[2.19] Clarify library usage for no-name declarations update (#4390)

fixes #4314 staged: https://lib-dir-docs.web.app Changes: - Added [code sample](https://lib-dir-docs.web.app/guides/language/effective-dart/documentation#consider-writing-a-library-level-doc-comment) to existing library-level doc comment section - Added [Don't use library directives unless attaching doc comments or annotations](https://lib-dir-docs.web.app/guides/language/effective-dart/usage#dont-use-library-directives-unless-attaching-doc-comments-or-annotations) section - Added [Don't explicitly name libraries](https://lib-dir-docs.web.app/guides/language/effective-dart/usage#dont-explicitly-name-libraries) section - Reduced [`part of`](https://lib-dir-docs.web.app/guides/language/effective-dart/usage#do-use-strings-in-part-of-directives) section - Removed / modified unnecessary notes about the library directive on the [Creating packages](https://lib-dir-docs.web.app/guides/libraries/create-library-packages) page I thought about completely removing the `part of` section in Effective Dart: Usage, in favor of the more general new section "Don't explicitly name libraries", because it's not even recommended to use `part of` at all anymore. The new section could add a code example similar to the one for `part of` that more generally illustrates "Use URI strings to refer to libraries", and the linter rule could point to that section instead. Thoughts? @kevmoo @pq etc. Co-authored-by: Parker Lougheed <parlough@gmail.com> Co-authored-by: Kevin Moore <kevmoo@users.noreply.github.com>
dart-lang · Dec 20, 2022 · 458bcdb · pq · Dec 21, 2022 · pq
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diff --git a/examples/misc/lib/effective_dart/docs_good.dart b/examples/misc/lib/effective_dart/docs_good.dart
@@ -1,6 +1,12 @@
-// ignore_for_file: type_annotate_public_apis, unused_element, strict_raw_type
+// ignore_for_file: type_annotate_public_apis, unused_element, strict_raw_type, undefined_annotation, experiment_not_enabled, missing_identifier
 // ignore_for_file: no_leading_underscores_for_local_identifiers, use_function_type_syntax_for_parameters
 
+// #docregion library-doc
+/// A really great test library.
+@TestOn('browser')
+library;
+// #enddocregion library-doc
+
 import 'package:examples_util/ellipsis.dart';
 
 void miscDeclAnalyzedButNotTested() {

diff --git a/examples/misc/lib/effective_dart/usage_bad.dart b/examples/misc/lib/effective_dart/usage_bad.dart
@@ -7,6 +7,14 @@
 // ignore_for_file: prefer_adjacent_string_concatenation, prefer_is_not_empty, prefer_interpolation_to_compose_strings
 // ignore_for_file: unnecessary_this, always_declare_return_types, no_leading_underscores_for_local_identifiers
 // ignore_for_file: deprecated_colon_for_default_value
+// ignore_for_file: experiment_not_enabled, missing_identifier
+
+// #docregion library-dir
+
+library my_library;
+
+// #enddocregion library-dir
+
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';

diff --git a/src/_guides/language/effective-dart/documentation.md b/src/_guides/language/effective-dart/documentation.md
@@ -125,9 +125,17 @@ functionality provided within. Consider including:
 * Links to the most important or most commonly used classes and functions.
 * Links to external references on the domain the library is concerned with.
 
-You document a library by placing a doc comment right above the `library`
-directive at the start of the file. If the library doesn't have a `library`
-directive, you can add one just to hang the doc comment off of it.
+To document a library, place a doc comment before
+the `library` directive and any annotations that might be attached
+at the start of the file.
+
+{:.good}
+<?code-excerpt "docs_good.dart (library-doc)"?>
+{% prettify dart tag=pre+code %}
+/// A really great test library.
+@TestOn('browser')
+library;
+{% endprettify %}
 
 ### CONSIDER writing doc comments for private APIs.
 

diff --git a/src/_guides/language/effective-dart/style.md b/src/_guides/language/effective-dart/style.md
@@ -98,11 +98,11 @@ extension SmartIterable<T> on Iterable<T> { ... }
 
