src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java - bazel - Git at Google

 // Copyright 2014 The Bazel Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //    http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package com.google.devtools.build.lib.syntax;

 import com.google.common.base.Preconditions;
 import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.util.SpellChecker;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import javax.annotation.Nullable;

 /**
  * A class for doing static checks on files, before evaluating them.
  *
  * <p>We implement the semantics discussed in
  * https://github.com/bazelbuild/proposals/blob/master/docs/2018-06-18-name-resolution.md
  *
  * <p>When a variable is defined, it is visible in the entire block. For example, a global variable
  * is visible in the entire file; a variable in a function is visible in the entire function block
  * (even on the lines before its first assignment).
  *
  * <p>Validation is a mutation of the syntax tree, as it attaches scope information to Identifier
  * nodes. (In the future, it will attach additional information to functions to support lexical
  * scope, and even compilation of the trees to bytecode.) Validation errors are reported in the
  * analogous manner to scan/parse errors: for a StarlarkFile, they are appended to {@code
  * StarlarkFile.errors}; for an expression they are reported by an SyntaxError exception. It is
  * legal to validate a file that already contains scan/parse errors, though it may lead to secondary
  * validation errors.
  */
 // TODO(adonovan): make this class private. Call it through the EvalUtils facade.
 public final class ValidationEnvironment extends NodeVisitor {

   enum Scope {
     /** Symbols defined inside a function or a comprehension. */
     Local("local"),
     /** Symbols defined at a module top-level, e.g. functions, loaded symbols. */
     Module("global"),
     /** Predefined symbols (builtins) */
     Universe("builtin");

     private final String qualifier;

     private Scope(String qualifier) {
       this.qualifier = qualifier;
     }

     String getQualifier() {
       return qualifier;
     }
   }

   /**
    * Module is a static abstraction of a Starlark module. It describes the set of variable names for
    * use during name resolution.
    */
   public interface Module {

     // TODO(adonovan): opt: for efficiency, turn this into a predicate, not an enumerable set,
     // and look up bindings as they are needed, not preemptively.
     // Otherwise we must do work proportional to the number of bindings in the
     // environment, not the number of free variables of the file/expression.
     //
     // A single method will then suffice:
     //   Scope resolve(String name) throws Undeclared
     // This requires that the Module retain its semantics.

     /** Returns the set of names defined by this module. The caller must not modify the set. */
     Set<String> getNames();

     /**
      * Returns (optionally) a more specific error for an undeclared name than the generic message.
      * This hook allows the module to implement "semantics-restricted" names without any knowledge
      * in this file.
      */
     @Nullable
     String getUndeclaredNameError(StarlarkSemantics semantics, String name);
   }

   private static final Identifier PREDECLARED = new Identifier("");

   private static class Block {
     private final Map<String, Identifier> variables = new HashMap<>();
     private final Scope scope;
     @Nullable private final Block parent;

     Block(Scope scope, @Nullable Block parent) {
       this.scope = scope;
       this.parent = parent;
     }
   }

   private final List<Event> errors;
   private final StarlarkSemantics semantics;
   private final Module module;
   private Block block;
   private int loopCount;

   // In BUILD files, we have a slightly different behavior for legacy reasons.
   // TODO(adonovan): eliminate isBuildFile. It is necessary because the prelude is implemented
   // by inserting shared Statements, which must not be mutated, into each StarlarkFile.
   // Instead, we should implement the prelude by executing it like a .bzl module
   // and putting its members in the initial environment of the StarlarkFile.
   // In the meantime, let's move this flag into Module (GlobalFrame).
   private final boolean isBuildFile;

   private ValidationEnvironment(
       List<Event> errors, Module module, StarlarkSemantics semantics, boolean isBuildFile) {
     this.errors = errors;
     this.module = module;
     this.semantics = semantics;
     this.isBuildFile = isBuildFile;
     block = new Block(Scope.Universe, null);
     for (String name : module.getNames()) {
       block.variables.put(name, PREDECLARED);
     }
   }

   void addError(Location loc, String message) {
     errors.add(Event.error(loc, message));
   }

