SwiftPM 5.2 introduced a regression that caused a semantic change in how package manifests declaring a tools version earlier than 5.3 were parsed. Specifically, files with an unknown source type are not ignored, as they were in SwiftPM 5.1.x and earlier. This leads to issues such as the one reported here: https://swiftpm.slack.com/archives/C14QU6CUU/p1587442104122300

Author: abertelrud

Sources/PackageLoading/TargetSourcesBuilder.swift

@@ -175,7 +175,10 @@ public struct TargetSourcesBuilder {
                     if let ext = path.extension,
                       FileRuleDescription.header.fileTypes.contains(ext) {
                         matchedRule = Rule(rule: .header, localization: nil)
-                    } else {
+                    } else if toolsVersion >= .v5_3 {
+                        matchedRule = Rule(rule: .compile, localization: nil)
+                    } else if let ext = path.extension,
+                      SupportedLanguageExtension.validExtensions(toolsVersion: toolsVersion).contains(ext) {
                         matchedRule = Rule(rule: .compile, localization: nil)
                     }
                     // The source file might have been declared twice so

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -1659,6 +1659,30 @@ class PackageBuilderTests: XCTestCase {
         }
     }
 
+    func testUnknownSourceFilesUnderDeclaredSourcesIgnoredInV5_2Manifest() throws {
+        // Files with unknown suffixes under declared sources are not considered valid sources in 5.2 manifest.
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/Sources/lib/movie.mkv",
+            "/Sources/lib/lib.c",
+            "/Sources/lib/include/lib.h"
+        )
+
+        let manifest = Manifest.createManifest(
+            name: "pkg",
+            v: .v5_2,
+            targets: [
+                TargetDescription(name: "lib", dependencies: [], path: "./Sources/lib", sources: ["."]),
+            ]
+        )
+
+        PackageBuilderTester(manifest, in: fs) { package, _ in
+            package.checkModule("lib") { module in
+                module.checkSources(root: "/Sources/lib", paths: "lib.c")
+                module.check(includeDir: "/Sources/lib/include")
+            }
+        }
+    }
+
     func testBuildSettings() {
         let fs = InMemoryFileSystem(emptyFiles:
             "/Sources/exe/main.swift",