Reuse module resolutions in project references

Author: gluxon

src/compiler/program.ts

@@ -1221,6 +1221,9 @@ namespace ts {
             function doesStructuralPermitResolutionsReuse(): boolean {
                 switch (structuralIsReused) {
                     case StructureIsReused.Not: return false;
+                    case StructureIsReused.SafeProjectReferenceModules:
+                        return file.resolvedModules !== undefined &&
+                            isSourceOfProjectReferenceRedirect(file.originalFileName);
                     case StructureIsReused.SafeModules: return true;
                     case StructureIsReused.Completely: return true;
 
@@ -1293,7 +1296,10 @@ namespace ts {
 
         function tryReuseStructureFromOldProgram(): StructureIsReused {
             if (!oldProgram) {
-                return StructureIsReused.Not;
+                // During initial program creation, root files may import files from project
+                // references that were previously loaded. Those resolutions are safe to reuse
+                // since another program instance kept them up to date.
+                return StructureIsReused.SafeProjectReferenceModules;
             }
 
             // check properties that can affect structure of the program or module resolution strategy

src/compiler/types.ts

@@ -3824,9 +3824,10 @@ namespace ts {
     /* @internal */
     /** "Structure" refers to the SourceFile graph of a Program linked by module resolutions. */
     export const enum StructureIsReused {
-        Not         = 0,      // The entire Program must be (re)created.
-        SafeModules = 1 << 0, // SourceFile objects need to be rediscovered, but module resolutions can be reused.
-        Completely  = 1 << 1, // SourceFile objects and module resolutions can be reused from an old Program.
+        Not                         = 0,      // The entire Program must be (re)created.
+        SafeProjectReferenceModules = 1 << 0, // SourceFile objects need to be rediscovered, but module resolutions within project reference sources may be reused.
+        SafeModules                 = 1 << 1, // SourceFile objects need to be rediscovered, but module resolutions can be reused.
+        Completely                  = 1 << 2, // SourceFile objects and module resolutions can be reused from an old Program.
     }
 
     export type CustomTransformerFactory = (context: TransformationContext) => CustomTransformer;