Let contributors F5 the F# VS extension without a matching-Roslyn VS

DEVGUIDE.md

@@ -371,6 +371,9 @@ First, ensure that `VisualFSharpDebug` is the startup project.
 
 Then, use the **f5** or **ctrl+f5** keyboard shortcuts to test your tooling changes. The former will debug a new instance of Visual Studio. The latter will launch a new instance of Visual Studio, but with your changes installed.
 
+> [!TIP]
+> If the F# extension fails to load when you F5 (because the Roslyn version in the repo is ahead of the one in your Visual Studio), launch VS with `.\start-vs-VisualFSharpSln.ps1` instead of opening the solution directly. It builds the extension against the Roslyn your VS actually ships, so you don't need an internal/nightly VS or a local Roslyn build. Requires VS with Roslyn 5.10 or newer.
+
 Alternatively, you can do this entirely via the command line if you prefer that:
 
 ```shell

start-vs-VisualFSharpSln.ps1

@@ -0,0 +1,64 @@
+# Launch VS on VisualFSharp.slnx, building the F# VS extension against the Roslyn your installed VS
+# ships (not the newer one flowed into the repo) so F5 loads it. See DEVGUIDE.md. Needs VS Roslyn >= 5.10.
+# -RoslynVersion forces a version (e.g. if the exact VS build isn't on a feed); any 5.Y.* binds identically.
+[CmdletBinding()]
+param(
+    [string]$RoslynVersion,
+    [string]$DevEnv,
+    [string]$Solution = 'VisualFSharp.slnx',
+    [switch]$DryRun
+)
+Set-StrictMode -Version Latest; $ErrorActionPreference = 'Stop'; $root = $PSScriptRoot
+
+if (-not $DevEnv) {
+    $vswhere = Join-Path ${env:ProgramFiles(x86)} 'Microsoft Visual Studio\Installer\vswhere.exe'
+    $DevEnv = if ($env:DevEnvDir) { Join-Path $env:DevEnvDir 'devenv.exe' }
+              elseif (Test-Path $vswhere) { & $vswhere -latest -prerelease -property productPath 2>$null }
+}
+if (-not ($DevEnv -and (Test-Path $DevEnv))) { throw 'devenv.exe not found; run from a VS Developer prompt or pass -DevEnv.' }
+
+if (-not $RoslynVersion) {
+    $dll = "$(Split-Path $DevEnv)\CommonExtensions\Microsoft\VBCSharp\LanguageServices\Microsoft.CodeAnalysis.dll"
+    if (-not (Test-Path $dll)) { throw "Can't detect your VS Roslyn version; pass -RoslynVersion." }
+    $RoslynVersion = ([System.Diagnostics.FileVersionInfo]::GetVersionInfo($dll).ProductVersion -split '\+')[0]
+}
+$minor = [version](($RoslynVersion -split '-')[0])
+if ($minor -lt [version]'5.10.0') {
+    throw "Your VS ships Roslyn $RoslynVersion but the repo references packages from >= 5.10 (unified ExternalAccess, #20099). Update VS or deploy a local Roslyn."
+}
+Write-Host "Building the F# extension against Roslyn $RoslynVersion ($DevEnv)."
+
+# Repoint every Roslyn package (versions set in eng/Version.Details.props) via a props file MSBuild
+# imports after it through the CustomAfterMicrosoftCommonProps hook the launched VS inherits.
+$names = 'MicrosoftCodeAnalysis', 'MicrosoftCodeAnalysisCompilers', 'MicrosoftCodeAnalysisCSharp',
+    'MicrosoftCodeAnalysisEditorFeatures', 'MicrosoftCodeAnalysisEditorFeaturesText', 'MicrosoftCodeAnalysisFeatures',
+    'MicrosoftVisualStudioLanguageServices', 'MicrosoftVisualStudioLanguageServicesExternalAccess'
+$override = Join-Path $root 'artifacts\RoslynOverride.props'
+New-Item -ItemType Directory -Force (Split-Path $override) | Out-Null
+"<Project><PropertyGroup>$(-join ($names | ForEach-Object { "<${_}Version>$RoslynVersion</${_}Version>" }))</PropertyGroup></Project>" |
+    Set-Content -LiteralPath $override -Encoding UTF8
+
+# Apply to THIS process only, restoring in finally so it can't leak into a later build.cmd/CI run
+# (which must keep the flowed Roslyn); the launched VS snapshots the env for its F5/restore builds.
+$vars = @{
+    CustomAfterMicrosoftCommonProps = $override
+    DOTNET_ROOT                     = Join-Path $root '.dotnet'
+    'DOTNET_ROOT(x86)'              = Join-Path $root '.dotnet\x86'
+    PATH                            = "$(Join-Path $root '.dotnet');$env:PATH"
+    RunNetFrameworkApiCompat        = 'false'
+    RunRefApiCompat                 = 'false'
+}
+$saved = @{}; foreach ($k in $vars.Keys) { $saved[$k] = [Environment]::GetEnvironmentVariable($k) }
+try {
+    foreach ($k in $vars.Keys) { Set-Item -LiteralPath "Env:\$k" -Value $vars[$k] }
+    if ($DryRun) { Write-Host "DryRun: $override"; return }
+    & (Join-Path $root 'Restore.cmd')
+    if ($LASTEXITCODE) { throw "Restore failed for Roslyn $RoslynVersion; try another 5.$($minor.Minor).* build via -RoslynVersion." }
+    Start-Process $DevEnv "`"$(Join-Path $root $Solution)`""
+    Write-Host 'Launched VS. Set VisualFSharpDebug as the startup project, then F5 / Ctrl+F5.'
+}
+finally {
+    foreach ($k in $saved.Keys) {
+        if ($null -eq $saved[$k]) { Remove-Item -LiteralPath "Env:\$k" -ErrorAction SilentlyContinue } else { Set-Item -LiteralPath "Env:\$k" -Value $saved[$k] }
+    }
+}