Fix the conversion of private names to public names

lib/src/migrators/module.dart

@@ -6,6 +6,7 @@
 
 import 'package:args/args.dart';
 import 'package:collection/collection.dart';
+import 'package:charcode/charcode.dart';
 import 'package:path/path.dart' as p;
 import 'package:sass_api/sass_api.dart';
 import 'package:source_span/source_span.dart';
@@ -191,6 +192,9 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
           "Can't access _configuredVariables when not visiting a dependency."));
   Set<MemberDeclaration<VariableDeclaration>>? __configuredVariables;
 
+  /// The number of variables that have been generated from whole cloth.
+  var _generatedVariables = 0;
+
   /// A mapping between member declarations and references.
   ///
   /// This performs an initial pass to determine how a declaration seen in the
@@ -263,7 +267,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
     var forwardsByUrl = <Uri, Map<String, Set<MemberDeclaration>>>{};
     var hiddenByUrl = <Uri, Set<MemberDeclaration>>{};
     for (var declaration in references.globalDeclarations) {
-      var private = declaration.name.startsWith('-');
+      var private = _isPrivate(declaration.name);
 
       // Whether this member will be exposed by the regular entrypoint.
       var visibleAtEntrypoint = !private &&
@@ -352,11 +356,11 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
     if (declaration.isForwarded) return;
 
     var name = declaration.name;
-    if (name.startsWith('-') &&
+    if (_isPrivate(name) &&
         references.referencedOutsideDeclaringStylesheet(declaration)) {
-      // Remove leading `-` since private members can't be accessed outside
-      // the module they're declared in.
-      name = name.substring(1);
+      // Private members can't be accessed outside the module they're declared
+      // in.
+      name = _privateToPublic(name);
     }
     name = _unprefix(name);
     if (name != declaration.name) {
@@ -365,6 +369,27 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
     }
   }
 
+  /// Returns whether [identifier] is a private member name.
+  ///
+  /// Assumes [identifier] is a valid CSS identifier.
+  bool _isPrivate(String identifier) {
+    return identifier.startsWith('-') || identifier.startsWith('_');
+  }
+
+  /// Converts a private identifier to a public one.
+  String _privateToPublic(String identifier) {
+    assert(_isPrivate(identifier));
+    for (var i = 0; i < identifier.length; i++) {
+      var char = identifier.codeUnitAt(i);
+      if (char != $dash && char != $underscore) {
+        return identifier.substring(i);
+      }
+    }
+
+    _generatedVariables++;
+    return 'var${_generatedVariables}';
+  }
+
   /// Returns whether the member named [name] should be forwarded in the
   /// entrypoint.
   ///
@@ -1025,8 +1050,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
         newName = declaration.name.substring(importOnlyPrefix.length);
       }
 
-      if (_shouldForward(declaration.name) &&
-          !declaration.name.startsWith('-')) {
+      if (_shouldForward(declaration.name) && !_isPrivate(declaration.name)) {
         var subprefix = "";
         if (importOnlyPrefix != null) {
           var prefix = _prefixFor(declaration.name);
@@ -1036,7 +1060,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
         }
         if (declaration.name != newName) _needsImportOnly = true;
         shownByPrefix.putIfAbsent(subprefix, () => {}).add(declaration);
-      } else if (!newName.startsWith('-')) {
+      } else if (!_isPrivate(newName)) {
         hidden.add(declaration);
       }
     }
@@ -1076,7 +1100,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
         if (declaration is ImportOnlyMemberDeclaration) {
           name = name.substring(declaration.importOnlyPrefix.length);
         }
-        if (name.startsWith('-')) name = name.substring(1);
+        if (_isPrivate(name)) name = _privateToPublic(name);
         name = _unprefix(name);
         if (subprefix.isNotEmpty) name = '$subprefix$name';
         if (declaration.member is VariableDeclaration) name = '\$$name';
@@ -1205,7 +1229,7 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
   void _renameReference(FileSpan span, MemberDeclaration declaration) {
     var newName = renamedMembers[declaration];
     if (newName != null) {
-      if (newName.startsWith('-') &&
+      if (_isPrivate(newName) &&
           declaration.name.endsWith(newName.substring(1))) {
         addPatch(patchDelete(span,
             start: 1, end: declaration.name.length - newName.length + 1));
@@ -1228,11 +1252,16 @@ class _ModuleMigrationVisitor extends MigrationVisitor {
   ///
   /// Otherwise, returns [name] unaltered.
   String _unprefix(String name) {
-    var isPrivate = name.startsWith('-');
-    var unprivateName = isPrivate ? name.substring(1) : name;
+    var isPrivate = _isPrivate(name);
+    var unprivateName = isPrivate ? _privateToPublic(name) : name;
     var prefix = _prefixFor(unprivateName);
     if (prefix == null) return name;
-    return (isPrivate ? '-' : '') + unprivateName.substring(prefix.length);
+
+    var withoutPrefix = unprivateName.substring(prefix.length);
+    if (_isPrivate(withoutPrefix)) {
+      withoutPrefix = _privateToPublic(withoutPrefix);
+    }
+    return (isPrivate ? '-' : '') + withoutPrefix;
   }
 
