From 16f90cac4b78d8a9207ecbbcf515518bc01cb7b2 Mon Sep 17 00:00:00 2001
From: JoostK <joost.koehoorn@gmail.com>
Date: Sat, 14 Nov 2020 20:37:27 +0100
Subject: [PATCH] perf(compiler-cli): ensure module resolution cache is reused
 for type-check program (#39693)

The Angular compiler creates two `ts.Program`s; one for emit and one for
template type-checking. The creation of the type-check program could
benefit from reusing the `ts.ModuleResolutionCache` that was primed
during the creation of the emit program. This requires that the compiler
host implements `resolveModuleNames`, as otherwise TypeScript will setup
a `ts.ModuleResolutionHost` of its own for both programs.

This commit ensures that `resolveModuleNames` is always implemented,
even if the originally provided compiler host does not. This is
beneficial for the `ngc` binary.

PR Close #39693
---
 .../compiler-cli/src/ngtsc/core/src/host.ts   | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/packages/compiler-cli/src/ngtsc/core/src/host.ts b/packages/compiler-cli/src/ngtsc/core/src/host.ts
index 24683b131b2..1dfeba4f187 100644
--- a/packages/compiler-cli/src/ngtsc/core/src/host.ts
+++ b/packages/compiler-cli/src/ngtsc/core/src/host.ts
@@ -101,6 +101,12 @@ export class NgCompilerHost extends DelegatingCompilerHost implements
     this.constructionDiagnostics = diagnostics;
     this.inputFiles = [...inputFiles, ...shimAdapter.extraInputFiles];
     this.rootDirs = rootDirs;
+
+    if (this.resolveModuleNames === undefined) {
+      // In order to reuse the module resolution cache during the creation of the type-check
+      // program, we'll need to provide `resolveModuleNames` if the delegate did not provide one.
+      this.resolveModuleNames = this.createCachedResolveModuleNamesFunction();
+    }
   }
 
   /**
@@ -263,4 +269,17 @@ export class NgCompilerHost extends DelegatingCompilerHost implements
   get unifiedModulesHost(): UnifiedModulesHost|null {
     return this.fileNameToModuleName !== undefined ? this as UnifiedModulesHost : null;
   }
+
+  private createCachedResolveModuleNamesFunction(): ts.CompilerHost['resolveModuleNames'] {
+    const moduleResolutionCache = ts.createModuleResolutionCache(
+        this.getCurrentDirectory(), this.getCanonicalFileName.bind(this));
+
+    return (moduleNames, containingFile, reusedNames, redirectedReference, options) => {
+      return moduleNames.map(moduleName => {
+        const module = ts.resolveModuleName(
+            moduleName, containingFile, options, this, moduleResolutionCache, redirectedReference);
+        return module.resolvedModule;
+      });
+    };
+  }
 }