Explicity track changes, so they continue across multiple mutations.

Author: chsienki

src/Compilers/CSharp/Test/Semantic/SourceGeneration/GeneratorDriverTests.cs

@@ -3082,6 +3082,10 @@ public void Post_Init_Trees_Are_Reparsed_When_ParseOptions_Change()
             parseOptions = parseOptions.WithLanguageVersion(LanguageVersion.CSharp1);
             compilation = CreateCompilation(source, options: TestOptions.DebugDll, parseOptions: parseOptions);
             driver = driver.WithUpdatedParseOptions(parseOptions);
+
+            // change some other options to ensure the parseOption change tracking flows correctly
+            driver = driver.AddAdditionalTexts(ImmutableArray<AdditionalText>.Empty);
+
             driver = driver.RunGeneratorsAndUpdateCompilation(compilation, out compilation, out diagnostics);
             diagnostics.Verify();
 

src/Compilers/Core/Portable/SourceGeneration/GeneratorDriver.cs

@@ -123,7 +123,7 @@ public GeneratorDriver ReplaceAdditionalText(AdditionalText oldText, AdditionalT
         public GeneratorDriver ReplaceAdditionalTexts(ImmutableArray<AdditionalText> newTexts) => FromState(_state.With(additionalTexts: newTexts));
 
         public GeneratorDriver WithUpdatedParseOptions(ParseOptions newOptions) => newOptions is object
-                                                                                   ? FromState(_state.With(parseOptions: newOptions))
+                                                                                   ? FromState(_state.With(parseOptions: newOptions, parseOptionsChanged: true))
                                                                                    : throw new ArgumentNullException(nameof(newOptions));
 
         public GeneratorDriver WithUpdatedAnalyzerConfigOptions(AnalyzerConfigOptionsProvider newOptions) => newOptions is object
@@ -282,7 +282,7 @@ internal GeneratorDriverState RunGeneratorsCore(Compilation compilation, Diagnos
                 }
             }
 
-            state = state.With(stateTable: driverStateBuilder.ToImmutable(), syntaxStore: syntaxStoreBuilder.ToImmutable(), generatorStates: stateBuilder.ToImmutableAndFree(), runTime: timer.Elapsed);
+            state = state.With(stateTable: driverStateBuilder.ToImmutable(), syntaxStore: syntaxStoreBuilder.ToImmutable(), generatorStates: stateBuilder.ToImmutableAndFree(), runTime: timer.Elapsed, parseOptionsChanged: false);
             return state;
         }
 

src/Compilers/Core/Portable/SourceGeneration/GeneratorDriverState.cs

@@ -95,6 +95,9 @@ internal GeneratorDriverState(ParseOptions parseOptions,
 
         internal readonly bool TrackIncrementalSteps;
 
+        /// <summary>
+        /// Tracks if the <see cref="ParseOptions"/> have been changed meaning post init trees will need to be re-parsed.
+        /// </summary>
         internal readonly bool ParseOptionsChanged;
 
         internal GeneratorDriverState With(
@@ -107,7 +110,8 @@ internal GeneratorDriverState With(
             ParseOptions? parseOptions = null,
             AnalyzerConfigOptionsProvider? optionsProvider = null,
             IncrementalGeneratorOutputKind? disabledOutputs = null,
-            TimeSpan? runTime = null)
+            TimeSpan? runTime = null,
+            bool? parseOptionsChanged = null)
         {
             return new GeneratorDriverState(
                 parseOptions ?? this.ParseOptions,
@@ -121,7 +125,7 @@ internal GeneratorDriverState With(
                 disabledOutputs ?? this.DisabledOutputs,
                 runTime ?? this.RunTime,
                 this.TrackIncrementalSteps,
-                parseOptions is not null
+                parseOptionsChanged ?? this.ParseOptionsChanged
                 );
         }
     }