PR Summary

ScriptBlockAst.GetScriptBlock() is the public way to get a new script block from an ast that is not tied to the session state the original was created in. This lets a module take a script block from the caller and run it somewhere else, such as another runspace or a callback on a different thread, without it reaching back into the caller's session state.

This doesn't work when the caller passes a function's script block, for example ${function:Foo} or (Get-Command Foo).ScriptBlock. Its Ast is a FunctionDefinitionAst, which has no GetScriptBlock() method. The only public workaround is Ast.Body.GetScriptBlock() or creating a new ScriptBlock from the Extent string. The first method drops the parameters declared after the function name and the filter keyword, so the result behaves differently from the function. The second works but strips the Ast file information so things like error messages no longer have a file position and $PSCommandPath/$PSScriptRoot won't work anymore. Code that needs the full function has to use reflection to reach the internal ScriptBlock(IParameterMetadataProvider, bool) constructor.

For example, a module that runs user script blocks in its own runspace can now handle both kinds of input in the same way with this PR:

# Works today
$sbk = { param($Prefix) "$Prefix $value" }
$value = 'caller'

$ps = [PowerShell]::Create()
$null = $ps.AddScript('param ($s) $value = "other"; & ($s.Ast.GetScriptBlock()) -Prefix V').
    AddArgument($sbk)
$ps.Invoke()   # V other

# PR allows this to work
function Get-Value ($Prefix) { "$Prefix $value" }
$value = 'caller'

$ps = [PowerShell]::Create()
$null = $ps.AddScript('param ($s) $value = "other"; & ($s.Ast.GetScriptBlock()) -Prefix V').
    AddArgument(${function:Get-Value})
$ps.Invoke()   # V other

Without unbinding, passing ${function:Get-Value} directly outputs "V caller". That script block runs against the original runspace's session state from another thread, which is not safe.

PR Context

This change adds FunctionDefinitionAst.GetScriptBlock(). Like the ScriptBlockAst version, it runs the post parse checks if they have not been done and throws if the ast has errors. It then returns a new unbound script block built from the whole function definition. The result keeps the parameters declared after the function name and the filter behaviour, matching how the engine builds the script block when it defines the function.

The returned script block takes the current language mode, the same as ScriptBlockAst.GetScriptBlock(). Ast.Body.GetScriptBlock() was already public, so this gives constrained script nothing new and is still up to the caller to validate and enforce properly.

PR Checklist

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• Breaking changes
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  • Documentation needed
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• Testing - New and feature
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  • Make sure you've added a new test if existing tests do not effectively test the code changed