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.EventHandler;
 import com.google.devtools.build.lib.events.Location;
 import java.util.HashSet;
 import java.util.List;
 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).
  */
 public final class ValidationEnvironment extends SyntaxTreeVisitor {

   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;
     }

     public String getQualifier() {
       return qualifier;
     }
   }

   private static class Block {
     private final Set<String> variables = new HashSet<>();
     private final Scope scope;
     @Nullable private final Block parent;

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

   /**
    * We use an unchecked exception around EvalException because the SyntaxTreeVisitor doesn't let
    * visit methods throw checked exceptions. We might change that later.
    */
   private static class ValidationException extends RuntimeException {
     EvalException exception;

     ValidationException(EvalException e) {
       exception = e;
     }

     ValidationException(Location location, String message, String url) {
       exception = new EvalException(location, message, url);
     }

     ValidationException(Location location, String message) {
       exception = new EvalException(location, message);
     }
   }

   private final Environment env;
   private Block block;
   private int loopCount;

   /** Create a ValidationEnvironment for a given global Environment (containing builtins). */
   ValidationEnvironment(Environment env) {
     Preconditions.checkArgument(env.isGlobal());
     this.env = env;
     block = new Block(Scope.Universe, null);
     Set<String> builtinVariables = env.getVariableNames();
     block.variables.addAll(builtinVariables);
   }

   /**
    * 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).getLValue());
         break;
       case AUGMENTED_ASSIGNMENT:
         collectDefinitions(((AugmentedAssignmentStatement) stmt).getLValue());
         break;
       case IF:
         IfStatement ifStmt = (IfStatement) stmt;
         for (IfStatement.ConditionalStatements cond : ifStmt.getThenBlocks()) {
           collectDefinitions(cond.getStatements());
         }
         collectDefinitions(ifStmt.getElseBlock());
         break;
       case FOR:
         ForStatement forStmt = (ForStatement) stmt;
         collectDefinitions(forStmt.getVariable());
         collectDefinitions(forStmt.getBlock());
         break;
       case FUNCTION_DEF:
         Identifier fctName = ((FunctionDefStatement) stmt).getIdentifier();
         declare(fctName.getName(), fctName.getLocation());
         break;
       case LOAD:
         for (Identifier id : ((LoadStatement) stmt).getSymbols()) {
           declare(id.getName(), id.getLocation());
         }
         break;
       case CONDITIONAL:
       case EXPRESSION:
       case FLOW:
       case PASS:
       case RETURN:
         // nothing to declare
     }
   }

   private void collectDefinitions(LValue left) {
     for (Identifier id : left.boundIdentifiers()) {
       declare(id.getName(), id.getLocation());
     }
   }

   @Override
   public void visit(Identifier node) {
     @Nullable Block b = blockThatDefines(node.getName());
     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'.
       FlagGuardedValue result = env.getRestrictedBindings().get(node.getName());
       if (result != null) {
         throw new ValidationException(result.getEvalExceptionFromAttemptingAccess(
             node.getLocation(), env.getSemantics(), node.getName()));
       }
       throw new ValidationException(node.createInvalidIdentifierException(getAllSymbols()));
     }
     node.setScope(b.scope);
   }

   private void validateLValue(Location loc, Expression expr) {
     if (expr instanceof IndexExpression) {
       visit(expr);
     } else if (expr instanceof ListLiteral) {
       for (Expression e : ((ListLiteral) expr).getElements()) {
         validateLValue(loc, e);
       }
     } else if (!(expr instanceof Identifier)) {
       throw new ValidationException(loc, "cannot assign to '" + expr + "'");
     }
   }

   @Override
   public void visit(LValue node) {
     validateLValue(node.getLocation(), node.getExpression());
   }

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

   @Override
   public void visit(ForStatement node) {
     loopCount++;
     super.visit(node);
     Preconditions.checkState(loopCount > 0);
     loopCount--;
   }

   @Override
   public void visit(FlowStatement node) {
     if (loopCount <= 0) {
       throw new ValidationException(
           node.getLocation(), node.getKind().getName() + " 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(AbstractComprehension node) {
     openBlock(Scope.Local);
     for (AbstractComprehension.Clause clause : node.getClauses()) {
       if (clause.getLValue() != null) {
         collectDefinitions(clause.getLValue());
       }
     }
     super.visit(node);
     closeBlock();
   }

