Disallow uninitialized non-nullable variables

Declaring a non-nullable variable without initializing it is a static
error. If a constant is declared instead (either run-time or
compile-time) the error handling logic is the same as before.

BUG=
R=jmesserly@google.com

Review URL: https://codereview.chromium.org/2219653005 .

pkg/analyzer/lib/src/generated/error.dart

@@ -2743,6 +2743,7 @@ abstract class ErrorCode {
     StaticTypeWarningCode.NON_BOOL_EXPRESSION,
     StaticTypeWarningCode.NON_BOOL_NEGATION_EXPRESSION,
     StaticTypeWarningCode.NON_BOOL_OPERAND,
+    StaticTypeWarningCode.NON_NULLABLE_FIELD_NOT_INITIALIZED,
     StaticTypeWarningCode.NON_TYPE_AS_TYPE_ARGUMENT,
     StaticTypeWarningCode.RETURN_OF_INVALID_TYPE,
     StaticTypeWarningCode.TYPE_ARGUMENT_NOT_MATCHING_BOUNDS,
@@ -4280,6 +4281,14 @@ class StaticTypeWarningCode extends ErrorCode {
       const StaticTypeWarningCode('NON_BOOL_OPERAND',
           "The operands of the '{0}' operator must be assignable to 'bool'");
 
+  /**
+   *
+   */
+  static const StaticTypeWarningCode NON_NULLABLE_FIELD_NOT_INITIALIZED =
+      const StaticTypeWarningCode(
+          'NON_NULLABLE_FIELD_NOT_INITIALIZED',
+          "Variable '{0}' of non-nullable type '{1}' must be initialized");
+
   /**
    * 15.8 Parameterized Types: It is a static type warning if <i>A<sub>i</sub>,
    * 1 &lt;= i &lt;= n</i> does not denote a type in the enclosing lexical scope.

pkg/analyzer/lib/src/task/strong/checker.dart

@@ -628,6 +628,20 @@ class CodeChecker extends RecursiveAstVisitor {
     node.visitChildren(this);
   }
 
+  @override
+  Object visitVariableDeclaration(VariableDeclaration node) {
+    if (!node.isConst &&
+        !node.isFinal &&
+        node.initializer == null &&
+        rules.isNonNullableType(node?.element?.type)) {
+      _recordMessage(
+          node,
+          StaticTypeWarningCode.NON_NULLABLE_FIELD_NOT_INITIALIZED,
+          [node.name, node?.element?.type]);
+    }
+    return super.visitVariableDeclaration(node);
+  }
+
   @override
   void visitWhileStatement(WhileStatement node) {
     checkBoolean(node.condition);

pkg/analyzer/test/src/task/strong/non_null_checker_test.dart

@@ -48,12 +48,6 @@ main() {
     checkFile('int x = null;');
   }
 
-  void test_uninitialized_nonnullable() {
-    // If `int`s are non-nullable, then this code should throw an error.
-    addFile('int x;');
-    check(nonnullableTypes: <String>['dart:core,int']);
-  }
-
   void test_initialize_nonnullable_with_null() {
     addFile('int x = /*error:INVALID_ASSIGNMENT*/null;');
     check(nonnullableTypes: <String>['dart:core,int']);
@@ -76,6 +70,50 @@ main() {
     check(nonnullableTypes: <String>['dart:core,int']);
   }
 
+  void test_uninitialized_nonnullable_local_variable() {
+    // Ideally, we will do flow analysis and throw an error only if a variable
+    // is used before it has been initialized.
+    addFile('main() { int /*error:NON_NULLABLE_FIELD_NOT_INITIALIZED*/x; }');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
+
+  void test_uninitialized_nonnullable_top_level_variable_declaration() {
+    // If `int`s are non-nullable, then this code should throw an error.
+    addFile('int /*error:NON_NULLABLE_FIELD_NOT_INITIALIZED*/x;');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
+
+  void test_uninitialized_nonnullable_field_declaration() {
+    addFile('''
+void foo() {}
+
+class A {
+  // Ideally, we should allow x to be init in the constructor, but that requires
+  // too much complication in the checker, so for now we throw a static error at
+  // the declaration site.
+  int /*error:NON_NULLABLE_FIELD_NOT_INITIALIZED*/x;
+
+  A();
+}
+''');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
+
+  void test_prefer_final_to_non_nullable_error() {
+    addFile('main() { final int /*error:FINAL_NOT_INITIALIZED*/x; }');
+    addFile('final int /*error:FINAL_NOT_INITIALIZED*/x;');
+    addFile('''
+void foo() {}
+
+class A {
+  final int x;
+
+  /*warning:FINAL_NOT_INITIALIZED_CONSTRUCTOR_1*/A();
+}
+''');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
+
   // Default example from NNBD document.
   final String defaultNnbdExample = '''
 class Point {