Indicate the original configuration location for multi-config errors

lib/src/async_environment.dart

@@ -799,7 +799,7 @@ class AsyncEnvironment {
         configuration[name] = ConfiguredValue(values[name], null, nodes[name]);
       }
     }
-    return Configuration(configuration, isImplicit: true);
+    return Configuration.implicit(configuration);
   }
 
   /// Returns a module that represents the top-level members defined in [this],

lib/src/configuration.dart

@@ -4,6 +4,7 @@
 
 import 'dart:collection';
 
+import 'ast/node.dart';
 import 'ast/sass.dart';
 import 'configured_value.dart';
 import 'util/limited_map_view.dart';
@@ -19,6 +20,11 @@ class Configuration {
   Map<String, ConfiguredValue> get values => UnmodifiableMapView(_values);
   final Map<String, ConfiguredValue> _values;
 
+  /// The node whose span indicates where the configuration was declared.
+  ///
+  /// This is `null` for implicit configurations.
+  final AstNode nodeWithSpan;
+
   /// Whether or not this configuration is implicit.
   ///
   /// Implicit configurations are created when a file containing a `@forward`
@@ -31,8 +37,18 @@ class Configuration {
   /// silently ignored in this case.
   final bool isImplicit;
 
-  Configuration(Map<String, ConfiguredValue> values, {this.isImplicit = false})
-      : _values = values;
+  /// Creates an explicit configuration with the given [values].
+  Configuration(Map<String, ConfiguredValue> values, this.nodeWithSpan)
+      : _values = values,
+        isImplicit = false;
+
+  /// Creates an implicit configuration with the given [values].
+  ///
+  /// See [isImplicit] for details.
+  Configuration.implicit(Map<String, ConfiguredValue> values)
+      : _values = values,
+        nodeWithSpan = null,
+        isImplicit = true;
 
   /// The empty configuration, which indicates that the module has not been
   /// configured.
@@ -41,6 +57,7 @@ class Configuration {
   /// ignored if the module has already been loaded.
   const Configuration.empty()
       : _values = const {},
+        nodeWithSpan = null,
         isImplicit = true;
 
   bool get isEmpty => values.isEmpty;
@@ -67,6 +84,8 @@ class Configuration {
     } else if (forward.hiddenVariables?.isNotEmpty ?? false) {
       newValues = LimitedMapView.blocklist(newValues, forward.hiddenVariables);
     }
-    return Configuration(newValues, isImplicit: isImplicit);
+    return isImplicit
+        ? Configuration.implicit(newValues)
+        : Configuration(newValues, nodeWithSpan);
   }
 }

lib/src/environment.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 6e375d7e8ccaafe80c33ec5a220bc4acf5dbc3d0
+// Checksum: e0f39c1b98cb2adf98116bd51bf03d492be953c3
 //
 // ignore_for_file: unused_import
 
@@ -804,7 +804,7 @@ class Environment {
         configuration[name] = ConfiguredValue(values[name], null, nodes[name]);
       }
     }
-    return Configuration(configuration, isImplicit: true);
+    return Configuration.implicit(configuration);
   }
 
   /// Returns a module that represents the top-level members defined in [this],

lib/src/visitor/async_evaluate.dart

@@ -437,7 +437,7 @@ class _EvaluateVisitor
 
             values[name] = ConfiguredValue(value, span);
           });
-          configuration = Configuration(values);
+          configuration = Configuration(values, _callableNode);
         }
 
         await _loadModule(url, "load-css()", _callableNode,
@@ -625,10 +625,15 @@ class _EvaluateVisitor
                 "\"with\".";
 
         var existingNode = _moduleNodes[url];
-        throw existingNode == null
+        var secondarySpans = {
+          if (existingNode != null) existingNode.span: "original load",
+          if (configuration == null)
+            _configuration.nodeWithSpan.span: "configuration"
+        };
+
+        throw secondarySpans.isEmpty
             ? _exception(message)
-            : _multiSpanException(
-                message, "new load", {existingNode.span: "original load"});
+            : _multiSpanException(message, "new load", secondarySpans);
       }
 
       return alreadyLoaded;
@@ -1274,7 +1279,7 @@ class _EvaluateVisitor
           _expressionNode(variable.expression));
     }
 
-    return Configuration(newValues);
+    return Configuration(newValues, node);
   }
 
   /// Remove configured values from [upstream] that have been removed from
@@ -1876,7 +1881,7 @@ class _EvaluateVisitor
                   (await variable.expression.accept(this)).withoutSlash(),
                   variable.span,
                   _expressionNode(variable.expression))
-          });
+          }, node);
 
     await _loadModule(node.url, "@use", node, (module) {
       _environment.addModule(module, node, namespace: node.namespace);

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: b2a6756fe0f47ed5aec3b92b74f021198d7a770a
+// Checksum: eb095e782e2983223945d189caadc649b081a676
 //
 // ignore_for_file: unused_import
 
@@ -442,7 +442,7 @@ class _EvaluateVisitor
 
             values[name] = ConfiguredValue(value, span);
           });
-          configuration = Configuration(values);
+          configuration = Configuration(values, _callableNode);
         }
 
         _loadModule(url, "load-css()", _callableNode,
@@ -628,10 +628,15 @@ class _EvaluateVisitor
                 "\"with\".";
 
         var existingNode = _moduleNodes[url];
-        throw existingNode == null
+        var secondarySpans = {
+          if (existingNode != null) existingNode.span: "original load",
+          if (configuration == null)
+            _configuration.nodeWithSpan.span: "configuration"
+        };
+
+        throw secondarySpans.isEmpty
             ? _exception(message)
-            : _multiSpanException(
-                message, "new load", {existingNode.span: "original load"});
+            : _multiSpanException(message, "new load", secondarySpans);
       }
 
       return alreadyLoaded;
@@ -1272,7 +1277,7 @@ class _EvaluateVisitor
           _expressionNode(variable.expression));
     }
 
-    return Configuration(newValues);
+    return Configuration(newValues, node);
   }
 
   /// Remove configured values from [upstream] that have been removed from
@@ -1866,7 +1871,7 @@ class _EvaluateVisitor
                   variable.expression.accept(this).withoutSlash(),
                   variable.span,
                   _expressionNode(variable.expression))
-          });
+          }, node);
 
     _loadModule(node.url, "@use", node, (module) {
       _environment.addModule(module, node, namespace: node.namespace);