Ban passing @extern files under --js flag in a TypedAST compilation

This is something we could in theory support, but has no known use cases at the moment.

PiperOrigin-RevId: 505173097

Author: lauraharker

src/com/google/javascript/jscomp/Compiler.java

@@ -86,6 +86,7 @@
 import com.google.javascript.rhino.JSDocInfo;
 import com.google.javascript.rhino.Node;
 import com.google.javascript.rhino.StaticScope;
+import com.google.javascript.rhino.StaticSourceFile.SourceKind;
 import com.google.javascript.rhino.jstype.JSTypeRegistry;
 import java.io.IOException;
 import java.io.InputStream;
@@ -2932,7 +2933,7 @@ ControlFlowGraph<Node> computeCFG() {
    */
   @VisibleForTesting
   final SourceFile SYNTHETIC_EXTERNS_FILE =
-      SourceFile.fromCode(SYNTHETIC_FILE_NAME_PREFIX + "externs] ", "");
+      SourceFile.fromCode(SYNTHETIC_FILE_NAME_PREFIX + "externs] ", "", SourceKind.EXTERN);
 
   private @Nullable CompilerInput syntheticExternsInput; // matches SYNTHETIC_EXTERNS_FILE
 

src/com/google/javascript/jscomp/serialization/TypedAstDeserializer.java

@@ -23,6 +23,7 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.base.Optional;
+import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
@@ -35,14 +36,17 @@
 import com.google.javascript.jscomp.parsing.parser.FeatureSet;
 import com.google.javascript.rhino.IR;
 import com.google.javascript.rhino.Node;
+import com.google.javascript.rhino.StaticSourceFile.SourceKind;
 import com.google.protobuf.CodedInputStream;
 import com.google.protobuf.ExtensionRegistry;
 import com.google.protobuf.InvalidProtocolBufferException;
 import com.google.protobuf.WireFormat;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.function.Function;
@@ -279,9 +283,28 @@ private void deserializeTypedAst(
 
   private ImmutableList<SourceFile> toFileShard(TypedAst typedAstProto) {
     ImmutableList.Builder<SourceFile> fileShardBuilder = ImmutableList.builder();
-    for (SourceFileProto p : typedAstProto.getSourceFilePool().getSourceFileList()) {
-      fileShardBuilder.add(
-          filePoolBuilder.computeIfAbsent(p.getFilename(), (n) -> SourceFile.fromProto(p)));
+    HashSet<Integer> externFileIndices = new HashSet<>();
+    for (LazyAst lazyAst : typedAstProto.getExternAstList()) {
+      externFileIndices.add(lazyAst.getSourceFile() - 1);
+    }
+    List<SourceFileProto> protos = typedAstProto.getSourceFilePool().getSourceFileList();
+    for (int i = 0; i < protos.size(); i++) {
+      SourceFileProto p = protos.get(i);
+      SourceFile file =
+          filePoolBuilder.computeIfAbsent(p.getFilename(), (n) -> SourceFile.fromProto(p));
+      // Verify that the TypedAST SourceFileProto files and the 'requiredInputFiles' SourceFiles
+      // agree on which files are externs vs. code. In non-TypedAST builds, we allow passing @extern
+      // files under the --js flag. This means 'SourceFile' objects may have a non-EXTERN
+      // SourceKind but point to a file with @externs.
+      // For TypedAST builds, we could support this, but it's an uncommon pattern and trickier to
+      // support than ban.
+      if (externFileIndices.contains(i)) {
+        Preconditions.checkState(
+            file.getKind() == SourceKind.EXTERN,
+            "TypedAST compilations must pass all extern files as externs, not js, but found %s",
+            file.getName());
+      }
+      fileShardBuilder.add(file);
     }
     return fileShardBuilder.build();
   }

test/com/google/javascript/jscomp/CommandLineRunnerTest.java

@@ -237,7 +237,7 @@ public void test3StageCompile() throws Exception {
             "--source_map_include_content",
             "--translations_file",
             msgBundle.toString(),
-            "--js",
+            "--externs",
             externsFile.toString(),
             "--js",
             srcFile.toString());