   /**
    * First pass: add all definitions to the current block. This is done because symbols are
    * sometimes used before their definition point (e.g. a functions are not necessarily declared in
    * order).
    */
   private void collectDefinitions(Iterable<Statement> stmts) {
     for (Statement stmt : stmts) {
       collectDefinitions(stmt);
     }
   }

   private void collectDefinitions(Statement stmt) {
     switch (stmt.kind()) {
       case ASSIGNMENT:
         collectDefinitions(((AssignmentStatement) stmt).getLHS());
         break;
       case IF:
         IfStatement ifStmt = (IfStatement) stmt;
         collectDefinitions(ifStmt.getThenBlock());
         if (ifStmt.getElseBlock() != null) {
           collectDefinitions(ifStmt.getElseBlock());
         }
         break;
       case FOR:
         ForStatement forStmt = (ForStatement) stmt;
         collectDefinitions(forStmt.getLHS());
         collectDefinitions(forStmt.getBlock());
         break;
       case DEF:
         DefStatement def = (DefStatement) stmt;
         declare(def.getIdentifier());
         break;
       case LOAD:
         LoadStatement load = (LoadStatement) stmt;

         // The global reassignment check is not yet enabled for BUILD files,
         // but we apply it to load statements as a special case.
         // Because (for now) its error message is better than the general
         // message emitted by 'declare', we'll apply it to non-BUILD files too.
         Set<String> names = new HashSet<>();
         for (LoadStatement.Binding b : load.getBindings()) {
           if (!names.add(b.getLocalName().getName())) {
             addError(
                 b.getLocalName().getStartLocation(),
                 String.format(
                     "load statement defines '%s' more than once", b.getLocalName().getName()));
           }
         }

         for (LoadStatement.Binding b : load.getBindings()) {
           declare(b.getLocalName());
         }
         break;
       case EXPRESSION:
       case FLOW:
       case RETURN:
         // nothing to declare
     }
   }

   private void collectDefinitions(Expression lhs) {
     for (Identifier id : Identifier.boundIdentifiers(lhs)) {
       declare(id);
     }
   }

   private void assign(Expression lhs) {
     if (lhs instanceof Identifier) {
       if (!isBuildFile) {
         ((Identifier) lhs).setScope(block.scope);
       }
       // no-op
     } else if (lhs instanceof IndexExpression) {
       visit(lhs);
     } else if (lhs instanceof ListExpression) {
       for (Expression elem : ((ListExpression) lhs).getElements()) {
         assign(elem);
       }
     } else {
       addError(lhs.getStartLocation(), "cannot assign to '" + lhs + "'");
     }
   }

   @Override
   public void visit(Identifier node) {
     String name = node.getName();
     @Nullable Block b = blockThatDefines(name);
     if (b == null) {
       // The identifier might not exist because it was restricted (hidden) by the current semantics.
       // If this is the case, output a more helpful error message than 'not found'.
       String error = module.getUndeclaredNameError(semantics, name);
       if (error == null) {
         // generic error
         error = createInvalidIdentifierException(node.getName(), getAllSymbols());
       }
       addError(node.getStartLocation(), error);
       return;
     }
     // TODO(laurentlb): In BUILD files, calling setScope will throw an exception. This happens
     // because some AST nodes are shared across multipe ASTs (due to the prelude file).
     if (!isBuildFile) {
       node.setScope(b.scope);
     }
   }

   private static String createInvalidIdentifierException(String name, Set<String> candidates) {
     if (name.equals("$error$")) {
       return "contains syntax error(s)";
     }

     String error = getErrorForObsoleteThreadLocalVars(name);
     if (error != null) {
       return error;
     }

     String suggestion = SpellChecker.didYouMean(name, candidates);
     return "name '" + name + "' is not defined" + suggestion;
   }

   static String getErrorForObsoleteThreadLocalVars(String name) {
     if (name.equals("PACKAGE_NAME")) {
       return "The value 'PACKAGE_NAME' has been removed in favor of 'package_name()', "
           + "please use the latter ("
           + "https://docs.bazel.build/versions/master/skylark/lib/native.html#package_name). ";
     }
     if (name.equals("REPOSITORY_NAME")) {
       return "The value 'REPOSITORY_NAME' has been removed in favor of 'repository_name()', please"
           + " use the latter ("
           + "https://docs.bazel.build/versions/master/skylark/lib/native.html#repository_name).";
     }
     return null;
   }