   @Override
   public void visit(FunctionDefStatement node) {
     for (Parameter<Expression, Expression> param : node.getParameters()) {
       if (param.isOptional()) {
         visit(param.getDefaultValue());
       }
     }
     openBlock(Scope.Local);
     for (Parameter<Expression, Expression> param : node.getParameters()) {
       if (param.hasName()) {
         declare(param.getName(), param.getLocation());
       }
     }
     collectDefinitions(node.getStatements());
     visitAll(node.getStatements());
     closeBlock();
   }

   @Override
   public void visit(IfStatement node) {
     if (block.scope != Scope.Local) {
       throw new ValidationException(
           node.getLocation(),
           "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(AugmentedAssignmentStatement node) {
     if (node.getLValue().getExpression() instanceof ListLiteral) {
       throw new ValidationException(
           node.getLocation(), "cannot perform augmented assignment on a list or tuple expression");
     }
     // Other bad cases are handled when visiting the LValue node.
     super.visit(node);
   }

   /** Declare a variable and add it to the environment. */
   private void declare(String varname, Location location) {
     if (block.scope == Scope.Module && block.variables.contains(varname)) {
       // Symbols defined in the module scope cannot be reassigned.
       throw new ValidationException(
           location,
           String.format("Variable %s is read only", varname),
           "https://bazel.build/versions/master/docs/skylark/errors/read-only-variable.html");
     }
     block.variables.add(varname);
   }

   /** 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.contains(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);
     }
     return all;
   }

   /** Throws ValidationException if a load() appears after another kind of statement. */
   private static 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;
         }
         throw new ValidationException(
             statement.getLocation(),
             "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.getLocation();
       }
     }
   }

   /** Validates the AST and runs static checks. */
   private void validateAst(List<Statement> statements) {
     // Check that load() statements are on top.
     if (env.getSemantics().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();
   }

   public static void validateAst(Environment env, List<Statement> statements) throws EvalException {
     try {
       ValidationEnvironment venv = new ValidationEnvironment(env);
       venv.validateAst(statements);
       // Check that no closeBlock was forgotten.
       Preconditions.checkState(venv.block.parent == null);
     } catch (ValidationException e) {
       throw e.exception;
     }
   }

   public static boolean validateAst(
       Environment env, List<Statement> statements, EventHandler eventHandler) {
     try {
       validateAst(env, statements);
       return true;
     } catch (EvalException e) {
       if (!e.isDueToIncompleteAST()) {
         eventHandler.handle(Event.error(e.getLocation(), e.getMessage()));
       }
       return false;
     }
   }

   /** 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);
   }

   /**
    * Checks that the AST is using the restricted syntax.
    *
    * <p>Restricted syntax is used by Bazel BUILD files. It forbids function definitions, *args, and
    * **kwargs. This creates a better separation between code and data.
    */
   public static boolean checkBuildSyntax(
       List<Statement> statements, final EventHandler eventHandler) {
     // Wrap the boolean inside an array so that the inner class can modify it.
     final boolean[] success = new boolean[] {true};
     // TODO(laurentlb): Merge with the visitor above when possible (i.e. when BUILD files use it).
     SyntaxTreeVisitor checker =
         new SyntaxTreeVisitor() {

           private void error(ASTNode node, String message) {
             eventHandler.handle(Event.error(node.getLocation(), message));
             success[0] = false;
           }

           @Override
           public void visit(FunctionDefStatement node) {
             error(
                 node,
                 "function definitions are not allowed in BUILD files. You may move the function to "
                     + "a .bzl file and load it.");
           }

           @Override
           public void visit(ForStatement node) {
             error(
                 node,
                 "for statements are not allowed in BUILD files. You may inline the loop, move it "
                     + "to a function definition (in a .bzl file), or as a last resort use a list "
                     + "comprehension.");
           }

           @Override
           public void visit(IfStatement node) {
             error(
                 node,
                 "if statements are not allowed in BUILD files. You may move conditional logic to a "
                     + "function definition (in a .bzl file), or for simple cases use an if "
                     + "expression.");
           }

           @Override
           public void visit(FuncallExpression node) {
             for (Argument.Passed arg : node.getArguments()) {
               if (arg.isStarStar()) {
                 error(
                     node,
                     "**kwargs arguments are not allowed in BUILD files. Pass the arguments in "
                         + "explicitly.");
               } else if (arg.isStar()) {
                 error(
                     node,
                     "*args arguments are not allowed in BUILD files. Pass the arguments in "
                         + "explicitly.");
               }
             }
           }
         };
     checker.visitAll(statements);
     return success[0];
   }
 }
	// 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.EventHandler;
	import com.google.devtools.build.lib.events.Location;
	import java.util.HashSet;
	import java.util.List;
	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).
	*/
	public final class ValidationEnvironment extends SyntaxTreeVisitor {

