Merge pull request #30116 from owenv/property-wrapper-note

[Diagnostics] Tweak @propertyWrapper diagnostic wording and add an educational note

Author: owenv

include/swift/AST/DiagnosticsSema.def

@@ -4797,7 +4797,7 @@ ERROR(property_wrapper_wrong_initial_value_init, none,
       "'wrappedValue' property type (%2) or an @autoclosure thereof",
       (DeclName, Type, Type))
 ERROR(property_wrapper_failable_init, none,
-      "%0 cannot be failable", (DeclName))
+      "property wrapper initializer %0 cannot be failable", (DeclName))
 ERROR(property_wrapper_type_requirement_not_accessible,none,
       "%select{private|fileprivate|internal|public|open}0 %1 %2 cannot have "
       "more restrictive access than its enclosing property wrapper type %3 "

include/swift/AST/EducationalNotes.def

@@ -34,4 +34,13 @@ EDUCATIONAL_NOTES(invalid_dynamic_callable_type,
 EDUCATIONAL_NOTES(missing_dynamic_callable_kwargs_method,
                   "dynamic-callable-requirements.md")
 
+EDUCATIONAL_NOTES(property_wrapper_no_value_property,
+                  "property-wrapper-requirements.md")
+EDUCATIONAL_NOTES(property_wrapper_wrong_initial_value_init,
+                  "property-wrapper-requirements.md")
+EDUCATIONAL_NOTES(property_wrapper_failable_init,
+                  "property-wrapper-requirements.md")
+EDUCATIONAL_NOTES(property_wrapper_type_requirement_not_accessible,
+                  "property-wrapper-requirements.md")
+
 #undef EDUCATIONAL_NOTES

test/decl/var/property_wrappers.swift

@@ -112,7 +112,7 @@ struct MultipleInitialValues<Value> {
 struct InitialValueFailable<Value> {
   var wrappedValue: Value
 
-  init?(wrappedValue initialValue: Value) { // expected-error{{'init(wrappedValue:)' cannot be failable}}
+  init?(wrappedValue initialValue: Value) { // expected-error{{property wrapper initializer 'init(wrappedValue:)' cannot be failable}}
     return nil
   }
 }
@@ -121,7 +121,7 @@ struct InitialValueFailable<Value> {
 struct InitialValueFailableIUO<Value> {
   var wrappedValue: Value
 
-  init!(wrappedValue initialValue: Value) {  // expected-error{{'init(wrappedValue:)' cannot be failable}}
+  init!(wrappedValue initialValue: Value) {  // expected-error{{property wrapper initializer 'init(wrappedValue:)' cannot be failable}}
     return nil
   }
 }

userdocs/diagnostics/property-wrapper-requirements.md

@@ -0,0 +1,7 @@
+Property Wrapper Implementation Requirements
+---
+If a type is marked with the `@propertyWrapper` attribute, it must meet certain requirements to be a valid property wrapper.
+
+First, all property wrapper types must have a property named `wrappedValue`. This property cannot be static and must have the same access level as the property wrapper type. If the property wrapper provides a `projectedValue` property, it is subject to the same requirements.
+
+Second, none of a property wrapper's initializers may be failable. Additionally, if a property wrapper initializer has a `wrappedValue` parameter, the type of that parameter must either be the same as the type of the `wrappedValue` property or an `@autoclosure` of that type.