   @Override
   public void visit(ReturnStatement node) {
     if (block.scope != Scope.Local) {
       addError(node.getStartLocation(), "return statements must be inside a function");
     }
     super.visit(node);
   }

   @Override
   public void visit(ForStatement node) {
     if (block.scope != Scope.Local) {
       addError(
           node.getStartLocation(),
           "for loops are not allowed at the top level. You may move it inside a function "
               + "or use a comprehension, [f(x) for x in sequence]");
     }
     loopCount++;
     visit(node.getCollection());
     assign(node.getLHS());
     visitBlock(node.getBlock());
     Preconditions.checkState(loopCount > 0);
     loopCount--;
   }

   @Override
   public void visit(LoadStatement node) {
     if (block.scope == Scope.Local) {
       addError(node.getStartLocation(), "load statement not at top level");
     }
     super.visit(node);
   }

   @Override
   public void visit(FlowStatement node) {
     if (node.getKind() != TokenKind.PASS && loopCount <= 0) {
       addError(node.getStartLocation(), node.getKind() + " statement must be inside a for loop");
     }
     super.visit(node);
   }

   @Override
   public void visit(DotExpression node) {
     visit(node.getObject());
     // Do not visit the field.
   }

   @Override
   public void visit(Comprehension node) {
     openBlock(Scope.Local);
     for (Comprehension.Clause clause : node.getClauses()) {
       if (clause instanceof Comprehension.For) {
         Comprehension.For forClause = (Comprehension.For) clause;
         collectDefinitions(forClause.getVars());
       }
     }
     // TODO(adonovan): opt: combine loops
     for (Comprehension.Clause clause : node.getClauses()) {
       if (clause instanceof Comprehension.For) {
         Comprehension.For forClause = (Comprehension.For) clause;
         visit(forClause.getIterable());
         assign(forClause.getVars());
       } else {
         Comprehension.If ifClause = (Comprehension.If) clause;
         visit(ifClause.getCondition());
       }
     }
     visit(node.getBody());
     closeBlock();
   }

   @Override
   public void visit(DefStatement node) {
     if (block.scope == Scope.Local) {
       addError(
           node.getStartLocation(),
           "nested functions are not allowed. Move the function to the top level.");
     }
     for (Parameter param : node.getParameters()) {
       if (param instanceof Parameter.Optional) {
         visit(param.getDefaultValue());
       }
     }
     openBlock(Scope.Local);
     for (Parameter param : node.getParameters()) {
       if (param.getIdentifier() != null) {
         declare(param.getIdentifier());
       }
     }
     collectDefinitions(node.getStatements());
     visitAll(node.getStatements());
     closeBlock();
   }

   @Override
   public void visit(IfStatement node) {
     if (block.scope != Scope.Local) {
       addError(
           node.getStartLocation(),
           "if statements are not allowed at the top level. You may move it inside a function "
               + "or use an if expression (x if condition else y).");
     }
     super.visit(node);
   }

   @Override
   public void visit(AssignmentStatement node) {
     visit(node.getRHS());

     // Disallow: [e, ...] += rhs
     // Other bad cases are handled in assign.
     if (node.isAugmented() && node.getLHS() instanceof ListExpression) {
       addError(
           node.getStartLocation(),
           "cannot perform augmented assignment on a list or tuple expression");
     }

     assign(node.getLHS());
   }

   /** Declare a variable and add it to the environment. */
   private void declare(Identifier id) {
     Identifier prev = block.variables.putIfAbsent(id.getName(), id);

