Fix a tools version loading failure that occurred when the regular manifest doesn't contain a tools-version. (#3208)

Usually the main manifest is the latest, but there are examples of packages where it's the opposite:  the main version is the old one, and newer ones have version suffixes.  In rare cases the main manifest is so old that it doesn't have a tools version, and the new logic for tools version loading incorrectly reported an error in those cases.

rdar://73267827

Author: abertelrud

Sources/PackageLoading/ToolsVersionLoader.swift

@@ -99,7 +99,21 @@ extension Manifest {
             else {
                 versionSpecificManifestToolsVersion = try toolsVersionLoader.load(file: versionSpecificManifest, fileSystem: fileSystem)
             }
-            let regularManifestToolsVersion = try toolsVersionLoader.load(file: regularManifest, fileSystem: fileSystem)
+
+            // Try to get the tools version of the regular manifest.  At the comment marker is missing, we default to
+            // tools version 3.1.0 (as documented).
+            let regularManifestToolsVersion: ToolsVersion
+            do {
+                regularManifestToolsVersion = try toolsVersionLoader.load(file: regularManifest, fileSystem: fileSystem)
+            }
+            catch {
+                if case ToolsVersionLoader.Error.malformedToolsVersionSpecification(.commentMarker(.isMissing)) = error {
+                    regularManifestToolsVersion = .v3
+                }
+                else {
+                    throw error
+                }
+            }
 
             // Compare the tools version of this manifest with the regular
             // manifest and use the version-specific manifest if it has

Tests/PackageLoadingTests/PD4_2LoadingTests.swift

@@ -360,6 +360,7 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
     func testVersionSpecificLoadingOfVersion3Manifest() throws {
         // Create a temporary FS to hold the package manifests.
         let fs = InMemoryFileSystem()
+        
         // Write a regular manifest with a tools version comment, and a `[email protected]` manifest without one.
         let packageDir = AbsolutePath.root
         let manifestContents = "import PackageDescription\nlet package = Package(name: \"Trivial\")"
@@ -372,6 +373,17 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
         // Check we can load the manifest.
         let manifest = try manifestLoader.load(package: packageDir, baseURL: "/foo", toolsVersion: .v4_2, packageKind: .root, fileSystem: fs)
         XCTAssertEqual(manifest.name, "Trivial")
+        
+        // Switch it around so that the main manifest is now the one that doesn't have a comment.
+        try fs.writeFileContents(
+            packageDir.appending(component: Manifest.basename + ".swift"),
+            bytes: ByteString(encodingAsUTF8: manifestContents))
+        try fs.writeFileContents(
+            packageDir.appending(component: Manifest.basename + "@swift-4.swift"),
+            bytes: ByteString(encodingAsUTF8: "// swift-tools-version:4.0\n" + manifestContents))
+        // Check we can load the manifest.
+        let manifest2 = try manifestLoader.load(package: packageDir, baseURL: "/foo", toolsVersion: .v4_2, packageKind: .root, fileSystem: fs)
+        XCTAssertEqual(manifest2.name, "Trivial")
     }
 
     func testRuntimeManifestErrors() throws {