ExternalReferenceWalker processes possible value documentation for property list sections (#1061)

* ExternalReferenceWalker processes markdown content
inside of PropertyListPossibleValuesSection's

* Update Tests/SwiftDocCTests/Infrastructure/ExternalReferenceResolverTests.swift

Simplify unit test

Co-authored-by: Sofía Rodríguez <sofilin2000@icloud.com>

---------

Co-authored-by: Sofía Rodríguez <sofilin2000@icloud.com>

Author: patshaughnessy

Sources/SwiftDocC/Semantics/ExternalLinks/ExternalReferenceWalker.swift

@@ -229,6 +229,12 @@ struct ExternalReferenceWalker: SemanticVisitor {
                 for markup in parameter.contents { visitMarkup(markup) }
             }
         }
+
+        for possibleValuesSection in symbol.possibleValuesSectionVariants.allValues.map(\.variant) {
+            for possibleValue in possibleValuesSection.possibleValues {
+                for markup in possibleValue.contents { visitMarkup(markup) }
+            }
+        }
     }
 
     mutating func visitDeprecationSummary(_ summary: DeprecationSummary) {

Tests/SwiftDocCTests/Infrastructure/ExternalReferenceResolverTests.swift

@@ -1434,4 +1434,50 @@ class ExternalReferenceResolverTests: XCTestCase {
         }
         XCTAssertEqual(externalLinkCount, 2, "Did not resolve the 2 expected external links.")
     }
+
+    func testPossibleValuesWithExternalLink() throws {
+
+        // Create some example documentation using the symbol graph file located under
+        // Tests/SwiftDocCTests/Test Bundles/DictionaryData.docc, and the following
+        // documentation extension markup.
+        let documentationExtension = TextFile(
+            name: "Genre.md",
+            utf8Content: """
+                    # ``DictionaryData/Genre``
+                    
+                    Genre object.
+                    
+                    The artist's genre.
+                    
+                    - PossibleValues:
+                      - Classic Rock: Something about classic rock with a link: <doc://com.external.testbundle/something/related/to/this/allowed/value>.
+                      - Folk: Something about folk music with a link: <doc://com.external.testbundle/something/related/to/this/allowed/value>.
+                    """)
+        let symbol = try exampleDocumentation(
+            copying: "DictionaryData",
+            documentationExtension: documentationExtension,
+            path: "/documentation/DictionaryData/Genre"
+        )
+
+        let section = try XCTUnwrap(symbol.possibleValuesSectionVariants.firstValue)
+        XCTAssertEqual(section.possibleValues.count, 3)
+
+        // Check that the two keys with external links in the markup above were found
+        // and processed by the test external reference resolver.
+        var externalLinkCount = 0
+        for possibleValue in section.possibleValues {
+            let value = possibleValue.contents.first?.format().trimmingCharacters(in: .whitespaces)
+            if possibleValue.value == "Classic Rock" {
+                let stringValue = try XCTUnwrap(value)
+                XCTAssertEqual(stringValue, "Something about classic rock with a link: <doc://com.external.testbundle/externally/resolved/path>.")
+                externalLinkCount += 1
+            } else if possibleValue.value == "Folk" {
+                let stringValue = try XCTUnwrap(value)
+                XCTAssertEqual(stringValue, "Something about folk music with a link: <doc://com.external.testbundle/externally/resolved/path>.")
+                externalLinkCount += 1
+            }
+        }
+        XCTAssertEqual(externalLinkCount, 2, "Did not resolve the 2 expected external links.")
+    }
+
 }