With "Unquoted Imports" disallow unquoted part of library names.

accepted/future-releases/unquoted-imports/feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom
 
 Status: Accepted
 
-Version 0.4 (see [CHANGELOG](#CHANGELOG) at end)
+Version 0.5 (see [CHANGELOG](#CHANGELOG) at end)
 
 Experiment flag: unquoted-imports
 
@@ -341,20 +341,80 @@ after the directives outside of the path. These are all valid:
 
 ```dart
 import /* Weird but OK. */ some/path;
-export some/path; // Hi there.
-part some/path // Before the semicolon? Really?
+import some/path; // Hi there.
+export some/path // Before the semicolon? Really?
     ;
 ```
 
 The syntax that results from the above few sections is simple to tokenize and
 parse while looking like a single opaque "unquoted string" to users and tools.
 
+### Part of directives
+
+In Dart today, `part of` directives can already contain unquoted dotted
+identifiers, like:
+
+```dart
+// some_lib.dart
+library some.lib;
+
+part 'some_part.dart';
+
+// some_part.dart
+part of some.lib;
+```
+
+This is a legacy syntax from when library names were more widely used in Dart.
+Library names in `part of` directives have been deprecated for many years
+because the syntax doesn't work well with many tools. How is a given tool
+supposed to know where to find the library that happens to contain a `library`
+directive with that name? The quoted URI syntax was added later specifically to
+address that point and users are encouraged by documentation and lints to use
+the quoted syntax.
+
+Looking at a corpus of 122,420 files:
+
+```
+-- Directive (443733 total) --
+ 352744 ( 79.495%): import   =========================================
+  55471 ( 12.501%): export   =======
+  17823 (  4.017%): part     ===
+  17695 (  3.988%): part of  ===
+```
+
+So `part of` directives are fairly rare to begin with. Of them, most use the
+recommended URI syntax and would not be affected by this change:
+
+```
+-- Part of (17695 total) --
+  13229 ( 74.761%): uri           ===================================
+   4466 ( 25.239%): library name  ============
+```
+
+If we reinterpreted that existing syntax to refer to another package instead of
+a library within the current package, user breakage is likely and would be very
+confusing.
+
+To avoid potential user confusion and breakage, the new syntax in this proposal
+isn't allowed in `part or` or `part` directives. We disallow it in `part of`
+to avoid confusion with the legacy syntax, and we disallow it in `part` to be
+symmetric with `part of`.
+
+Further, we make a **language-versioned breaking change** and remove support for
+unquoted library identifiers in `part of` directives completely. That way, a
+user never sees syntax in a `part of` file that *looks* like a package path but
+means something else.
+
+*Since part files should realistically always be part of the same package that
+contains the library that owns them, the new unquoted syntax is of limited use
+for part files anyway. Quoted relative URIs are shorter and more idiomatic.*
+
 ## Syntax
 
 The normative stuff starts now. Here is the proposal:
 