     // Symbols defined in the module scope cannot be reassigned.
     // TODO(laurentlb): Forbid reassignment in BUILD files too.
     if (prev != null && block.scope == Scope.Module && !isBuildFile) {
       addError(
           id.getStartLocation(),
           String.format(
               "cannot reassign global '%s' (read more at"
                   + " https://bazel.build/versions/master/docs/skylark/errors/read-only-variable.html)",
               id.getName()));
       if (prev != PREDECLARED) {
         addError(
             prev.getStartLocation(), String.format("'%s' previously declared here", id.getName()));
       }
     }
   }

   /** Returns the nearest Block that defines a symbol. */
   private Block blockThatDefines(String varname) {
     for (Block b = block; b != null; b = b.parent) {
       if (b.variables.containsKey(varname)) {
         return b;
       }
     }
     return null;
   }

   /** Returns the set of all accessible symbols (both local and global) */
   private Set<String> getAllSymbols() {
     Set<String> all = new HashSet<>();
     for (Block b = block; b != null; b = b.parent) {
       all.addAll(b.variables.keySet());
     }
     return all;
   }

   // Report an error if a load statement appears after another kind of statement.
   private void checkLoadAfterStatement(List<Statement> statements) {
     Location firstStatement = null;

     for (Statement statement : statements) {
       // Ignore string literals (e.g. docstrings).
       if (statement instanceof ExpressionStatement
           && ((ExpressionStatement) statement).getExpression() instanceof StringLiteral) {
         continue;
       }

       if (statement instanceof LoadStatement) {
         if (firstStatement == null) {
           continue;
         }
         addError(
             statement.getStartLocation(),
             "load() statements must be called before any other statement. "
                 + "First non-load() statement appears at "
                 + firstStatement
                 + ". Use --incompatible_bzl_disallow_load_after_statement=false to temporarily "
                 + "disable this check.");
       }

       if (firstStatement == null) {
         firstStatement = statement.getStartLocation();
       }
     }
   }

   private void validateToplevelStatements(List<Statement> statements) {
     // Check that load() statements are on top.
     if (!isBuildFile && semantics.incompatibleBzlDisallowLoadAfterStatement()) {
       checkLoadAfterStatement(statements);
     }

     openBlock(Scope.Module);

     // Add each variable defined by statements, not including definitions that appear in
     // sub-scopes of the given statements (function bodies and comprehensions).
     collectDefinitions(statements);

     // Second pass: ensure that all symbols have been defined.
     visitAll(statements);
     closeBlock();
   }

   /**
    * Performs static checks, including resolution of identifiers in {@code file} in the environment
    * defined by {@code module}. The StarlarkFile is mutated. Errors are appended to {@link
    * StarlarkFile#errors}. {@code isBuildFile} enables Bazel's legacy mode for BUILD files in which
    * reassignment at top-level is permitted.
    */
   public static void validateFile(
       StarlarkFile file, Module module, StarlarkSemantics semantics, boolean isBuildFile) {
     ValidationEnvironment venv =
         new ValidationEnvironment(file.errors, module, semantics, isBuildFile);
     if (semantics.incompatibleRestrictStringEscapes()) {
       file.addStringEscapeEvents();
     }
     venv.validateToplevelStatements(file.getStatements());
     // Check that no closeBlock was forgotten.
     Preconditions.checkState(venv.block.parent == null);
   }

   /**
    * Performs static checks, including resolution of identifiers in {@code expr} in the environment
    * defined by {@code module}. This operation mutates the Expression.
    */
   public static void validateExpr(Expression expr, Module module, StarlarkSemantics semantics)
       throws SyntaxError {
     List<Event> errors = new ArrayList<>();
     ValidationEnvironment venv =
         new ValidationEnvironment(errors, module, semantics, /*isBuildFile=*/ false);

     venv.visit(expr);

     if (!errors.isEmpty()) {
       throw new SyntaxError(errors);
     }
   }

   /** Open a new lexical block that will contain the future declarations. */
   private void openBlock(Scope scope) {
     block = new Block(scope, block);
   }

   /** Close a lexical block (and lose all declarations it contained). */
   private void closeBlock() {
     block = Preconditions.checkNotNull(block.parent);
   }

