Lint to test for private/implementation imports.

lib/src/rules/implementation_imports.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library linter.src.rules.implementation_imports;
+
+import 'package:analyzer/src/generated/ast.dart'
+    show AstVisitor, ImportDirective, SimpleAstVisitor;
+import 'package:linter/src/linter.dart';
+
+const desc = "Don't import implementation files from another package.";
+
+const details = '''
+**DON'T** import implementation files from another package.
+
+From the the [pub package layout doc]
+(https://www.dartlang.org/tools/pub/package-layout.html#implementation-files):
+
+The libraries inside `lib` are publicly visible: other packages are free to
+import them. But much of a package's code is internal implementation libraries
+that should only be imported and used by the package itself. Those go inside a
+subdirectory of `lib` called `src`. You can create subdirectories in there if
+it helps you organize things.
+
+You are free to import libraries that live in `lib/src` from within other Dart
+code in the same package (like other libraries in `lib`, scripts in `bin`,
+and tests) but you should never import from another package's `lib/src`
+directory. Those files are not part of the package’s public API, and they
+might change in ways that could break your code.
+
+**Bad:**
+
+```
+// In 'road_runner'
+import 'package:acme/lib/src/internals.dart;
+```
+''';
+
+bool isImplementation(Uri uri) {
+  List<String> segments = uri?.pathSegments ?? const [];
+  if (segments.length > 2) {
+    if (segments[1] == 'src') {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool isPackage(Uri uri) => uri?.scheme == 'package';
+
+bool samePackage(Uri uri1, Uri uri2) {
+  var segments1 = uri1.pathSegments;
+  var segments2 = uri2.pathSegments;
+  if (segments1.length < 1 || segments2.length < 1) {
+    return false;
+  }
+  return segments1[0] == segments2[0];
+}
+
+class ImplementationImports extends LintRule {
+  ImplementationImports()
+      : super(
+            name: 'implementation_imports',
+            description: desc,
+            details: details,
+            group: Group.style);
+
+  @override
+  AstVisitor getVisitor() => new Visitor(this);
+}
+
+class Visitor extends SimpleAstVisitor {
+  final LintRule rule;
+  Visitor(this.rule);
+
+  @override
+  visitImportDirective(ImportDirective node) {
+    Uri importUri = node?.source?.uri;
+    Uri sourceUri = node?.element?.source?.uri;
+
+    // Test for 'package:*/src/'.
+    if (!isImplementation(importUri)) {
+      return;
+    }
+
+    // If the source URI is not a `package` URI bail out.
+    if (!isPackage(sourceUri)) {
+      return;
+    }
+
+    if (!samePackage(importUri, sourceUri)) {
+      rule.reportLint(node.uri);
+    }
+  }
+}