   /// Returns the namespace that built-in module [module] is loaded under.

test/migrators/module/remove_prefix_flag/without_dash.hrx

@@ -0,0 +1,39 @@
+<==> arguments
+--remove-prefix=lib
+
+<==> input/entrypoint.scss
+$lib-a: 5;
+$lib_b: $lib-a + 2;
+
+@function lib-fn($lib-local) {
+  @return $lib-local;
+}
+
+@mixin lib-mixin {
+  lib-property: lib-value;
+}
+
+lib-selector {
+  lib-property: lib-fn(0);
+  @include lib-mixin;
+}
+
+<==> output/entrypoint.scss
+$a: 5;
+$b: $a + 2;
+
+@function fn($lib-local) {
+  @return $lib-local;
+}
+
+@mixin mixin {
+  lib-property: lib-value;
+}
+
+lib-selector {
+  lib-property: fn(0);
+  @include mixin;
+}
+
+<==> output/entrypoint.import.scss
+@forward "entrypoint" as lib-*;

test/migrators/module/scope/pseudoprivate/dash.hrx

@@ -0,0 +1,19 @@
+<==> arguments
+--migrate-deps
+<==> input/entrypoint.scss
+@import "library";
+a {
+  color: $-var;
+}
+
+<==> input/_library.scss
+$-var: red;
+
+<==> output/entrypoint.scss
+@use "library";
+a {
+  color: library.$var;
+}
+
+<==> output/_library.scss
+$var: red;

test/migrators/module/scope/pseudoprivate/dashes_only.hrx

@@ -0,0 +1,19 @@
+<==> arguments
+--migrate-deps
+<==> input/entrypoint.scss
+@import "library";
+a {
+  color: $---;
+}
+
+<==> input/_library.scss
+$---: red;
+
+<==> output/entrypoint.scss
+@use "library";
+a {
+  color: library.$var1;
+}
+
+<==> output/_library.scss
+$var1: red;

test/migrators/module/scope/pseudoprivate/double_dash.hrx

@@ -0,0 +1,19 @@
+<==> arguments
+--migrate-deps
+<==> input/entrypoint.scss
+@import "library";
+a {
+  color: $--var;
+}
+
+<==> input/_library.scss
+$--var: red;
+
+<==> output/entrypoint.scss
+@use "library";
+a {
+  color: library.$var;
+}
+
+<==> output/_library.scss
+$var: red;

test/migrators/module/scope/pseudoprivate/indirect.hrx

@@ -0,0 +1,21 @@
+<==> arguments
+--migrate-deps
+<==> input/entrypoint.scss
+@import "library";
+a {
+  color: $middle;
+}
+
+<==> input/_library.scss
+$-upstream: red;
+$middle: $-upstream;
+
+<==> output/entrypoint.scss
+@use "library";
+a {
+  color: library.$middle;
+}
+
+<==> output/_library.scss
+$-upstream: red;
+$middle: $-upstream;

test/migrators/module/scope/pseudoprivate/underscore.hrx

@@ -0,0 +1,19 @@
+<==> arguments
+--migrate-deps
+<==> input/entrypoint.scss
+@import "library";
+a {
+  color: $_var;
+}
+
+<==> input/_library.scss
+$_var: red;
+
+<==> output/entrypoint.scss
+@use "library";
+a {
+  color: library.$var;
+}
+
+<==> output/_library.scss
+$var: red;