Add a formatter check to disallow backtick line continuation

Summary

Our code formatter (.build/Invoke-CodeFormatterOnFiles.ps1) has no check that flags backtick (`) line continuation in .ps1 / .psm1 files. Backtick continuation is a well-known PowerShell anti-pattern — trailing whitespace after the backtick silently breaks the continuation, and the character is easy to miss in review. Splatting, breaking after a pipe (|), or breaking inside an open paren / brace is preferred in every case.

Neither PSSA (as configured in PSScriptAnalyzerSettings.psd1) nor any of the existing CodeFormatterChecks/ files (CheckContainsCurlyQuotes, CheckFileHasNewlineAtEndOfFile, CheckMarkdownFileHasNoBOM, CheckMultipleEmptyLines, CheckScriptFileHasBOM, CheckScriptFileHasComplianceHeader, CheckScriptFormat, CheckTokenTypeCasing) target backtick continuation. PSAvoidUsingBackticks is not a built-in PSSA rule (it lives in the community CommunityAnalyzerRules module) and is not referenced anywhere in this repo.

Evidence

Discovered while reviewing PR #2571.

• Transport/Get-TerrlExternalRecipientEstimate.ps1 — 9 backtick continuations (lines 316–319, 830–833, 879).
• Transport/Tests/Get-TerrlExternalRecipientEstimate.Tests.ps1 — 39 backtick continuations.

Every occurrence in the PR is either a cmdlet call that would be more readable as a splat, or an expression that could break naturally after | / (. Example:

# Before (line 316)
} elseif (Test-TerrlExchangeOnlineJournalReport `
        -SenderAddress $senderAddress `
        -Recipient $recipient `
        -JournalSet $journalSet `
        -JournalSenderSet $journalSenderSet) {

# After (splat)
$journalArgs = @{
    SenderAddress    = $senderAddress
    Recipient        = $recipient
    JournalSet       = $journalSet
    JournalSenderSet = $journalSenderSet
}
} elseif (Test-TerrlExchangeOnlineJournalReport @journalArgs) {

Proposed change

Add a new CheckBacktickLineContinuation.ps1 under .build/CodeFormatterChecks/ and wire it into .build/Invoke-CodeFormatterOnFiles.ps1:

$errorCount += (CheckBacktickLineContinuation $fileInfo $Save) ? 1 : 0

Detection strategy: tokenize with [System.Management.Automation.PSParser]::Tokenize (already used by CheckTokenTypeCasing) and flag any LineContinuation token, OR match the regex `\s*$ on non-blank source lines that are outside here-strings, comments, and string literals (token stream is safer than the regex — regex would produce false positives inside single-line here-strings or ` characters inside strings).

Reporting: each occurrence should be reported with file + 1-based line number so contributors can locate them quickly.

-Save behavior: none. Auto-fixing backticks to splats is not safe (requires choosing splat variable names, deciding on placement), so this check should be report-only and let the author refactor by hand.

Acceptance criteria

• CheckBacktickLineContinuation.ps1 exists and is dot-sourced by Invoke-CodeFormatterOnFiles.ps1.
• Invoke-CodeFormatterOnFiles calls it for each file and increments $errorCount on hits.
• Running against a repro script containing a backtick continuation returns a non-zero error count and prints file + line number.
• Backticks inside strings, here-strings (@" ... "@, @' ... '@), and comments do not trigger the check.
• Escaped characters that legitimately use backticks in strings ("`t", "`n", "`$var") are not flagged.
• -Save is a no-op for this check (auto-rewrite is not attempted).
• A follow-up sweep PR (or PRs, if scope requires) removes existing offenders across Admin/, Diagnostics/, Shared/, Transport/, Setup/, and the Tests/ folders.

Non-goals

• Auto-fix. Refactoring backtick continuations into splats or natural pipe/paren breaks is an authoring decision, not a formatter transformation.
• Blocking backticks in interactive PowerShell examples inside docs/ markdown files.
• Adding the CommunityAnalyzerRules module as a formatter dependency.

Repro

@'
Get-ChildItem -Path C:\Temp `
    -Recurse `
    -Filter *.log
'@ | Set-Content -Path .\repro.ps1 -Encoding utf8BOM

. .build\Invoke-CodeFormatterOnFiles.ps1
Invoke-CodeFormatterOnFiles -FilePaths .\repro.ps1  # currently returns 0 errors

Related

• #2572 — Type-token casing check (drafted alongside this one from PR #2571 review).

Read .build/Invoke-CodeFormatterOnFiles.ps1 and the existing .build/CodeFormatterChecks/ files, especially CheckTokenTypeCasing, to follow the formatter check pattern. Run the provided repro script through Invoke-CodeFormatterOnFiles and verify the new check reports each real continuation with its file and line number, ignores strings, here-strings, and comments, and leaves files unchanged with -Save.