-We add a new rule and hang it off the existing `uri` rule already used by import
-and export directives:
+We replace the existing `uri` rule with the following rule along with a couple
+of helper rules:
 
 ```
 uri               ::= stringLiteral | packagePath
@@ -371,22 +431,33 @@ between any of the `segmentComponent`, `/`, or `.` tokens in a `packagePath`.
 *In other words, there can be nothing except the terminals themselves from the
 first `segmentComponent` in the `packagePath` to the last.*
 
-*An import, export, or part directive can continue to use a `stringLiteral` for
-the quoted form (which is what they will do for relative references). But they
-can also use a `packagePath`, which is a slash-separated series of segments,
-each of which is a series of dot-separated components.*
+*An import or export directive can continue to use a `stringLiteral` for the
+quoted form (which is what they will do for relative references). But they can
+also use a `packagePath`, which is a slash-separated series of segments, each of
+which is a series of dot-separated components.*
+
+### Disallowing unquoted part and part-of directives.
 
-### Part directive lookahead
+We change the `partDirective` and `partHeader` rules to:
 
-*There are two directives for working with part files, `part` and `part of`.
-This means that when the parser sees `part of`, it doesn't immediately know if
-it is looking at a `part` directive followed by an unquoted identifier like
-`part of;` or `part of.some/other.thing;` versus a `part of` directive like
-`part of thing;` or `part of 'uri.dart';` It must lookahead past the `of`
-identifier to see if the next token is `;`, `.`, `/`, or another identifier.*
+```
+partDirective ::= metadata 'part' stringLiteral ';'
+partHeader    ::= metadata 'part' 'of' stringLiteral ';'
+```
+
+*As part of [augmentations][], we are also planning to [expand the power of part
+files][enhanced parts]. That includes supporting configurable part directives
+(but not configurable part-of. When/if that happens, this grammar will need to
+be tweaked to something like:*
+
+```
+partDirective          ::= metadata 'part' configurableQuotedUri ';'
+configurableQuotedUri  ::= stringLiteral configurationQuotedUri*
+configurationQuotedUri ::= 'if' '(' uriTest ')' stringLiteral
+```
 
-*This may add some complexity to parsing, but should be minor. Dart's grammar
-has other places that require much more (sometimes unbounded) lookahead.*
+[augmentations]: https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/feature-specification.md
+[enhanced parts]: https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/parts_with_imports.md
 
 ## Static semantics
 
@@ -461,8 +532,7 @@ There are no runtime semantics for this feature.
 
 ## Compatibility
 
-This feature is fully backwards compatible for `import`, `export`, and `part`
-directives.
+This feature is fully backwards compatible for `import` and `export` directives.
 
 For all directives, we still allow quoted "dart:" and "package:" imports. Users
 may be compelled to use the existing syntax in uncommon corner cases where the
@@ -474,73 +544,21 @@ migrated to the new style and the old quoted forms will be essentially vestigial
 syntax (similar to names after `library` directives). A future version of Dart
 may make a breaking change and remove support for the old syntax.
 
-### Part-of directives
-
-The `part of` directive allows a library name after `of` instead of a string
-literal. With this proposal, that syntax is now ambiguous. Is it interpreted
-as a library name, or as an unquoted URI that should be converted to a URI?
-In other words, given:
-
-```dart
-part of foo.bar;
-```
-
-Is the file saying it's a part of the library containing `library foo.bar;` or
-that it's part of the library found at URI `package:foo.bar/bar.dart`?
-
-Library names in `part of` directives have been deprecated for many years
-because the syntax doesn't work well with many tools. How is a given tool
-supposed to know where to find the library that happens to contain a `library`
-directive with that name? The quoted URI syntax was added later specifically to
-address that point and users are encouraged by documentation and lints to use
-the quoted syntax.
-
-Looking at a corpus of 122,420 files:
-
-```
--- Directive (443733 total) --
- 352744 ( 79.495%): import   =========================================
-  55471 ( 12.501%): export   =======
-  17823 (  4.017%): part     ===
-  17695 (  3.988%): part of  ===
-```
-
-So `part of` directives are fairly rare to begin with. Of them, most use the
-recommended URI syntax and would not be affected by this change:
-
-```
--- Part of (17695 total) --
-  13229 ( 74.761%): uri           ===================================
-   4466 ( 25.239%): library name  ============
-```
-
-In total, only about 1% of directives are `part of` with a library name:
-
-```
--- URI (443733 total) --
- 352744 ( 79.495%): import                     ===========================
-  55471 ( 12.501%): export                     =====
-  17823 (  4.017%): part                       ==
-  13229 (  2.981%): part of with uri           =
-   4466 (  1.006%): part of with library name  =
-```
-
-Given that, I propose that we make a **breaking change** and remove support for
-the long-deprecated library name syntax from `part of` directives. An unquoted
-series of identifiers after `part of` then gets unambiguously interpreted as
-this proposal's semantics. In other words, `part of foo.bar;` is part of the
-library at `package:foo/bar.dart`, not part of the library with name `foo.bar`.
-
-Users affected by the breakage can and should update their `part of` directive
-to point to the URI of the library that the file is a part of, using either the
-quoted or unquoted syntax.
+This proposal makes a **breaking change** to disallow unquoted library names in
+`part of` directives.
 
 ### Language versioning
 
 To avoid breaking existing `part of` directives, this change is language
-versioned. Only libraries whose language version is at or above the version that
-this proposal ships in can use this new unquoted syntax in `part of` or any
-other directive.
+versioned.
+
+Only libraries whose language version is at or above the version that this
+proposal ships in can use this new unquoted syntax in `import` and `export`
+directives.
+
+In language versions before the version this feature ships in, code may
+continue to use the old unquoted library name syntax in `part of` directives
+but users are encouraged to migrate away from that syntax.
 
 ## Tooling
 
@@ -570,6 +588,10 @@ new unquoted style whenever an existing directive could use it.
 
 ## Changelog
 
+### 0.5
+
+-   Require `part` and `part of` directives to use quoted paths (#4038).
+
 ### 0.4
 
 -   Allow reserved words and built-in identifiers as path components (#3984).