Do not pass /shared to compilers that cannot reach a compiler server

The dotnet-hosted Roslyn 2.10 build cannot start its VBCSCompiler server
under a current dotnet host, so with /shared every test compilation first
waited out the client's full 20-second new-server connection timeout
before falling back to a sub-second in-process compile. Since the 2.10
configurations were enabled on non-Windows (#3914), that added ~29
minutes to the Linux CI job and ~43 minutes on macOS: ~340 affected
tests at ~21s each, versus ~0.2s for the toolsets whose server works.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/Helpers/Tester.VB.cs

@@ -135,9 +135,8 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 					}
 				}
 
-				// note: the /shared switch is undocumented. It allows us to use the VBCSCompiler.exe compiler
-				// server to speed up testing
-				if (roslynVersion != "legacy")
+				// See UseCompilerServer for why /shared is not passed to every compiler.
+				if (roslynVersion != "legacy" && UseCompilerServer(roslynVersion))
 				{
 					otherOptions += "/shared ";
 				}

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -231,6 +231,23 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 				&& !(executesCompiledOutput && c.HasFlag(CompilerOptions.TargetNet40))).ToArray();
 		}
 
+		/// <summary>
+		/// Whether to pass the undocumented /shared switch, which makes the compiler use the
+		/// VBCSCompiler compiler server and so avoids paying compiler startup per invocation.
+		/// The dotnet-hosted Roslyn 2.x build cannot start its compiler server under a current
+		/// 'dotnet' host (the roll-forward in WrapCompiler applies to csc/vbc itself, not to
+		/// the server process the client tries to spawn), so with /shared every invocation
+		/// first waits out the client's full 20-second new-server connection timeout before
+		/// falling back to a sub-second in-process compile. Only the dotnet-hosted Roslyn 3.0+
+		/// builds get the switch on non-Windows platforms; the Mono-hosted 1.x compilers skip
+		/// it too rather than rely on the server protocol working under Mono.
+		/// </summary>
+		static bool UseCompilerServer(string roslynVersion)
+		{
+			return OperatingSystem.IsWindows()
+				|| Version.Parse(RoslynToolset.SanitizeVersion(roslynVersion)).Major > 2;
+		}
+
 		/// <summary>
 		/// Wraps an external compiler invocation. The .NET Framework builds of the Roslyn
 		/// compilers (csc.exe/vbc.exe) are directly executable on Windows and are hosted by
@@ -717,11 +734,12 @@ namespace System.Runtime.CompilerServices
 
 				HashSet<string> noWarn = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
 
-				// note: the /shared switch is undocumented. It allows us to use the VBCSCompiler.exe compiler
-				// server to speed up testing
 				if (roslynVersion != "legacy")
 				{
-					otherOptions += "/shared ";
+					if (UseCompilerServer(roslynVersion))
+					{
+						otherOptions += "/shared ";
+					}
 					var version = Version.Parse(RoslynToolset.SanitizeVersion(roslynVersion));
 					if (!targetNet40 && version.Major > 2)
 					{