[analyzer] Improve 'not a type' error message

pkg/analyzer/lib/src/dart/resolver/named_type_resolver.dart

@@ -396,14 +396,17 @@ class NamedTypeResolver with ScopeHelpers {
             importPrefixElement.getMethod(name) ??
             importPrefixElement.getSetter(name);
       }
+      var elementKind = element != null
+          ? _ErrorHelper._getElementKindDescription(element)
+          : 'not a type';
       var fragment = element?.firstFragment;
       var source = fragment?.libraryFragment?.source;
       var nameOffset = fragment?.nameOffset;
       diagnosticReporter.atOffset(
         offset: importPrefix.offset,
         length: nameToken.end - importPrefix.offset,
         diagnosticCode: diag.notAType,
-        arguments: ['${importPrefix.name.lexeme}.${nameToken.lexeme}'],
+        arguments: ['${importPrefix.name.lexeme}.${nameToken.lexeme}', elementKind],
         contextMessages: [
           if (source != null && nameOffset != null)
             DiagnosticMessageImpl(
@@ -663,14 +666,15 @@ class _ErrorHelper {
     if (element != null) {
       var errorRange = _getErrorRange(node);
       var name = node.name.lexeme;
+      var elementKind = _getElementKindDescription(element);
       var fragment = element.firstFragment;
       var source = fragment.libraryFragment?.source;
       var nameOffset = fragment.nameOffset;
       diagnosticReporter.atOffset(
         offset: errorRange.offset,
         length: errorRange.length,
         diagnosticCode: diag.notAType,
-        arguments: [name],
+        arguments: [name, elementKind],
         contextMessages: [
           if (source != null && nameOffset != null)
             DiagnosticMessageImpl(
@@ -758,4 +762,37 @@ class _ErrorHelper {
   static bool _isTypeInTypeArgumentList(NamedType node) {
     return node.parent is TypeArgumentList;
   }
+
+  /// Returns a human-readable description of what kind of element this is.
+  static String _getElementKindDescription(Element element) {
+    if (element is PropertyAccessorElement) {
+      if (element.isSetter) {
+        return 'a setter';
+      } else {
+        return 'a getter';
+      }
+    } else if (element is FunctionElement) {
+      return 'a function';
+    } else if (element is MethodElement) {
+      return 'a method';
+    } else if (element is ConstructorElement) {
+      return 'a constructor';
+    } else if (element is VariableElement) {
+      if (element is FieldElement) {
+        return 'a field';
+      } else if (element is TopLevelVariableElement) {
+        return 'a top-level variable';
+      } else if (element is LocalVariableElement) {
+        return 'a local variable';
+      } else if (element is ParameterElement) {
+        return 'a parameter';
+      }
+      return 'a variable';
+    } else if (element is ExtensionElement) {
+      return 'an extension';
+    } else if (element is PrefixElement) {
+      return 'a prefix';
+    }
+    return 'not a type';
+  }
 }

pkg/analyzer/lib/src/diagnostic/diagnostic.g.dart

@@ -12504,18 +12504,19 @@ notAssignedPotentiallyNonNullableLocalVariable = DiagnosticWithArguments(
 
 /// Parameters:
 /// String p0: the name that is not a type
+/// String p1: the kind of element (e.g., 'a setter', 'a function')
 const DiagnosticWithArguments<
-  LocatableDiagnostic Function({required String p0})
+  LocatableDiagnostic Function({required String p0, required String p1})
 >
 notAType = DiagnosticWithArguments(
   name: 'not_a_type',
-  problemMessage: "{0} isn't a type.",
+  problemMessage: "'{0}' is {1}, but a type was expected.",
   correctionMessage: "Try correcting the name to match an existing type.",
   hasPublishedDocs: true,
   type: DiagnosticType.COMPILE_TIME_ERROR,
   uniqueName: 'not_a_type',
   withArguments: _withArgumentsNotAType,
-  expectedTypes: [ExpectedType.string],
+  expectedTypes: [ExpectedType.string, ExpectedType.string],
 );
 
 /// Parameters:
@@ -20208,8 +20209,8 @@ _withArgumentsNotAssignedPotentiallyNonNullableLocalVariable({
   );
 }
 
-LocatableDiagnostic _withArgumentsNotAType({required String p0}) {
-  return LocatableDiagnosticImpl(diag.notAType, [p0]);
+LocatableDiagnostic _withArgumentsNotAType({required String p0, required String p1}) {
+  return LocatableDiagnosticImpl(diag.notAType, [p0, p1]);
 }
 
 LocatableDiagnostic _withArgumentsNotBinaryOperator({required String p0}) {

pkg/analyzer/messages.yaml

@@ -13427,7 +13427,8 @@ CompileTimeErrorCode:
     type: compileTimeError
     parameters:
       String p0: the name that is not a type
-    problemMessage: "#p0 isn't a type."
+      String p1: the kind of element (e.g., 'a setter', 'a function')
+    problemMessage: "'#p0' is #p1, but a type was expected."
     correctionMessage: Try correcting the name to match an existing type.
     hasPublishedDocs: true
     documentation: |-