Multifaceted approach to performantly enabling fileExists outside of the synchronize step in the emit host

src/compiler/program.ts

@@ -1239,7 +1239,14 @@ namespace ts {
                     (fileName, data, writeByteOrderMark, onError, sourceFiles) => host.writeFile(fileName, data, writeByteOrderMark, onError, sourceFiles)),
                 isEmitBlocked,
                 readFile: f => host.readFile(f),
-                fileExists: f => host.fileExists(f),
+                fileExists: f => {
+                    // Use local caches
+                    const path = toPath(f);
+                    if (getSourceFileByPath(path)) return true;
+                    if (contains(missingFilePaths, path)) return false;
+                    // Before falling back to the host
+                    return host.fileExists(f);
+                },
                 ...(host.directoryExists ? { directoryExists: f => host.directoryExists!(f) } : {}),
             };
         }

src/services/services.ts

@@ -1171,13 +1171,13 @@ namespace ts {
             }
 
             // Get a fresh cache of the host information
-            let hostCache = new HostCache(host, getCanonicalFileName);
+            let hostCache: HostCache | undefined = new HostCache(host, getCanonicalFileName);
             const rootFileNames = hostCache.getRootFileNames();
 
             const hasInvalidatedResolution: HasInvalidatedResolution = host.hasInvalidatedResolution || returnFalse;
 
             // If the program is already up-to-date, we can reuse it
-            if (isProgramUptoDate(program, rootFileNames, hostCache.compilationSettings(), path => hostCache.getVersion(path), fileExists, hasInvalidatedResolution, !!host.hasChangedAutomaticTypeDirectiveNames)) {
+            if (isProgramUptoDate(program, rootFileNames, hostCache.compilationSettings(), path => hostCache!.getVersion(path), fileExists, hasInvalidatedResolution, !!host.hasChangedAutomaticTypeDirectiveNames)) {
                 return;
             }
 
@@ -1204,7 +1204,7 @@ namespace ts {
                 readFile(fileName) {
                     // stub missing host functionality
                     const path = toPath(fileName, currentDirectory, getCanonicalFileName);
-                    const entry = hostCache.getEntryByPath(path);
+                    const entry = hostCache && hostCache.getEntryByPath(path);
                     if (entry) {
                         return isString(entry) ? undefined : getSnapshotText(entry.scriptSnapshot);
                     }
@@ -1246,7 +1246,7 @@ namespace ts {
 
             // hostCache is captured in the closure for 'getOrCreateSourceFile' but it should not be used past this point.
             // It needs to be cleared to allow all collected snapshots to be released
-            hostCache = undefined!;
+            hostCache = undefined;
 
             // We reset this cache on structure invalidation so we don't hold on to outdated files for long; however we can't use the `compilerHost` above,
             // Because it only functions until `hostCache` is cleared, while we'll potentially need the functionality to lazily read sourcemap files during
@@ -1260,7 +1260,7 @@ namespace ts {
 
             function fileExists(fileName: string): boolean {
                 const path = toPath(fileName, currentDirectory, getCanonicalFileName);
-                const entry = hostCache.getEntryByPath(path);
+                const entry = hostCache && hostCache.getEntryByPath(path);
                 return entry ?
                     !isString(entry) :
                     (!!host.fileExists && host.fileExists(fileName));
@@ -1278,11 +1278,11 @@ namespace ts {
             }
 
             function getOrCreateSourceFileByPath(fileName: string, path: Path, _languageVersion: ScriptTarget, _onError?: (message: string) => void, shouldCreateNewSourceFile?: boolean): SourceFile | undefined {
-                Debug.assert(hostCache !== undefined);
+                Debug.assert(hostCache !== undefined, "getOrCreateSourceFileByPath called after typical CompilerHost lifetime, check the callstack something with a reference to an old host.");
                 // The program is asking for this file, check first if the host can locate it.
                 // If the host can not locate the file, then it does not exist. return undefined
                 // to the program to allow reporting of errors for missing files.
-                const hostFileInformation = hostCache.getOrCreateEntryByPath(fileName, path);
+                const hostFileInformation = hostCache && hostCache.getOrCreateEntryByPath(fileName, path);
                 if (!hostFileInformation) {
                     return undefined;
                 }

tests/cases/fourslash/importTypesDeclarationDiagnosticsNoServerError.ts

@@ -0,0 +1,14 @@
+/// <reference path="fourslash.ts" />
+
+// @declaration: true
+// @Filename: node_modules/foo/index.d.ts
+////export function f(): I;
+////export interface I {
+////  x: number;
+////}
+// @Filename: a.ts
+////import { f } from "foo";
+////export const x = f();
+
+goTo.file(1);
+verify.getSemanticDiagnostics([]);