 }
	// Copyright 2014 The Bazel Authors. All rights reserved.
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package com.google.devtools.build.lib.syntax;

	import com.google.common.base.Preconditions;
	import com.google.devtools.build.lib.events.Event;
	import com.google.devtools.build.lib.events.Location;
	import com.google.devtools.build.lib.util.SpellChecker;
	import java.util.ArrayList;
	import java.util.HashMap;
	import java.util.HashSet;
	import java.util.List;
	import java.util.Map;
	import java.util.Set;
	import javax.annotation.Nullable;

	/**
	* A class for doing static checks on files, before evaluating them.
	*
	* <p>We implement the semantics discussed in
	* https://github.com/bazelbuild/proposals/blob/master/docs/2018-06-18-name-resolution.md
	*
	* <p>When a variable is defined, it is visible in the entire block. For example, a global variable
	* is visible in the entire file; a variable in a function is visible in the entire function block
	* (even on the lines before its first assignment).
	*
	* <p>Validation is a mutation of the syntax tree, as it attaches scope information to Identifier
	* nodes. (In the future, it will attach additional information to functions to support lexical
	* scope, and even compilation of the trees to bytecode.) Validation errors are reported in the
	* analogous manner to scan/parse errors: for a StarlarkFile, they are appended to {@code
	* StarlarkFile.errors}; for an expression they are reported by an SyntaxError exception. It is
	* legal to validate a file that already contains scan/parse errors, though it may lead to secondary
	* validation errors.
	*/
	// TODO(adonovan): make this class private. Call it through the EvalUtils facade.
	public final class ValidationEnvironment extends NodeVisitor {

	enum Scope {
	/** Symbols defined inside a function or a comprehension. */
	Local("local"),
	/** Symbols defined at a module top-level, e.g. functions, loaded symbols. */
	Module("global"),
	/** Predefined symbols (builtins) */
	Universe("builtin");

	private final String qualifier;

	private Scope(String qualifier) {
	this.qualifier = qualifier;
	}

	String getQualifier() {
	return qualifier;
	}
	}

	/**
	* Module is a static abstraction of a Starlark module. It describes the set of variable names for
	* use during name resolution.
	*/
	public interface Module {

	// TODO(adonovan): opt: for efficiency, turn this into a predicate, not an enumerable set,
	// and look up bindings as they are needed, not preemptively.
	// Otherwise we must do work proportional to the number of bindings in the
	// environment, not the number of free variables of the file/expression.
	//
	// A single method will then suffice:
	// Scope resolve(String name) throws Undeclared
	// This requires that the Module retain its semantics.

	/** Returns the set of names defined by this module. The caller must not modify the set. */
	Set<String> getNames();

	/**
	* Returns (optionally) a more specific error for an undeclared name than the generic message.
	* This hook allows the module to implement "semantics-restricted" names without any knowledge
	* in this file.
	*/
	@Nullable
	String getUndeclaredNameError(StarlarkSemantics semantics, String name);
	}

	private static final Identifier PREDECLARED = new Identifier("");

	private static class Block {
	private final Map<String, Identifier> variables = new HashMap<>();
	private final Scope scope;
	@Nullable private final Block parent;

	Block(Scope scope, @Nullable Block parent) {
	this.scope = scope;
	this.parent = parent;
	}
	}

	private final List<Event> errors;
	private final StarlarkSemantics semantics;
	private final Module module;
	private Block block;
	private int loopCount;

	// In BUILD files, we have a slightly different behavior for legacy reasons.
	// TODO(adonovan): eliminate isBuildFile. It is necessary because the prelude is implemented
	// by inserting shared Statements, which must not be mutated, into each StarlarkFile.
	// Instead, we should implement the prelude by executing it like a .bzl module
	// and putting its members in the initial environment of the StarlarkFile.
	// In the meantime, let's move this flag into Module (GlobalFrame).
	private final boolean isBuildFile;

	private ValidationEnvironment(
	List<Event> errors, Module module, StarlarkSemantics semantics, boolean isBuildFile) {
	this.errors = errors;
	this.module = module;
	this.semantics = semantics;
	this.isBuildFile = isBuildFile;
	block = new Block(Scope.Universe, null);
	for (String name : module.getNames()) {
	block.variables.put(name, PREDECLARED);
	}
	}

	void addError(Location loc, String message) {
	errors.add(Event.error(loc, message));
	}

