[5.5] Do not crash on relative file URLs in a dependency (#3604)

* Do not crash on relative file URLs in a dependency

Currently, SwiftPM crashes when someone specifies the URL of a package dependency as a relative file URL. We should instead be able to just make it absolute based on the location of the package itself.

(cherry picked from commit d771063)

* Error on relative file URLs

It turns out that relative file URLs actually do not exist, see [here](#3604 (comment)) for more discussion. Instead we will now error and guide users to using a local package reference.

rdar://80751251

(cherry picked from commit e6a7323)

Author: neonichu

Sources/PackageLoading/ManifestLoader.swift

@@ -15,7 +15,7 @@ import TSCUtility
 import Foundation
 public typealias FileSystem = TSCBasic.FileSystem
 
-public enum ManifestParseError: Swift.Error {
+public enum ManifestParseError: Swift.Error, Equatable {
     /// The manifest contains invalid format.
     case invalidManifestFormat(String, diagnosticFile: AbsolutePath?)
 

Sources/PackageLoading/PackageDescription4Loader.swift

@@ -345,7 +345,11 @@ extension PackageDependencyDescription {
             } else if dependencyLocation.hasPrefix(filePrefix) {
                 // FIXME: SwiftPM can't handle file locations with file:// scheme so we need to
                 // strip that. We need to design a Location data structure for SwiftPM.
-                return AbsolutePath(String(dependencyLocation.dropFirst(filePrefix.count))).pathString
+                let location = String(dependencyLocation.dropFirst(filePrefix.count))
+                if location.first != "/" {
+                    throw ManifestParseError.invalidManifestFormat("file:// URLs cannot be relative, did you mean to use `.package(path:)`?", diagnosticFile: nil)
+                }
+                return AbsolutePath(location).pathString
             } else if URL.scheme(dependencyLocation) == nil {
                 // If the dependency URL is not remote, try to "fix" it.
                 // If the URL has no scheme, we treat it as a path (either absolute or relative to the base URL).

Tests/PackageLoadingTests/PD4_2LoadingTests.swift

@@ -499,6 +499,26 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
         }
     }
 
+    func testURLContainsNotAbsolutePath() throws {
+        let stream = BufferedOutputByteStream()
+        stream <<< """
+        import PackageDescription
+        let package = Package(
+            name: "Trivial",
+            dependencies: [
+                .package(url: "file://../best", from: "1.0.0"),
+            ],
+            targets: [
+                .target(
+                    name: "foo",
+                    dependencies: []),
+            ]
+        )
+        """
+
+        XCTAssertManifestLoadThrows(ManifestParseError.invalidManifestFormat("file:// URLs cannot be relative, did you mean to use `.package(path:)`?", diagnosticFile: nil), stream.bytes)
+    }
+
     func testCacheInvalidationOnEnv() throws {
         try testWithTemporaryDirectory { path in
             let fs = localFileSystem