[Frontend] Ignore adjacent swiftmodule in compiler host modules

include/swift/AST/DiagnosticsFrontend.def

@@ -437,7 +437,8 @@ NOTE(sdk_version_pbm_version,none,
 NOTE(compiled_module_ignored_reason,none,
      "compiled module '%0' was ignored because %select{%error"
      "|it belongs to a framework in the SDK"
-     "|loading from module interfaces is preferred}1",
+     "|loading from module interfaces is preferred"
+     "|it's a compiler host module}1",
      (StringRef, unsigned))
 NOTE(out_of_date_module_here,none,
      "%select{compiled|cached|forwarding|prebuilt}0 module is out of date: '%1'",

include/swift/Basic/StringExtras.h

@@ -519,6 +519,9 @@ class NullTerminatedStringRef {
 /// where escaped Unicode characters lead to malformed/invalid JSON.
 void writeEscaped(llvm::StringRef Str, llvm::raw_ostream &OS);
 
+/// Whether the path components of `path` begin with those from `prefix`.
+bool pathStartsWith(StringRef prefix, StringRef path);
+
 } // end namespace swift
 
 #endif // SWIFT_BASIC_STRINGEXTRAS_H

lib/AST/Module.cpp

@@ -43,6 +43,7 @@
 #include "swift/Basic/Compiler.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
+#include "swift/Basic/StringExtras.h"
 #include "swift/Demangling/ManglingMacros.h"
 #include "swift/Parse/Token.h"
 #include "swift/Strings.h"
@@ -4235,18 +4236,6 @@ FrontendStatsTracer::getTraceFormatter<const SourceFile *>() {
   return &TF;
 }
 
-static bool prefixMatches(StringRef prefix, StringRef path) {
-  auto prefixIt = llvm::sys::path::begin(prefix),
-       prefixEnd = llvm::sys::path::end(prefix);
-  for (auto pathIt = llvm::sys::path::begin(path),
-            pathEnd = llvm::sys::path::end(path);
-       prefixIt != prefixEnd && pathIt != pathEnd; ++prefixIt, ++pathIt) {
-    if (*prefixIt != *pathIt)
-      return false;
-  }
-  return prefixIt == prefixEnd;
-}
-
 bool IsNonUserModuleRequest::evaluate(Evaluator &evaluator, ModuleDecl *mod) const {
   // stdlib is non-user by definition
   if (mod->isStdlibModule())
@@ -4274,5 +4263,6 @@ bool IsNonUserModuleRequest::evaluate(Evaluator &evaluator, ModuleDecl *mod) con
     return false;
 
   StringRef runtimePath = searchPathOpts.RuntimeResourcePath;
-  return (!runtimePath.empty() && prefixMatches(runtimePath, modulePath)) || (!sdkPath.empty() && prefixMatches(sdkPath, modulePath));
+  return (!runtimePath.empty() && pathStartsWith(runtimePath, modulePath)) ||
+      (!sdkPath.empty() && pathStartsWith(sdkPath, modulePath));
 }

lib/Basic/StringExtras.cpp

@@ -23,6 +23,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 
@@ -1417,3 +1418,15 @@ void swift::writeEscaped(llvm::StringRef Str, llvm::raw_ostream &OS) {
     }
   }
 }
+
+bool swift::pathStartsWith(StringRef prefix, StringRef path) {
+  auto prefixIt = llvm::sys::path::begin(prefix),
+       prefixEnd = llvm::sys::path::end(prefix);
+  for (auto pathIt = llvm::sys::path::begin(path),
+            pathEnd = llvm::sys::path::end(path);
+       prefixIt != prefixEnd && pathIt != pathEnd; ++prefixIt, ++pathIt) {
+    if (*prefixIt != *pathIt)
+      return false;
+  }
+  return prefixIt == prefixEnd;
+}

