Specify unnamed libraries (#2490)

Author: srawlins

accepted/future-releases/unnamed-libraries/feature-specification.md

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+# Unnamed Libraries
+
+Author: [email protected]
+
+Version 1.0
+
+Specification for issue [#1073](https://github.com/dart-lang/language/issues/1073)
+
+## Motivation
+
+Users would like to both document a library and associate metadata with a
+library without needing to decide on a library name.
+
+Declaring a library with a library declaration has become increasingly rare,
+with the availability of a 'part of' syntax with a URI string, and with the
+decline of the mirror system. Tools such as dartdoc and the test package
+attempt to support "library-level" documentation comments and annotations by
+looking at such elements associated with the first directive in a library, or
+the first declaration. Allowing users to write `library;` without a name gives
+a specific and meaningful syntax for library-level documentation and metadata.
+With this syntax, users do not need to conceive of a unique library naming
+scheme, nor do they need to write out names which are never used.
+
+## Specification
+
+With this feature, library directives are allowed to be written without a name:
+
+```dart
+// Existing named library syntax:
+library qualified.named.separated.by.dots;
+
+// New unnamed library syntax:
+library;
+```
+
+Prior to this feature, a library can be _explicitly named_ with a library
+directive, or _implicitly named_ when written without a library directive. An
+implicitly named library has the empty string as its name. With this feature, a
+library with a library directive without a name is an implicitly named library.
+
+### Grammar
+
+The language grammar is changed to allow library directives without name.
+
+The section containin
+
+> ```latex
+> <libraryName> ::= <metadata> \LIBRARY{} <dottedIdentifierList> `;'
+> ```
+
+becomes:
+
+> ```latex
+> <libraryName> ::= <metadata> \LIBRARY{} <dottedIdentifierList>? `;'
+> ```
+
+### Parts
+
+A library part specifies the library to which it belongs using the part-of
+directive, which accepts two ways of referring to a library. A part-of
+directive can specify a library by URI, which is the more common way, and does
+not require the library to be explicitly named. In an older style, a part-of
+directive can instead specify a library by its name. A part-of directive cannot
+refer by name to an implicitly named library.  Therefore, with this feature, a
+part-of directive using a library name cannot refer to a library with a library
+directive without a name.
+
+### `dart:mirrors`
+
+The mirror system has at least one mechanism that uses a library's name,
+`MirrorSystem.findLibrary`. This function cannot find an implicitly named
+library. Therefore it cannot find a library with a library directive without a
+name.
+
+## Summary
+
+We allow library directives without name.
+
+## Versions
+
+1.0, 2022-09-14: Initial version