[PackageLoading] Diagnose invalid values of build settings

<rdar://problem/52395869>

Author: aciidgh

Sources/PackageLoading/PackageDescription4Loader.swift

@@ -135,13 +135,27 @@ extension ManifestBuilder {
             condition = try parseCondition(conditionJSON)
         }
 
+        let value = try json.get([String].self, forKey: "value")
+
+        // Diagnose invalid values.
+        for item in value {
+            let groups = Self.invalidValueRegex.matchGroups(in: item).flatMap{ $0 }
+            if !groups.isEmpty {
+                let error = "the build setting '\(name)' contains invalid component(s): \(groups.joined(separator: " "))"
+                throw ManifestParseError.runtimeManifestErrors([error])
+            }
+        }
+
         return .init(
             tool: tool, name: name,
-            value: try json.get("value"),
+            value: value,
             condition: condition
         )
     }
 
+    /// Looks for Xcode-style build setting macros "$()".
+    private static let invalidValueRegex = try! RegEx(pattern: #"(\$\(.*?\))"#)
+
     func parseCondition(_ json: JSON) throws -> TargetBuildSettingDescription.Condition {
         let platformNames: [String]? = try? json.getArray("platforms").map({ try $0.get("name") })
         return .init(

Tests/PackageLoadingTests/PD5LoadingTests.swift

@@ -369,4 +369,29 @@ class PackageDescription5LoadingTests: XCTestCase {
             XCTAssertNotNil(contents)
         }
     }
+
+    func testInvalidBuildSettings() throws {
+        let stream = BufferedOutputByteStream()
+        stream <<< """
+            import PackageDescription
+            let package = Package(
+               name: "Foo",
+               targets: [
+                   .target(
+                       name: "Foo",
+                       cSettings: [
+                           .headerSearchPath("$(BYE)/path/to/foo/$(SRCROOT)/$(HELLO)"),
+                       ]
+                   ),
+               ]
+            )
+            """
+
+        do {
+            try loadManifestThrowing(stream.bytes) { _ in }
+            XCTFail("Unexpected success")
+        } catch ManifestParseError.runtimeManifestErrors(let errors) {
+            XCTAssertEqual(errors, ["the build setting 'headerSearchPath' contains invalid component(s): $(BYE) $(SRCROOT) $(HELLO)"])
+        }
+    }
 }

Tests/PackageLoadingTests/XCTestManifests.swift

@@ -110,6 +110,7 @@ extension PackageDescription5LoadingTests {
     static let __allTests__PackageDescription5LoadingTests = [
         ("testBasics", testBasics),
         ("testBuildSettings", testBuildSettings),
+        ("testInvalidBuildSettings", testInvalidBuildSettings),
         ("testPlatforms", testPlatforms),
         ("testSerializedDiagnostics", testSerializedDiagnostics),
         ("testSwiftLanguageVersion", testSwiftLanguageVersion),