Add new MissingTryBlock rule

[iRon7]

PR Summary

Although #2098 didn't get an up-for-graps label, I did create the suggested Add new MissingTryBlock rule to warn when catch or finally blocks are not preceded by a try block because:

• I think that a catch block that misses a try block is less valid than a try block that misses a catch block which causes an parse error by the engine
  • and therefore even deserves the severity Error
• A missing try block might easily get unnoticed during design time but will very likely fail during runtime
• It happened twice now that someone in our company tried to publish a script that was missing a try block

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[liamjpeters]

👋

This rule has a severity of Warning but is emitting a diagnostic of Error severity. Where this is the only diagnostic that your rule emits, these should match. I would suggest Warning - just because it's not the 99% use-case, doesn't mean I can't define a function called catch and have it accept a ScriptBlock if I wanted to.

This has the same copy-pasta as your other PR.

[Copilot]

Pull request overview

Adds a new built-in PSScriptAnalyzer rule to detect catch/finally blocks used without a preceding try, with accompanying documentation and Pester tests to validate behavior.

Changes:

• Introduces the new MissingTryBlock built-in rule implementation and localized strings.
• Adds rule documentation and registers it in the rules documentation index.
• Adds a dedicated Pester test suite for violation, compliant, and suppression scenarios.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
docs/Rules/README.md	Adds MissingTryBlock to the published rules index table.
docs/Rules/MissingTryBlock.md	New rule documentation page describing intent, guidance, and examples.
Tests/Rules/MissingTryBlock.tests.ps1	New Pester coverage for violations, compliant cases, and suppression behavior.
Rules/Strings.resx	Adds name/common-name/description/error message resources for the new rule.
Rules/MissingTryBlock.cs	Implements AST-based detection and emits diagnostics for missing try before catch/finally.

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[andyleejordan]

Thanks again @iRon7 for working through this with @liamjpeters. The severity mismatch is fixed and the implementation is tight (using StringConstantExpressionAst as the bareword detection is the right approach for this scenario).

One context note that may be useful: this rule was explicitly endorsed upstream in PowerShell/PowerShell#25491 by the engine working group as something that belongs in PSSA rather than the parser. Good signal that there's appetite for landing it.

Three things before I'm comfortable merging:

The // Find all FunctionDefinitionAst in the Ast comment. GitHub's review bot flagged this and you replied "Outdated?" — but the comment is still in Rules/MissingTryBlock.cs on the current head and it's misleading: the code is finding StringConstantExpressionAst, not FunctionDefinitionAst. Could you replace it with something like // Find bareword "catch" or "finally" tokens that are not part of a TryStatementAst?

Opt-in by default. Same convention point as the other rules: please derive from ConfigurableRule with Enable = false so users can opt in, and update docs/Rules/README.md.

Doc the false-positive. This rule will fire on user code like:

function catch { param([scriptblock]$body) & $body }
catch { Write-Host 'hi' }

which is legal PowerShell (catch is not a reserved word at the command position). Worth a short note in docs/Rules/MissingTryBlock.md so users know they may need to suppress in unusual cases. Doesn't change the implementation.

Once those are sorted, I'd like @liamjpeters to take a quick second look since he did the bulk of the original review.

Drafted by Copilot (Claude Opus 4.7)