Implement deprecations for strict function units

CHANGELOG.md

@@ -2,6 +2,25 @@
 
 * Fix indentation for selectors that span multiple lines in a `@media` query.
 
+* Emit a deprecation warning when passing `$alpha` values with units to
+  `color.adjust()` or `color.change()`. This will be an error in Dart Sass
+  2.0.0.
+
+* Emit a deprecation warning when passing a `$weight` value with no units or
+  with units other than `%` to `color.mix()`. This will be an error in Dart Sass
+  2.0.0.
+
+* Emit a deprecation warning when passing `$n` values with units to `list.nth()`
+  or `list.set-nth()`. This will be an error in Dart Sass 2.0.0.
+
+* Improve existing deprecation warnings to wrap `/`-as-division suggestions in
+  `calc()` expressions.
+
+### Dart API
+
+* Emit a deprecation warning when passing a `sassIndex` with units to
+  `Value.sassIndexToListIndex()`. This will be an error in Dart Sass 2.0.0.
+
 ## 1.55.0
 
 * **Potentially breaking bug fix:** Sass numbers are now universally stored as

lib/src/functions/color.dart

@@ -453,9 +453,19 @@ SassColor _updateComponents(List<Value> arguments,
   /// [max] should be 255 for RGB channels, 1 for the alpha channel, and 100
   /// for saturation, lightness, whiteness, and blackness.
   double? getParam(String name, num max,
-      {bool checkPercent = false, bool assertPercent = false}) {
+      {bool checkPercent = false,
+      bool assertPercent = false,
+      bool checkUnitless = false}) {
     var number = keywords.remove(name)?.assertNumber(name);
     if (number == null) return null;
+    if (!scale && checkUnitless) {
+      if (number.hasUnits) {
+        warn("\$$name: Passing a number with unit ${number.unitString} is "
+            "deprecated.\n"
+            "\n"
+            "To preserve current behavior: ${number.unitSuggestion(name)}");
+      }
+    }
     if (!scale && checkPercent) _checkPercent(number, name);
     if (scale || assertPercent) number.assertUnit("%", name);
     if (scale) max = 100;
@@ -465,7 +475,7 @@ SassColor _updateComponents(List<Value> arguments,
             change ? 0 : -max, max, name, checkPercent ? '%' : '');
   }
 
-  var alpha = getParam("alpha", 1);
+  var alpha = getParam("alpha", 1, checkUnitless: true);
   var red = getParam("red", 255);
   var green = getParam("green", 255);
   var blue = getParam("blue", 255);
@@ -656,12 +666,13 @@ void _checkAngle(SassNumber angle, String name) {
 
     var actualUnit = angle.numeratorUnits.first;
     message
-      ..writeln("To preserve current behavior: \$$name * 1deg/1$actualUnit")
+      ..writeln(
+          "To preserve current behavior: calc(\$$name * " "1deg/1$actualUnit)")
       ..writeln("To migrate to new behavior: 0deg + \$$name")
       ..writeln();
   } else {
     message
-      ..writeln("To preserve current behavior: \$$name${_removeUnits(angle)}")
+      ..writeln("To preserve current behavior: ${angle.unitSuggestion(name)}")
       ..writeln();
   }
 
@@ -676,18 +687,10 @@ void _checkPercent(SassNumber number, String name) {
   warn(
       "\$$name: Passing a number without unit % ($number) is deprecated.\n"
       "\n"
-      "To preserve current behavior: \$$name${_removeUnits(number)} * 1%",
+      "To preserve current behavior: ${number.unitSuggestion(name, '%')}",
       deprecation: true);
 }
 
-/// Returns the right-hand side of an expression that would remove all units
-/// from `$number` but leaves the value the same.
-///
-/// Used for constructing deprecation messages.
-String _removeUnits(SassNumber number) =>
-    number.denominatorUnits.map((unit) => " * 1$unit").join() +
-    number.numeratorUnits.map((unit) => " / 1$unit").join();
-
 /// Create an HWB color from the given [arguments].
 Value _hwb(List<Value> arguments) {
   var alpha = arguments.length > 3 ? arguments[3] : null;
@@ -805,6 +808,8 @@ double _percentageOrUnitless(SassNumber number, num max, String name) {
 
 /// Returns [color1] and [color2], mixed together and weighted by [weight].
 SassColor _mixColors(SassColor color1, SassColor color2, SassNumber weight) {
+  _checkPercent(weight, 'weight');
+
   // This algorithm factors in both the user-provided weight (w) and the
   // difference between the alpha values of the two colors (a) to decide how
   // to perform the weighted average of the two RGB values.

lib/src/value.dart

@@ -5,6 +5,7 @@
 import 'package:meta/meta.dart';
 
 import 'ast/selector.dart';
+import 'evaluation_context.dart';
 import 'exception.dart';
 import 'utils.dart';
 import 'value/boolean.dart';
@@ -120,7 +121,16 @@ abstract class Value {
   /// [asList]. If [sassIndex] came from a function argument, [name] is the
   /// argument name (without the `$`). It's used for error reporting.
   int sassIndexToListIndex(Value sassIndex, [String? name]) {
-    var index = sassIndex.assertNumber(name).assertInt(name);
+    var indexValue = sassIndex.assertNumber(name);
+    if (indexValue.hasUnits) {
+      warn("\$$name: Passing a number with unit ${indexValue.unitString} is "
+              "deprecated.\n"
+              "\n"
+              "To preserve current behavior: " +
+          indexValue.unitSuggestion(name ?? 'index'));
+    }
+
+    var index = indexValue.assertInt(name);
     if (index == 0) throw SassScriptException("List index may not be 0.", name);
     if (index.abs() > lengthAsList) {
       throw SassScriptException(

lib/src/value/number.dart

@@ -933,4 +933,21 @@ abstract class SassNumber extends Value {
     var innerMap = _conversions[unit];
     return innerMap == null ? 1 : 1 / innerMap.values.first;
   }
+
+  /// Returns a suggested Sass snippet for converting a variable named [name]
+  /// (without `%`) containing this number into a number with the same value and
+  /// the given [unit].
+  ///
+  /// If [unit] is null, this forces the number to be unitless.
+  ///
+  /// This is used for deprecation warnings when restricting which units are
+  /// allowed for a given function.
+  @internal
+  String unitSuggestion(String name, [String? unit]) {
+    var result = "\$$name" +
+        denominatorUnits.map((unit) => " * 1$unit").join() +
+        numeratorUnits.map((unit) => " / 1$unit").join() +
+        (unit == null ? "" : " * 1$unit");
+    return numeratorUnits.isEmpty ? result : "calc($result)";
+  }
 }