Only automatically link snippets to product libraries (#5929)

Snippets illustrate how to use a package's products by design, so they shouldn't link to library targets that aren't exported as products.

If snippets link to all library targets, one negative result would be inadvertently linking to test support libraries, which could cause failures due to automatic linking of `@rpath/libXCTestSwiftSupport.dylib`, a library not available to snippets at runtime.

rdar://102746374

Author: bitjammer

Sources/PackageLoading/PackageBuilder.swift

@@ -543,8 +543,9 @@ public final class PackageBuilder {
         if self.manifest.packageKind.isRoot {
             // Snippets: depend on all available library targets in the package.
             // TODO: Do we need to filter out targets that aren't available on the host platform?
+            let productTargets = Set(manifest.products.flatMap { $0.targets })
             let snippetDependencies = targets
-                .filter { $0.type == .library }
+                .filter { $0.type == .library && productTargets.contains($0.name) }
                 .map { Target.Dependency.target($0, conditions: []) }
             snippetTargets = try createSnippetTargets(dependencies: snippetDependencies)
         } else {

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -2281,6 +2281,46 @@ class PackageBuilderTests: XCTestCase {
             }
         }
     }
+    
+    func testSnippetsLinkProductLibraries() throws {
+        let root = AbsolutePath(path: "/Foo")
+        let internalSourcesDir = root.appending(components: "Sources", "Internal")
+        let productSourcesDir = root.appending(components: "Sources", "Product")
+        let snippetsDir = root.appending(components: "Snippets")
+        let fs = InMemoryFileSystem(emptyFiles:
+                                        internalSourcesDir.appending(component: "Internal.swift").pathString,
+            productSourcesDir.appending(component: "Product.swift").pathString,
+            snippetsDir.appending(component: "ASnippet.swift").pathString)
+        
+        let manifest = Manifest.createRootManifest(
+            name: "Foo", toolsVersion: .v5_7,
+            products: [
+                try ProductDescription(name: "Product", type: .library(.automatic), targets: ["Product"])
+            ],
+            targets: [
+                try TargetDescription(name: "Internal"),
+                try TargetDescription(name: "Product"),
+            ])
+        
+        PackageBuilderTester(manifest, path: root, in: fs) { result, diagnostics in
+            result.checkProduct("Product") { product in
+                product.check(type: .library(.automatic), targets: ["Product"])
+            }
+            result.checkProduct("ASnippet") { aSnippet in
+                aSnippet.check(type: .snippet, targets: ["ASnippet"])
+            }
+            result.checkModule("Internal") { foo in
+                foo.checkSources(sources: ["Internal.swift"])
+            }
+            result.checkModule("Product") { foo in
+                foo.checkSources(sources: ["Product.swift"])
+            }
+            result.checkModule("ASnippet") { aSnippet in
+                aSnippet.checkSources(sources: ["ASnippet.swift"])
+                aSnippet.check(targetDependencies: ["Product"])
+            }
+        }
+    }
 }
 
 final class PackageBuilderTester {