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -20,6 +20,7 @@
 #include "swift/AST/FileSystem.h"
 #include "swift/AST/Module.h"
 #include "swift/Basic/Platform.h"
+#include "swift/Basic/StringExtras.h"
 #include "swift/Frontend/CachingUtils.h"
 #include "swift/Frontend/Frontend.h"
 #include "swift/Frontend/ModuleInterfaceSupport.h"
@@ -233,6 +234,7 @@ struct ModuleRebuildInfo {
     NotIgnored,
     PublicFramework,
     InterfacePreferred,
+    CompilerHostModule,
   };
   struct CandidateModule {
     std::string path;
@@ -704,7 +706,28 @@ class ModuleInterfaceLoaderImpl {
   bool isInResourceDir(StringRef path) {
     StringRef resourceDir = ctx.SearchPathOpts.RuntimeResourcePath;
     if (resourceDir.empty()) return false;
-    return path.startswith(resourceDir);
+    return pathStartsWith(resourceDir, path);
+  }
+
+  bool isInResourceHostDir(StringRef path) {
+    StringRef resourceDir = ctx.SearchPathOpts.RuntimeResourcePath;
+    if (resourceDir.empty()) return false;
+
+    SmallString<128> hostPath;
+    llvm::sys::path::append(hostPath,
+                            resourceDir, "host");
+    return pathStartsWith(hostPath, path);
+  }
+
+  bool isInSystemFrameworks(StringRef path) {
+    StringRef sdkPath = ctx.SearchPathOpts.getSDKPath();
+    if (sdkPath.empty()) return false;
+
+    SmallString<128> publicFrameworksPath;
+    llvm::sys::path::append(publicFrameworksPath,
+                            sdkPath, "System", "Library", "Frameworks");
+
+    return pathStartsWith(publicFrameworksPath, path);
   }
 
   std::pair<std::string, std::string> getCompiledModuleCandidates() {
@@ -719,10 +742,12 @@ class ModuleInterfaceLoaderImpl {
     llvm::sys::path::append(publicFrameworksPath,
                             ctx.SearchPathOpts.getSDKPath(),
                             "System", "Library", "Frameworks");
-    if (!ctx.SearchPathOpts.getSDKPath().empty() &&
-        modulePath.startswith(publicFrameworksPath)) {
+    if (isInSystemFrameworks(modulePath)) {
       shouldLoadAdjacentModule = false;
       rebuildInfo.addIgnoredModule(modulePath, ReasonIgnored::PublicFramework);
+    } else if (isInResourceHostDir(modulePath)) {
+      shouldLoadAdjacentModule = false;
+      rebuildInfo.addIgnoredModule(modulePath, ReasonIgnored::CompilerHostModule);
     }
 
     switch (loadMode) {

test/ModuleInterface/ignore-adjacent-host-module.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/build/host)
+// RUN: %empty-directory(%t/cache)
+// RUN: split-file %s %t
+
+/// Modules loaded from within lib/swift/host should also be rebuilt from
+/// their interface (which actually means anything within resource-dir/host).
+
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift \
+// RUN:   -swift-version 5 -enable-library-evolution \
+// RUN:   -parse-stdlib -module-cache-path %t/cache \
+// RUN:   -o %t/build/host -emit-module-interface-path %t/build/host/Lib.swiftinterface
+
+// RUN: %target-swift-frontend -typecheck %t/Client.swift \
+// RUN:   -resource-dir %t/build -I %t/build/host \
+// RUN:   -parse-stdlib -module-cache-path %t/cache \
+// RUN:   -Rmodule-loading 2>&1 | %FileCheck %s
+
+// CHECK: remark: loaded module 'Lib'; source: '{{.*}}{{/|\\}}host{{/|\\}}Lib.swiftinterface', loaded: '{{.*}}{{/|\\}}cache{{/|\\}}Lib-{{.*}}.swiftmodule'
+
+//--- Lib.swift
+public func foo() {}
+
+//--- Client.swift
+import Lib
+foo()