	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;
	}

	public String getQualifier() {
	return qualifier;
	}
	}

	private static class Block {
	private final Set<String> variables = new HashSet<>();
	private final Scope scope;
	@Nullable private final Block parent;

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

	/**
	* We use an unchecked exception around EvalException because the SyntaxTreeVisitor doesn't let
	* visit methods throw checked exceptions. We might change that later.
	*/
	private static class ValidationException extends RuntimeException {
	EvalException exception;

	ValidationException(EvalException e) {
	exception = e;
	}

	ValidationException(Location location, String message, String url) {
	exception = new EvalException(location, message, url);
	}

	ValidationException(Location location, String message) {
	exception = new EvalException(location, message);
	}
	}

	private final Environment env;
	private Block block;
	private int loopCount;

	/** Create a ValidationEnvironment for a given global Environment (containing builtins). */
	ValidationEnvironment(Environment env) {
	Preconditions.checkArgument(env.isGlobal());
	this.env = env;
	block = new Block(Scope.Universe, null);
	Set<String> builtinVariables = env.getVariableNames();
	block.variables.addAll(builtinVariables);
	}

	/**
	* 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).getLValue());
	break;
	case AUGMENTED_ASSIGNMENT:
	collectDefinitions(((AugmentedAssignmentStatement) stmt).getLValue());
	break;
	case IF:
	IfStatement ifStmt = (IfStatement) stmt;
	for (IfStatement.ConditionalStatements cond : ifStmt.getThenBlocks()) {
	collectDefinitions(cond.getStatements());
	}
	collectDefinitions(ifStmt.getElseBlock());
	break;
	case FOR:
	ForStatement forStmt = (ForStatement) stmt;
	collectDefinitions(forStmt.getVariable());
	collectDefinitions(forStmt.getBlock());
	break;
	case FUNCTION_DEF:
	Identifier fctName = ((FunctionDefStatement) stmt).getIdentifier();
	declare(fctName.getName(), fctName.getLocation());
	break;
	case LOAD:
	for (Identifier id : ((LoadStatement) stmt).getSymbols()) {
	declare(id.getName(), id.getLocation());
	}
	break;
	case CONDITIONAL:
	case EXPRESSION:
	case FLOW:
	case PASS:
	case RETURN:
	// nothing to declare
	}
	}

	private void collectDefinitions(LValue left) {
	for (Identifier id : left.boundIdentifiers()) {
	declare(id.getName(), id.getLocation());
	}
	}

	@Override
	public void visit(Identifier node) {
	@Nullable Block b = blockThatDefines(node.getName());
	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'.
	FlagGuardedValue result = env.getRestrictedBindings().get(node.getName());
	if (result != null) {
	throw new ValidationException(result.getEvalExceptionFromAttemptingAccess(
	node.getLocation(), env.getSemantics(), node.getName()));
	}
	throw new ValidationException(node.createInvalidIdentifierException(getAllSymbols()));
	}
	node.setScope(b.scope);
	}

	private void validateLValue(Location loc, Expression expr) {
	if (expr instanceof IndexExpression) {
	visit(expr);
	} else if (expr instanceof ListLiteral) {
	for (Expression e : ((ListLiteral) expr).getElements()) {
	validateLValue(loc, e);
	}
	} else if (!(expr instanceof Identifier)) {
	throw new ValidationException(loc, "cannot assign to '" + expr + "'");
	}
	}

	@Override
	public void visit(LValue node) {
	validateLValue(node.getLocation(), node.getExpression());
	}

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

	@Override
	public void visit(ForStatement node) {
	loopCount++;
	super.visit(node);
	Preconditions.checkState(loopCount > 0);
	loopCount--;
	}

	@Override
	public void visit(FlowStatement node) {
	if (loopCount <= 0) {
	throw new ValidationException(
	node.getLocation(), node.getKind().getName() + " 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(AbstractComprehension node) {
	openBlock(Scope.Local);
	for (AbstractComprehension.Clause clause : node.getClauses()) {
	if (clause.getLValue() != null) {
	collectDefinitions(clause.getLValue());
	}
	}
	super.visit(node);
	closeBlock();
	}

