Add a deprecation warning for strict unary operations (#1800)

Closes #1721

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 1.55.0
 
+* Emit a deprecation warning for `$a -$b` and `$a +$b`, since these look like
+  they could be unary operations but they're actually parsed as binary
+  operations. Either explicitly write `$a - $b` or `$a (-$b)`. See
+  https://sass-lang.com/d/strict-unary for more details.
+
 ### Dart API
 
 * Add an optional `argumentName` parameter to `SassScriptException()` to make it

lib/src/callable/built_in.dart

@@ -72,8 +72,7 @@ class BuiltInCallable implements Callable, AsyncBuiltInCallable {
   ///
   /// If passed, [url] is the URL of the module in which the function is
   /// defined.
-  BuiltInCallable.overloadedFunction(
-      this.name, Map<String, Callback> overloads,
+  BuiltInCallable.overloadedFunction(this.name, Map<String, Callback> overloads,
       {Object? url})
       : _overloads = [
           for (var entry in overloads.entries)

lib/src/parse/stylesheet.dart

@@ -1788,6 +1788,34 @@ abstract class StylesheetParser extends Parser {
       } else {
         singleExpression_ = BinaryOperationExpression(operator, left, right);
         allowSlash = false;
+
+        if (operator == BinaryOperator.plus ||
+            operator == BinaryOperator.minus) {
+          if (scanner.string.substring(
+                      right.span.start.offset - 1, right.span.start.offset) ==
+                  operator.operator &&
+              isWhitespace(scanner.string.codeUnitAt(left.span.end.offset))) {
+            logger.warn(
+                "This operation is parsed as:\n"
+                "\n"
+                "    $left ${operator.operator} $right\n"
+                "\n"
+                "but you may have intended it to mean:\n"
+                "\n"
+                "    $left (${operator.operator}$right)\n"
+                "\n"
+                "Add a space after ${operator.operator} to clarify that it's "
+                "meant to be a binary operation, or wrap\n"
+                "it in parentheses to make it a unary operation. This will be "
+                "an error in future\n"
+                "versions of Sass.\n"
+                "\n"
+                "More info and automated migrator: "
+                "https://sass-lang.com/d/strict-unary",
+                span: singleExpression_!.span,
+                deprecation: true);
+          }
+        }
       }
     }
 

pkg/sass_api/pubspec.yaml

@@ -2,7 +2,7 @@ name: sass_api
 # Note: Every time we add a new Sass AST node, we need to bump the *major*
 # version because it's a breaking change for anyone who's implementing the
 # visitor interface(s).
-version: 3.1.0
+version: 3.1.0-dev
 description: Additional APIs for Dart Sass.
 homepage: https://github.com/sass/dart-sass