Only write width to library files

Files using the shared part builder will need to resolve format width
conflicts using some other mechanism. A shared part file can have
outputs contributed from different code generators that are unrelated,
and may have different behavior around formatting, so it is not possible
to write a single width comment that applies to the combined file.

example_usage/lib/library_source.info.dart

@@ -1,5 +1,5 @@
-// GENERATED CODE - DO NOT MODIFY BY HAND
 // dart format width=80
+// GENERATED CODE - DO NOT MODIFY BY HAND
 
 // **************************************************************************
 // MemberCountLibraryGenerator

source_gen/CHANGELOG.md

@@ -13,8 +13,8 @@
 - `LibraryBuilder`, `PartBuilder`, and `SharedPartBuilder` now take an optional
   `writeDescriptions` boolean. When set to `false`, headers and generator
   descriptions for the files will not be included in the builder output.
-- Include `//dart format width=80` comments in Dart unit files when they are
-  formatted with the default callback.
+- Include `//dart format width=80` comments in files generated by a
+  `LibraryBuilder` or `PartBuilder` and formatted with the default callback.
 
 ## 1.5.0
 

source_gen/lib/src/builder.dart

@@ -50,7 +50,7 @@ class _Builder extends Builder {
   /// [options] to allow output files to be generated into a different directory
   _Builder(
     this._generators, {
-    this.formatOutput = _defaultFormatOutput,
+    required this.formatOutput,
     String generatedExtension = '.g.dart',
     List<String> additionalOutputExtensions = const [],
     String? header,
@@ -236,7 +236,7 @@ class SharedPartBuilder extends _Builder {
   SharedPartBuilder(
     super.generators,
     String partId, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatOutput,
     super.additionalOutputExtensions,
     super.allowSyntaxErrors,
     super.writeDescriptions,
@@ -298,7 +298,7 @@ class PartBuilder extends _Builder {
   PartBuilder(
     super.generators,
     String generatedExtension, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatUnit,
     super.additionalOutputExtensions,
     super.writeDescriptions,
     super.header,
@@ -325,7 +325,8 @@ class LibraryBuilder extends _Builder {
   /// should be indicated in [additionalOutputExtensions].
   ///
   /// [formatOutput] is called to format the generated code. Defaults to
-  /// using the standard [DartFormatter].
+  /// using the standard [DartFormatter] and writing a comment specifying the
+  /// default format width of 80..
   ///
   /// [writeDescriptions] adds comments to the output used to separate the
   /// sections of the file generated from different generators, and reveals
@@ -341,7 +342,7 @@ class LibraryBuilder extends _Builder {
   /// libraries.
   LibraryBuilder(
     Generator generator, {
-    super.formatOutput,
+    super.formatOutput = _defaultFormatUnit,
     super.generatedExtension,
     super.additionalOutputExtensions,
     super.writeDescriptions,
@@ -408,9 +409,13 @@ Future<bool> _hasAnyTopLevelAnnotations(
 
 const defaultFileHeader = '// GENERATED CODE - DO NOT MODIFY BY HAND';
 
-String _defaultFormatOutput(String code, Version version) {
+String _defaultFormatOutput(String code, Version version) =>
+    DartFormatter(languageVersion: version).format(code);
+
+/// Prefixes a dart format width and formats [code].
+String _defaultFormatUnit(String code, Version version) {
   code = '$dartFormatWidth\n$code';
-  return DartFormatter(languageVersion: version).format(code);
+  return _defaultFormatOutput(code, version);
 }
 
 final _headerLine = '// '.padRight(77, '*');