Enforce use of `throw` stmt in early error tests

Previously, test consumers were encouraged to insert a `throw` statement
as the first statement of tests for early errors. This recommendation
made tests harder to consume, and as an optional transformation,
consumers may have ignored it or simply been unaware it was made. By
explicitly including such a `throw` statement, the tests become more
literal, making them easier to consume and more transparent in their
expectations.

Document expectation for all tests for early errors to include an
explicit `throw` statement. Extend linting script to verify that
contributors are automatically notified of violations and to ensure that
future contributions satisfy this expectation.

CONTRIBUTING.md

@@ -187,7 +187,7 @@ function $DONE(arg) {
 
 ## Handling Errors and Negative Test Cases
 
-Expectations for **parsing errors** should be declared using [the `negative` frontmatter flag](#negative):
+Expectations for **parsing errors** should be declared using [the `negative` frontmatter flag](#negative). They must also include the exact `throw` statement specified in this example (in order to guarantee that implementations do not execute the code):
 
 ```javascript
 /*---
@@ -196,6 +196,8 @@ negative:
   type: SyntaxError
 ---*/
 
+throw "Test262: This statement should not be evaluated.";
+
 var var = var;
 ```
 

INTERPRETING.md

@@ -147,8 +147,7 @@ attribute is a YAML dictonary with two keys:
 
 - `phase` - the stage of the test interpretation process that the error is
   expected to be produced; either "early" (meaning, "prior to evaluation") or
-  "runtime" (meaning, "during evaluation"); in the case of "early", additional
+  "runtime" (meaning, "during evaluation")
-  test transformation may be required--see below
 - `type` - the name of the constructor of the expected error
 
 If a test configured with the `negative` attribute completes without throwing
@@ -167,14 +166,6 @@ negative:
 unresolvable;
 ```
 
-Consumers are free to assert the "early" phase as they see fit.
-
-For example, it is possible to insert a `throw` statement with a unique error
-type at the beginning of the test file. In this case, the statement should be
-inserted *after* the directive desribed in the section titled "Strict Mode"
-(where appropriate), though it must *not* be inserted for tests containing the
-"raw" flag.
-
 ### `includes`
 
 One or more files whose content must be evaluated in the test realm's global

tools/lint/lib/checks/frontmatter.py

@@ -29,14 +29,3 @@ class CheckFrontmatter(Check):
         unrecognized = fields - _VALID_FIELDS
         if len(unrecognized) > 0:
             return 'Unrecognized fields: %s' % ', '.join(list(unrecognized))
-
-        if 'negative' in meta:
-            negative = meta['negative']
-            if not isinstance(negative, dict):
-                return '"negative" must be a dictionary with fields "type" and "phase"'
-
-            if not 'type' in negative:
-                return '"negative" must specify a "type" field'
-
-            if not 'phase' in negative:
-                return '"negative" must specify a "phase" field'

tools/lint/lib/checks/negative.py

@@ -0,0 +1,27 @@
+import re
+from ..check import Check
+
+_THROW_STMT = re.compile(
+      r'^throw "Test262: This statement should not be evaluated\.";$',
+      re.MULTILINE)
+
+class CheckNegative(Check):
+    '''Ensure tests have the expected YAML-formatted metadata.'''
+    ID = 'NEGATIVE'
+
+    def run(self, name, meta, source):
+        if meta is None or meta.get('negative') is None:
+            return
+
+        negative = meta['negative']
+        if not isinstance(negative, dict):
+            return '"negative" must be a dictionary with fields "type" and "phase"'
+
+        if not 'type' in negative:
+            return '"negative" must specify a "type" field'
+
+        if not 'phase' in negative:
+            return '"negative" must specify a "phase" field'
+
+        if negative["phase"] == "early" and not _THROW_STMT.search(source):
+            return 'Negative tests of type "early" must include a `throw` statement'

tools/lint/lint.py

@@ -9,6 +9,7 @@ from lib.collect_files import collect_files
 from lib.checks.features import CheckFeatures
 from lib.checks.frontmatter import CheckFrontmatter
 from lib.checks.license import CheckLicense
+from lib.checks.negative import CheckNegative
 from lib.eprint import eprint
 import lib.frontmatter
 import lib.whitelist
@@ -21,7 +22,10 @@ parser.add_argument('path',
         nargs='+',
         help='file name or directory of files to lint')
 
-checks = [CheckFrontmatter(), CheckFeatures('features.txt'), CheckLicense()]
+checks = [
+        CheckFrontmatter(), CheckFeatures('features.txt'), CheckLicense(),
+        CheckNegative()
+    ]
 
 def lint(file_names):
     errors = dict()

tools/lint/test/fixtures/negative_early_throw_bad_value.js

@@ -0,0 +1,15 @@
+NEGATIVE
+^ expected errors | v input
+// Copyright (C) 2017 Mike Pennisi. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-assignment-operators-static-semantics-early-errors
+description: Minimal test
+negative:
+  type: SyntaxError
+  phase: early
+---*/
+
+throw "Test262: This statement should not be evaluated!";
+
+!!!

tools/lint/test/fixtures/negative_early_throw_missing.js

@@ -1,3 +1,4 @@
+NEGATIVE
 ^ expected errors | v input
 // Copyright (C) 2017 Mike Pennisi. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.

tools/lint/test/fixtures/negative_missing_phase.js

@@ -1,4 +1,4 @@
-FRONTMATTER
+NEGATIVE
 ^ expected errors | v input
 // Copyright (C) 2017 Mike Pennisi. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.

tools/lint/test/fixtures/negative_missing_type.js

@@ -1,4 +1,4 @@
-FRONTMATTER
+NEGATIVE
 ^ expected errors | v input
 // Copyright (C) 2017 Mike Pennisi. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.

tools/lint/test/fixtures/negative_string.js

@@ -1,4 +1,4 @@
-FRONTMATTER
+NEGATIVE
 ^ expected errors | v input
 // Copyright (C) 2017 Mike Pennisi. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.

tools/lint/test/fixtures/negative_valid_early.js

@@ -0,0 +1,14 @@
+^ expected errors | v input
+// Copyright (C) 2017 Mike Pennisi. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-assignment-operators-static-semantics-early-errors
+description: Minimal test
+negative:
+  type: SyntaxError
+  phase: early
+---*/
+
+throw "Test262: This statement should not be evaluated.";
+
+!!!

tools/lint/test/fixtures/negative_valid_runtime.js

@@ -0,0 +1,13 @@
+^ expected errors | v input
+// Copyright (C) 2017 Mike Pennisi. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-assignment-operators-static-semantics-early-errors
+description: Minimal test
+negative:
+  type: ReferenceError
+  phase: runtime
+---*/
+
+x;
+let x;