Validate project reference path

Sources/ProjectSpec/SpecValidation.swift

@@ -178,6 +178,12 @@ extension Project {
             }
         }
 
+        for projectReference in projectReferences {
+            if !(basePath + projectReference.path).exists {
+                errors.append(.invalidProjectReferencePath(projectReference))
+            }
+        }
+
         for scheme in schemes {
             for buildTarget in scheme.build.targets {
                 switch buildTarget.target.location {

Sources/ProjectSpec/SpecValidationError.swift

@@ -30,6 +30,7 @@ public struct SpecValidationError: Error, CustomStringConvertible {
         case missingDefaultConfig(configName: String)
         case invalidPerConfigSettings
         case invalidProjectReference(scheme: String, reference: String)
+        case invalidProjectReferencePath(ProjectReference)
 
         public var description: String {
             switch self {
@@ -75,6 +76,8 @@ public struct SpecValidationError: Error, CustomStringConvertible {
                 return "Settings that are for a specific config must go in \"configs\". \"base\" can be used for common settings"
             case let .invalidProjectReference(scheme, project):
                 return "Scheme \(scheme.quoted) has invalid project reference \(project.quoted)"
+            case let .invalidProjectReferencePath(reference):
+                return "Project reference \(reference.name) has a project file path that doesn't exist \"\(reference.path)\""
             }
         }
     }

Tests/ProjectSpecTests/ProjectSpecTests.swift

@@ -226,6 +226,13 @@ class ProjectSpecTests: XCTestCase {
                 try expectValidationError(project, .invalidProjectReference(scheme: "scheme1", reference: "invalidProjectRef"))
             }
 
+            $0.it("fails with invalid project reference path") {
+                var project = baseProject
+                let reference = ProjectReference(name: "InvalidProj", path: "invalid_path")
+                project.projectReferences = [reference]
+                try expectValidationError(project, .invalidProjectReferencePath(reference))
+            }
+
             $0.it("allows missing optional file") {
                 var project = baseProject
                 project.targets = [Target(