mercurial argument completion, works in powershell 5.1, should work with TabExpansionPlusPlus; faster (and more limited) than https://github.com/JeremySkinner/posh-hg

hg.ArgumentCompleters.ps1

# http://www.wtfpl.net/txt/copying/

function isHgDirectory() {
  if(test-path ".hg") {
    return (Get-Item .).fullname
  }

  $d = (Get-Item .).parent
  while ($d -ne $NULL) {
      $p = $d.fullname + "\.hg"
      if (Test-Path $p) {
          return $d.fullname
      } else {
          $d = $d.parent
      }
    }

    return $false
}

function hgStatus($filter) {
  $hgdir = isHgDirectory
  $arg = '-' + $filter
  if ($hgdir) {
    (hg st -n $arg) | ForEach-Object {
      $p = Join-Path $hgdir $_
      [System.Management.Automation.CompletionResult]::new($p, $p, 'ParameterValue', $p)
    }
  }
}

$global:hgcommandset = (hg debugcomplete) | Sort-Object -Unique
function HgArgumentCompletion {
  param($wordToComplete, $commandAst, $cursorPosition)
  $ct = $commandAst.CommandElements.Count
  if ($ct -eq 1) {
    $hgcommandset | ForEach-Object {
      [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
    }
  } elseif ($ct -eq 2 -and $commandAst.CommandElements[1].ToString() -eq $wordToComplete) {
    $hgcommandset | Where-Object { $_ -like  "${wordToComplete}*" }| ForEach-Object {
      [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
    }
  } elseif ($ct -gt 2 -and $wordToComplete -match '^[mardcui]+$') {
    hgStatus $wordToComplete
  } else {
    $cmd = $commandAst.CommandElements[1].ToString()
    $c = "${wordToComplete}*"
    if ($c -eq "*") {
      $c = "--*"
    }
    (hg debugcomplete -o $cmd) | Sort-Object -Unique | Where-Object { $_ -like  $c }| ForEach-Object {
      [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
    }
  }
}

Register-ArgumentCompleter -Native -CommandName hg -ScriptBlock $function:HgArgumentCompletion