	/**
	* First pass: add all definitions to the current block. This is done because symbols are
	* sometimes used before their definition point (e.g. a functions are not necessarily declared in
	* order).
	*/
	private void collectDefinitions(Iterable<Statement> stmts) {
	for (Statement stmt : stmts) {
	collectDefinitions(stmt);
	}
	}

	private void collectDefinitions(Statement stmt) {
	switch (stmt.kind()) {
	case ASSIGNMENT:
	collectDefinitions(((AssignmentStatement) stmt).getLHS());
	break;
	case IF:
	IfStatement ifStmt = (IfStatement) stmt;
	collectDefinitions(ifStmt.getThenBlock());
	if (ifStmt.getElseBlock() != null) {
	collectDefinitions(ifStmt.getElseBlock());
	}
	break;
	case FOR:
	ForStatement forStmt = (ForStatement) stmt;
	collectDefinitions(forStmt.getLHS());
	collectDefinitions(forStmt.getBlock());
	break;
	case DEF:
	DefStatement def = (DefStatement) stmt;
	declare(def.getIdentifier());
	break;
	case LOAD:
	LoadStatement load = (LoadStatement) stmt;

	// The global reassignment check is not yet enabled for BUILD files,
	// but we apply it to load statements as a special case.
	// Because (for now) its error message is better than the general
	// message emitted by 'declare', we'll apply it to non-BUILD files too.
	Set<String> names = new HashSet<>();
	for (LoadStatement.Binding b : load.getBindings()) {
	if (!names.add(b.getLocalName().getName())) {
	addError(
	b.getLocalName().getStartLocation(),
	String.format(
	"load statement defines '%s' more than once", b.getLocalName().getName()));
	}
	}

	for (LoadStatement.Binding b : load.getBindings()) {
	declare(b.getLocalName());
	}
	break;
	case EXPRESSION:
	case FLOW:
	case RETURN:
	// nothing to declare
	}
	}

	private void collectDefinitions(Expression lhs) {
	for (Identifier id : Identifier.boundIdentifiers(lhs)) {
	declare(id);
	}
	}

	private void assign(Expression lhs) {
	if (lhs instanceof Identifier) {
	if (!isBuildFile) {
	((Identifier) lhs).setScope(block.scope);
	}
	// no-op
	} else if (lhs instanceof IndexExpression) {
	visit(lhs);
	} else if (lhs instanceof ListExpression) {
	for (Expression elem : ((ListExpression) lhs).getElements()) {
	assign(elem);
	}
	} else {
	addError(lhs.getStartLocation(), "cannot assign to '" + lhs + "'");
	}
	}

	@Override
	public void visit(Identifier node) {
	String name = node.getName();
	@Nullable Block b = blockThatDefines(name);
	if (b == null) {
	// The identifier might not exist because it was restricted (hidden) by the current semantics.
	// If this is the case, output a more helpful error message than 'not found'.
	String error = module.getUndeclaredNameError(semantics, name);
	if (error == null) {
	// generic error
	error = createInvalidIdentifierException(node.getName(), getAllSymbols());
	}
	addError(node.getStartLocation(), error);
	return;
	}
	// TODO(laurentlb): In BUILD files, calling setScope will throw an exception. This happens
	// because some AST nodes are shared across multipe ASTs (due to the prelude file).
	if (!isBuildFile) {
	node.setScope(b.scope);
	}
	}

	private static String createInvalidIdentifierException(String name, Set<String> candidates) {
	if (name.equals("$error$")) {
	return "contains syntax error(s)";
	}

	String error = getErrorForObsoleteThreadLocalVars(name);
	if (error != null) {
	return error;
	}