 [extensions]: /guides/language/extension-methods
 
-### DO name libraries, packages, directories, and source files using `lowercase_with_underscores`. {#do-name-libraries-and-source-files-using-lowercase_with_underscores}
+<a id="do-name-libraries-and-source-files-using-lowercase_with_underscores"></a>
+### DO name packages, directories, and source files using `lowercase_with_underscores`. {#do-name-packages-and-file-system-entities-using-lowercase-with-underscores}
 
-{% include linter-rule-mention.md rule1="library_names" rule2="file_names" %}
+{% include linter-rule-mention.md rule1="file_names" rule2="package_names" %}
 <!-- source for rules (update these if you update the guideline):
-https://github.com/dart-lang/linter/blob/master/lib/src/rules/library_names.dart
 https://github.com/dart-lang/linter/blob/master/lib/src/rules/file_names.dart -->
 
 Some file systems are not case-sensitive, so many projects require filenames to
@@ -112,27 +112,20 @@ a valid Dart identifier, which may be helpful if the language later supports
 symbolic imports.
 
 {:.good}
-<?code-excerpt "style_lib_good.dart" replace="/foo\///g"?>
-{% prettify dart tag=pre+code %}
-library peg_parser.source_scanner;
-
-import 'file_system.dart';
-import 'slider_menu.dart';
-{% endprettify %}
+```text
+my_package
+└─ lib
+   └─ file_system.dart
+   └─ slider_menu.dart
+```
 
 {:.bad}
-<?code-excerpt "style_lib_good.dart" replace="/foo\///g;/file./file-/g;/slider_menu/SliderMenu/g;/source_scanner/SourceScanner/g;/peg_parser/pegparser/g"?>
-{% prettify dart tag=pre+code %}
-library pegparser.SourceScanner;
-
-import 'file-system.dart';
-import 'SliderMenu.dart';
-{% endprettify %}
-
-{{site.alert.note}}
-  This guideline specifies *how* to name a library *if you choose to name it*. 
-  It is fine to _omit_ the library directive in a file if you want.
-{{site.alert.end}}
+```text
+mypackage
+└─ lib
+   └─ file-system.dart
+   └─ SliderMenu.dart
+```
 
 
 ### DO name import prefixes using `lowercase_with_underscores`.
@@ -323,6 +316,31 @@ defaultTimeout
 kDefaultTimeout
 {% endprettify %}
 
+### DON'T explicitly name libraries
+
+Appending a name to the `library` directive is technically possible,
+but is a legacy feature and discouraged. 
+
+Dart generates a unique tag for each library
+based on its path and filename.
+Naming libraries overrides this generated URI.
+Without the URI, it can be harder for tools to find
+the main library file in question. 
+
+{:.bad}
+<?code-excerpt "usage_bad.dart (library-dir)"?>
+{% prettify dart tag=pre+code %}
+
+library my_library;
+{% endprettify %}
+
+{:.good}
+<?code-excerpt "docs_good.dart (library-doc)"?>
+{% prettify dart tag=pre+code %}
+/// A really great test library.
+@TestOn('browser')
+library;
+{% endprettify %}
 
 ## Ordering
 

diff --git a/src/_guides/language/effective-dart/usage.md b/src/_guides/language/effective-dart/usage.md
@@ -19,21 +19,25 @@ but *maintainers* of it sure will.
 
 These guidelines help you compose your program out of multiple files in a
 consistent, maintainable way. To keep these guidelines brief, they use "import"
-to cover `import` and `export` directives. The guidelines apply equally to both.
+to cover `import` and `export` directives. The guidelines apply equally to both. 
 
 ### DO use strings in `part of` directives.
 
 Many Dart developers avoid using `part` entirely. They find it easier to reason
 about their code when each library is a single file. If you do choose to use
 `part` to split part of a library out into another file, Dart requires the other
-file to in turn indicate which library it's a part of. For legacy reasons, Dart
-allows this `part of` directive to use the *name* of the library it's a part of.
-That makes it harder for tools to physically find the main library file, and can
-make it ambiguous which library the part is actually part of.
+file to in turn indicate which library it's a part of. 
 
