutputFile
 
 notifications:
   - provider: Webhook

src/rustup-win-installer/msi/Completion.Tests.ps1

@@ -0,0 +1,21 @@
+if (Get-Command -Name rustup -ErrorAction Stop) {
+	# Enable rustup tab completion
+	rustup completions powershell | Out-String | Invoke-Expression
+}
+
+function Get-Result([string]$Text, [int]$CursorPosition = $Text.Length, [hashtable]$Options) {
+	[System.Management.Automation.CommandCompletion]::CompleteInput($Text, $CursorPosition, $Options).CompletionMatches
+}
+
+Describe 'rustup flags and subcommands' {
+	$result = Get-Result 'rustup '
+
+	# Keep it simple because over-specify testing might be risky
+	$result.Count | Should -BeGreaterThan 10
+	$result.CompletionText | Should -Contain 'completions'
+	$result.CompletionText | Should -Contain 'install'
+	$result.CompletionText | Should -Contain 'uninstall'
+	$result.CompletionText | Should -Contain 'update'
+	$result.CompletionText | Should -Contain '--help'
+	$result.CompletionText | Should -Contain '--verbose'
+}