	String suggestion = SpellChecker.didYouMean(name, candidates);
	return "name '" + name + "' is not defined" + suggestion;
	}

	static String getErrorForObsoleteThreadLocalVars(String name) {
	if (name.equals("PACKAGE_NAME")) {
	return "The value 'PACKAGE_NAME' has been removed in favor of 'package_name()', "
	+ "please use the latter ("
	+ "https://docs.bazel.build/versions/master/skylark/lib/native.html#package_name). ";
	}
	if (name.equals("REPOSITORY_NAME")) {
	return "The value 'REPOSITORY_NAME' has been removed in favor of 'repository_name()', please"
	+ " use the latter ("
	+ "https://docs.bazel.build/versions/master/skylark/lib/native.html#repository_name).";
	}
	return null;
	}

	@Override
	public void visit(ReturnStatement node) {
	if (block.scope != Scope.Local) {
	addError(node.getStartLocation(), "return statements must be inside a function");
	}
	super.visit(node);
	}

	@Override
	public void visit(ForStatement node) {
	if (block.scope != Scope.Local) {
	addError(
	node.getStartLocation(),
	"for loops are not allowed at the top level. You may move it inside a function "
	+ "or use a comprehension, [f(x) for x in sequence]");
	}
	loopCount++;
	visit(node.getCollection());
	assign(node.getLHS());
	visitBlock(node.getBlock());
	Preconditions.checkState(loopCount > 0);
	loopCount--;
	}

	@Override
	public void visit(LoadStatement node) {
	if (block.scope == Scope.Local) {
	addError(node.getStartLocation(), "load statement not at top level");
	}
	super.visit(node);
	}

	@Override
	public void visit(FlowStatement node) {
	if (node.getKind() != TokenKind.PASS && loopCount <= 0) {
	addError(node.getStartLocation(), node.getKind() + " statement must be inside a for loop");
	}
	super.visit(node);
	}

	@Override
	public void visit(DotExpression node) {
	visit(node.getObject());
	// Do not visit the field.
	}

	@Override
	public void visit(Comprehension node) {
	openBlock(Scope.Local);
	for (Comprehension.Clause clause : node.getClauses()) {
	if (clause instanceof Comprehension.For) {
	Comprehension.For forClause = (Comprehension.For) clause;
	collectDefinitions(forClause.getVars());
	}
	}
	// TODO(adonovan): opt: combine loops
	for (Comprehension.Clause clause : node.getClauses()) {
	if (clause instanceof Comprehension.For) {
	Comprehension.For forClause = (Comprehension.For) clause;
	visit(forClause.getIterable());
	assign(forClause.getVars());
	} else {
	Comprehension.If ifClause = (Comprehension.If) clause;
	visit(ifClause.getCondition());
	}
	}
	visit(node.getBody());
	closeBlock();
	}

	@Override
	public void visit(DefStatement node) {
	if (block.scope == Scope.Local) {
	addError(
	node.getStartLocation(),
	"nested functions are not allowed. Move the function to the top level.");
	}
	for (Parameter param : node.getParameters()) {
	if (param instanceof Parameter.Optional) {
	visit(param.getDefaultValue());
	}
	}
	openBlock(Scope.Local);
	for (Parameter param : node.getParameters()) {
	if (param.getIdentifier() != null) {
	declare(param.getIdentifier());
	}
	}
	collectDefinitions(node.getStatements());
	visitAll(node.getStatements());
	closeBlock();
	}

	@Override
	public void visit(IfStatement node) {
	if (block.scope != Scope.Local) {
	addError(
	node.getStartLocation(),
	"if statements are not allowed at the top level. You may move it inside a function "
	+ "or use an if expression (x if condition else y).");
	}
	super.visit(node);
	}

	@Override
	public void visit(AssignmentStatement node) {
	visit(node.getRHS());

	// Disallow: [e, ...] += rhs
	// Other bad cases are handled in assign.
	if (node.isAugmented() && node.getLHS() instanceof ListExpression) {
	addError(
	node.getStartLocation(),
	"cannot perform augmented assignment on a list or tuple expression");
	}

	assign(node.getLHS());
	}

	/** Declare a variable and add it to the environment. */
	private void declare(Identifier id) {
	Identifier prev = block.variables.putIfAbsent(id.getName(), id);

