[Diags] Add an edu note explaining StringInterpolationProtocol informal requirements

include/swift/AST/EducationalNotes.def

@@ -46,4 +46,11 @@ EDUCATIONAL_NOTES(property_wrapper_failable_init,
 EDUCATIONAL_NOTES(property_wrapper_type_requirement_not_accessible,
                   "property-wrapper-requirements.md")
 
+EDUCATIONAL_NOTES(missing_append_interpolation,
+                  "string-interpolation-conformance.md")
+EDUCATIONAL_NOTES(append_interpolation_static,
+                  "string-interpolation-conformance.md")
+EDUCATIONAL_NOTES(append_interpolation_void_or_discardable,
+                  "string-interpolation-conformance.md")
+
 #undef EDUCATIONAL_NOTES

test/Parse/invalid_string_interpolation_protocol.swift

@@ -2,13 +2,13 @@
 
 // Has a lot of invalid 'appendInterpolation' methods
 public struct BadStringInterpolation: StringInterpolationProtocol {
-  // expected-error@-1{{type conforming to 'StringInterpolationProtocol' does not implement a valid 'appendInterpolation' method}}
+  // expected-error@-1{{type conforming to 'StringInterpolationProtocol' does not implement a valid 'appendInterpolation' method}} {{educational-notes=string-interpolation-conformance}}
 
   public init(literalCapacity: Int, interpolationCount: Int) {}
   public mutating func appendLiteral(_: String) {}
 
   public static func appendInterpolation(static: ()) {
-    // expected-warning@-1{{'appendInterpolation' method will never be used because it is static}} {{10-17=}}
+    // expected-warning@-1{{'appendInterpolation' method will never be used because it is static}} {{10-17=}} {{educational-notes=string-interpolation-conformance}}
   }
 
   private func appendInterpolation(private: ()) {
@@ -20,7 +20,7 @@ public struct BadStringInterpolation: StringInterpolationProtocol {
   }
 
   public func appendInterpolation(intResult: ()) -> Int {
-    // expected-warning@-1{{'appendInterpolation' method does not return 'Void' or have a discardable result}} {{10-10=@discardableResult }}
+    // expected-warning@-1{{'appendInterpolation' method does not return 'Void' or have a discardable result}} {{10-10=@discardableResult }} {{educational-notes=string-interpolation-conformance}}
   }
 }
 

userdocs/diagnostics/string-interpolation-conformance.md

@@ -0,0 +1,16 @@
+# Conforming to `StringInterpolationProtocol`
+A type conforming to `ExpressibleByStringInterpolation` uses a helper type called `StringInterpolation` to perform its interpolation. Many types can use `DefaultStringInterpolation`, which implements `String`'s interpolation behavior. Types can also implement custom behavior by providing their own type conforming to `StringInterpolationProtocol`.
+
+In addition to its formal requirements, `init(literalCapacity:interpolationCount:)` and `appendLiteral(_:)`, `StringInterpolationProtocol` has an additional, informal requirement, `appendInterpolation`. String interpolations using `\()` syntax are translated into calls to matching `appendInterpolation` methods.
+
+`StringInterpolationProtocol` conformers must provide at least one `appendInterpolation` method which:
+
+- Is an instance method, as opposed to a `static` or `class` method
+- Does not specify a return type, explicitly returns `Void`, or is marked with the `@discardableResult` attribute
+- Is at least as accessible as its containing type
+
+There are no restrictions on an `appendInterpolation` method's argument list, generic parameters, availability, or error-throwing behavior.
+
+If `appendInterpolation` is overloaded, the Swift compiler will choose an appropriate overload using the labels and argument types of each interpolation. When choosing an overload, any accessible `appendInterpolation` instance method may be used, even if it does not meet all of the requirements above. However, if a `StringInterpolationProtocol` conformer doesn't have any `appendInterpolation` methods which meet all of the requirments, an error will be reported at compile time.
+
+To learn more about customizing string interpolation behavior, see the standard library documentation of the `ExpressibleByStringInterpolation` and `StringInterpolationProtocol` protocols.