	@Override
	public void visit(FunctionDefStatement node) {
	for (Parameter<Expression, Expression> param : node.getParameters()) {
	if (param.isOptional()) {
	visit(param.getDefaultValue());
	}
	}
	openBlock(Scope.Local);
	for (Parameter<Expression, Expression> param : node.getParameters()) {
	if (param.hasName()) {
	declare(param.getName(), param.getLocation());
	}
	}
	collectDefinitions(node.getStatements());
	visitAll(node.getStatements());
	closeBlock();
	}

	@Override
	public void visit(IfStatement node) {
	if (block.scope != Scope.Local) {
	throw new ValidationException(
	node.getLocation(),
	"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(AugmentedAssignmentStatement node) {
	if (node.getLValue().getExpression() instanceof ListLiteral) {
	throw new ValidationException(
	node.getLocation(), "cannot perform augmented assignment on a list or tuple expression");
	}
	// Other bad cases are handled when visiting the LValue node.
	super.visit(node);
	}

	/** Declare a variable and add it to the environment. */
	private void declare(String varname, Location location) {
	if (block.scope == Scope.Module && block.variables.contains(varname)) {
	// Symbols defined in the module scope cannot be reassigned.
	throw new ValidationException(
	location,
	String.format("Variable %s is read only", varname),
	"https://bazel.build/versions/master/docs/skylark/errors/read-only-variable.html");
	}
	block.variables.add(varname);
	}

	/** 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.contains(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);
	}
	return all;
	}

	/** Throws ValidationException if a load() appears after another kind of statement. */
	private static 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;
	}
	throw new ValidationException(
	statement.getLocation(),
	"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.getLocation();
	}
	}
	}

	/** Validates the AST and runs static checks. */
	private void validateAst(List<Statement> statements) {
	// Check that load() statements are on top.
	if (env.getSemantics().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();
	}

	public static void validateAst(Environment env, List<Statement> statements) throws EvalException {
	try {
	ValidationEnvironment venv = new ValidationEnvironment(env);
	venv.validateAst(statements);
	// Check that no closeBlock was forgotten.
	Preconditions.checkState(venv.block.parent == null);
	} catch (ValidationException e) {
	throw e.exception;
	}
	}

	public static boolean validateAst(
	Environment env, List<Statement> statements, EventHandler eventHandler) {
	try {
	validateAst(env, statements);
	return true;
	} catch (EvalException e) {
	if (!e.isDueToIncompleteAST()) {
	eventHandler.handle(Event.error(e.getLocation(), e.getMessage()));
	}
	return false;
	}
	}

	/** 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);
	}

	/**
	* Checks that the AST is using the restricted syntax.
	*
	* <p>Restricted syntax is used by Bazel BUILD files. It forbids function definitions, *args, and
	* **kwargs. This creates a better separation between code and data.
	*/
	public static boolean checkBuildSyntax(
	List<Statement> statements, final EventHandler eventHandler) {
	// Wrap the boolean inside an array so that the inner class can modify it.
	final boolean[] success = new boolean[] {true};
	// TODO(laurentlb): Merge with the visitor above when possible (i.e. when BUILD files use it).
	SyntaxTreeVisitor checker =
	new SyntaxTreeVisitor() {

	private void error(ASTNode node, String message) {
	eventHandler.handle(Event.error(node.getLocation(), message));
	success[0] = false;
	}

	@Override
	public void visit(FunctionDefStatement node) {
	error(
	node,
	"function definitions are not allowed in BUILD files. You may move the function to "
	+ "a .bzl file and load it.");
	}

	@Override
	public void visit(ForStatement node) {
	error(
	node,
	"for statements are not allowed in BUILD files. You may inline the loop, move it "
	+ "to a function definition (in a .bzl file), or as a last resort use a list "
	+ "comprehension.");
	}

	@Override
	public void visit(IfStatement node) {
	error(
	node,
	"if statements are not allowed in BUILD files. You may move conditional logic to a "
	+ "function definition (in a .bzl file), or for simple cases use an if "
	+ "expression.");
	}

	@Override
	public void visit(FuncallExpression node) {
	for (Argument.Passed arg : node.getArguments()) {
	if (arg.isStarStar()) {
	error(
	node,
	"**kwargs arguments are not allowed in BUILD files. Pass the arguments in "
	+ "explicitly.");
	} else if (arg.isStar()) {
	error(
	node,
	"*args arguments are not allowed in BUILD files. Pass the arguments in "
	+ "explicitly.");
	}
	}
	}
	};
	checker.visitAll(statements);
	return success[0];
	}
	}