	// Symbols defined in the module scope cannot be reassigned.
	// TODO(laurentlb): Forbid reassignment in BUILD files too.
	if (prev != null && block.scope == Scope.Module && !isBuildFile) {
	addError(
	id.getStartLocation(),
	String.format(
	"cannot reassign global '%s' (read more at"
	+ " https://bazel.build/versions/master/docs/skylark/errors/read-only-variable.html)",
	id.getName()));
	if (prev != PREDECLARED) {
	addError(
	prev.getStartLocation(), String.format("'%s' previously declared here", id.getName()));
	}
	}
	}

	/** Returns the nearest Block that defines a symbol. */
	private Block blockThatDefines(String varname) {
	for (Block b = block; b != null; b = b.parent) {
	if (b.variables.containsKey(varname)) {
	return b;
	}
	}
	return null;
	}

	/** Returns the set of all accessible symbols (both local and global) */
	private Set<String> getAllSymbols() {
	Set<String> all = new HashSet<>();
	for (Block b = block; b != null; b = b.parent) {
	all.addAll(b.variables.keySet());
	}
	return all;
	}

	// Report an error if a load statement appears after another kind of statement.
	private void checkLoadAfterStatement(List<Statement> statements) {
	Location firstStatement = null;

	for (Statement statement : statements) {
	// Ignore string literals (e.g. docstrings).
	if (statement instanceof ExpressionStatement
	&& ((ExpressionStatement) statement).getExpression() instanceof StringLiteral) {
	continue;
	}

	if (statement instanceof LoadStatement) {
	if (firstStatement == null) {
	continue;
	}
	addError(
	statement.getStartLocation(),
	"load() statements must be called before any other statement. "
	+ "First non-load() statement appears at "
	+ firstStatement
	+ ". Use --incompatible_bzl_disallow_load_after_statement=false to temporarily "
	+ "disable this check.");
	}

	if (firstStatement == null) {
	firstStatement = statement.getStartLocation();
	}
	}
	}

	private void validateToplevelStatements(List<Statement> statements) {
	// Check that load() statements are on top.
	if (!isBuildFile && semantics.incompatibleBzlDisallowLoadAfterStatement()) {
	checkLoadAfterStatement(statements);
	}

	openBlock(Scope.Module);

	// Add each variable defined by statements, not including definitions that appear in
	// sub-scopes of the given statements (function bodies and comprehensions).
	collectDefinitions(statements);

	// Second pass: ensure that all symbols have been defined.
	visitAll(statements);
	closeBlock();
	}

	/**
	* Performs static checks, including resolution of identifiers in {@code file} in the environment
	* defined by {@code module}. The StarlarkFile is mutated. Errors are appended to {@link
	* StarlarkFile#errors}. {@code isBuildFile} enables Bazel's legacy mode for BUILD files in which
	* reassignment at top-level is permitted.
	*/
	public static void validateFile(
	StarlarkFile file, Module module, StarlarkSemantics semantics, boolean isBuildFile) {
	ValidationEnvironment venv =
	new ValidationEnvironment(file.errors, module, semantics, isBuildFile);
	if (semantics.incompatibleRestrictStringEscapes()) {
	file.addStringEscapeEvents();
	}
	venv.validateToplevelStatements(file.getStatements());
	// Check that no closeBlock was forgotten.
	Preconditions.checkState(venv.block.parent == null);
	}

	/**
	* Performs static checks, including resolution of identifiers in {@code expr} in the environment
	* defined by {@code module}. This operation mutates the Expression.
	*/
	public static void validateExpr(Expression expr, Module module, StarlarkSemantics semantics)
	throws SyntaxError {
	List<Event> errors = new ArrayList<>();
	ValidationEnvironment venv =
	new ValidationEnvironment(errors, module, semantics, /isBuildFile=/ false);

	venv.visit(expr);

	if (!errors.isEmpty()) {
	throw new SyntaxError(errors);
	}
	}

	/** Open a new lexical block that will contain the future declarations. */
	private void openBlock(Scope scope) {
	block = new Block(scope, block);
	}

	/** Close a lexical block (and lose all declarations it contained). */
	private void closeBlock() {
	block = Preconditions.checkNotNull(block.parent);
	}

	}