-The preferred, modern syntax is to use a URI string that points directly to the
-library file, just like you use in other directives. If you have some library,
-`my_library.dart`, that contains:
+Dart allows the `part of` directive to use the *name* of a library.
+Naming libraries is a legacy feature that is now [discouraged][]. 
+Library names can introduce ambiguity
+when determining which library a part belongs to.
+
+[discouraged]: /guides/language/effective-dart/style#dont-explicitly-name-libraries
+
+The preferred syntax syntax is to use a URI string that points
+directly to the library file. 
+If you have some library, `my_library.dart`, that contains:
 
 <?code-excerpt "my_library.dart"?>
 {% prettify dart tag=pre+code %}
@@ -42,15 +46,15 @@ library my_library;
 part 'some/other/file.dart';
 {% endprettify %}
 
-Then the part file should look like:
+Then the part file should use the library file's URI string:
 
 {:.good}
 <?code-excerpt "some/other/file.dart"?>
 {% prettify dart tag=pre+code %}
 part of '../../my_library.dart';
 {% endprettify %}
 
-And not:
+Not the library name:
 
 {:.bad}
 <?code-excerpt "some/other/file_2.dart"?>

diff --git a/src/_guides/language/language-tour.md b/src/_guides/language/language-tour.md
@@ -4107,7 +4107,7 @@ The `import` and `library` directives can help you create a
 modular and shareable code base. Libraries not only provide APIs, but
 are a unit of privacy: identifiers that start with an underscore (`_`)
 are visible only inside the library. *Every Dart app is a library*, even
-if it doesn’t use a `library` directive.
+if it doesn’t use a [`library`](#library-directive) directive.
 
 Libraries can be distributed using [packages](/guides/packages).
 
@@ -4148,7 +4148,6 @@ import 'package:test/test.dart';
   *URLs* (uniform resource locators) are a common kind of URI.
 {{site.alert.end}}
 
-
 #### Specifying a library prefix
 
 If you import two libraries that have conflicting identifiers, then you
@@ -4241,6 +4240,20 @@ Keep in mind the following when you use deferred loading:
   using <code>deferred as <em>namespace</em></code>.
   The `loadLibrary()` function returns a [`Future`](/guides/libraries/library-tour#future).
 
+#### The `library` directive {#library-directive}
+
+To specify library-level [doc comments][] or [metadata annotations][],
+attach them to a `library` declaration at the start of the file.
+
+<?code-excerpt "misc/lib/effective_dart/docs_good.dart (library-doc)"?>
+{% prettify dart tag=pre+code %}
+/// A really great test library.
+@TestOn('browser')
+library;
+{% endprettify %}
+
+[doc comments]: /guides/language/effective-dart/documentation#consider-writing-a-library-level-doc-comment
+[metadata annotations]: /guides/language/language-tour#metadata
 
 ### Implementing libraries
 
@@ -4251,7 +4264,6 @@ for advice on how to implement a library package, including:
 * How to organize library source code.
 * How to use the `export` directive.
 * When to use the `part` directive.
-* When to use the `library` directive.
 * How to use conditional imports and exports to implement
   a library that supports multiple platforms.
 

diff --git a/src/_guides/libraries/create-library-packages.md b/src/_guides/libraries/create-library-packages.md
@@ -50,14 +50,6 @@ lib directory
   To make APIs under lib/src public, you can export lib/src files
   from a file that's directly under lib.
 
-{{site.alert.note}}
-  When the `library` directive isn't specified, a unique
-  tag is generated for each library based on its path and filename.
-  Therefore, we suggest that you omit the `library` directive from
-  your code unless you plan to
-  [generate library-level documentation](#documenting-a-library).
-{{site.alert.end}}
-
 ## Organizing a library package
 
 Library packages are easiest to maintain, extend, and test
@@ -272,11 +264,11 @@ void updateBadge() {
 For an example of generated docs, see the
 [shelf documentation.]({{site.pub-api}}/shelf/latest)
 
-{{site.alert.note}}
-  To include any library-level documentation in the generated docs,
-  you must specify the `library` directive.
-  See [issue 1082.](https://github.com/dart-lang/dartdoc/issues/1082)
-{{site.alert.end}}
+To include any *library-level* documentation in the generated docs,
+add a `library` directive and attach the comment directly above it.
+For the how-and-why of documenting libraries, see
+[Effective Dart: Documentation](/guides/language/effective-dart/documentation#consider-writing-a-library-level-doc-comment).
+
 
 ## Distributing an open source library {#distributing-a-library}