Make Generated syntax trees restore to project-level nullability (#35018)

* Make Generated syntax trees restore to project nullability

docs/features/nullable-reference-types.md

@@ -361,9 +361,7 @@ A syntax tree is determined to be generated if meets one or more of the followin
     - `<autogenerated`
     - `<auto-generateed`
 
-Newer, nullable-aware generators, may then opt-in to nullable analysis by including a generated `#nullable enable` at the beginning of the generated code.
-
-Within the context of a generated syntax tree `#nullable restore` will always set the state to "disable".
+Newer, nullable-aware generators may then opt-in to nullable analysis by including a generated `#nullable restore` at the beginning of the code, ensuring that the generated syntax tree will be analyzed according to the project-level nullable context.
 
 ## Public APIs
 There are a few questions that an API consumer would want to answer:

src/Compilers/CSharp/Portable/Syntax/NullableDirectiveMap.cs

@@ -55,7 +55,8 @@ private NullableDirectiveMap(ImmutableArray<(int Position, bool? State)> directi
                 index = ~index - 1;
             }
 
-            bool? state = null;
+            // Generated files have an initial nullable context that is "disabled"
+            bool? state = (_isGeneratedCode) ? false : (bool?)null;
             if (index >= 0)
             {
                 Debug.Assert(_directives[index].Position <= position);
@@ -63,12 +64,6 @@ private NullableDirectiveMap(ImmutableArray<(int Position, bool? State)> directi
                 state = _directives[index].State;
             }
 
-            if (state == null && _isGeneratedCode)
-            {
-                // Generated files have a default nullable context that is "disabled".
-                state = false;
-            }
-
             return state;
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -5232,22 +5232,25 @@ static void F(object x)
         }
 
         [Fact]
-        public void Directive_GloballyEnabled_GeneratedCode_RestoreToDisabled()
+        public void Directive_GloballyEnabled_GeneratedCode_RestoreToProjectDefault()
         {
             var source =
 @"
 // <autogenerated />
-#nullable enable
 partial class Program
 {
 #nullable restore
     static void F(object x)
     {
-        x = null; // no warning, restored to disabled
+        x = null; // warning, restored to project default
     }
 }";
             var comp = CreateCompilation(source, options: TestOptions.DebugDll.WithNullableContextOptions(NullableContextOptions.Enable));
-            comp.VerifyDiagnostics();
+            comp.VerifyDiagnostics(
+                 // (8,13): warning CS8600: Converting null literal or possible null value to non-nullable type.
+                 //         x = null; // warning
+                 Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "null").WithLocation(8, 13)
+                );
         }
 
         [Fact]