Add tab completion instructions for PowerShell

[matt9ucci]

For Windows PowerShell and PowerShell Core (Linux/Mac/Win) users.

[kinnison]

I'm okay with this, but I'd like someone else who has Windows to hand to give it a 👍 too

[dwijnand]

@matt9ucci do you know if one could programmatically test installed PowerShell completions?

I'd (always) prefer a test, but I'm cool with this landing as is, and forward-fixing anything that comes up.

[matt9ucci]

do you know if one could programmatically test installed PowerShell completions?

Yes. 😃
Try the following test code:

# Test rustup
if (!(Get-Command rustup)) {
    Write-Error 'rustup is not installed'
}

# Enable rustup tab completion
rustup completions powershell | Out-String | Invoke-Expression

# Get completion texts for 'rustup ' (e.g. completions, component, default, doc, etc.)
$text = 'rustup '
$cursorPosition = $text.Length
$options = $null
$completionTexts = [System.Management.Automation.CommandCompletion]::CompleteInput($text, $cursorPosition, $options).CompletionMatches.CompletionText

# Verify the result of completion
if ($completionTexts -contains 'completions') {
    Write-Host 'rustup tab completion is successfully enabled'
} else {
    Write-Error 'rustup tab completion is not enabled'
}

It is just an easy version. If we need "full" unit testing, using the Pester is the best solution (for reference, see my Pester test cases for DockerCompletion module)

[dwijnand]

Cool. It doesn't look like we have anything like that running on AppVeyor. Is setting that up something that would be of interest to you?

[kinnison]

I'm not sure we CI completions at all right now. I'd be satisfied with just one other person who can use PowerShell verifying that the instructions provided for the README work and are appropriately idiomatic.

[matt9ucci]

Hi all. I have done with Pester test script and appveyor.yml, and submitted the PR #1629. Please review it.
If you do not need such a test script, don't hesitate to close the PR.