[Build] Add linker search path to lib dir in the toolchain

<rdar://problem/48696675> Add toolchain's usr/lib directory in the linker search path

Author: aciidgh

Sources/Build/BuildPlan.swift

@@ -612,6 +612,9 @@ public final class ProductBuildDescription {
     /// The build parameters.
     let buildParameters: BuildParameters
 
+    /// The file system reference.
+    let fs: FileSystem
+
     /// The path to the product binary produced.
     public var binary: AbsolutePath {
         return buildParameters.buildPath.appending(outname)
@@ -670,10 +673,11 @@ public final class ProductBuildDescription {
     }
 
     /// Create a build description for a product.
-    init(product: ResolvedProduct, buildParameters: BuildParameters) {
+    init(product: ResolvedProduct, buildParameters: BuildParameters, fs: FileSystem) {
         assert(product.type != .library(.automatic), "Automatic type libraries should not be described.")
         self.product = product
         self.buildParameters = buildParameters
+        self.fs = fs
     }
 
     /// Strips the arguments which should *never* be passed to Swift compiler
@@ -759,6 +763,15 @@ public final class ProductBuildDescription {
         // User arguments (from -Xlinker and -Xswiftc) should follow generated arguments to allow user overrides
         args += buildParameters.linkerFlags
         args += stripInvalidArguments(buildParameters.swiftCompilerFlags)
+
+        // Add toolchain's libdir at the very end (even after the user -Xlinker arguments).
+        //
+        // This will allow linking to libraries shipped in the toolchain.
+        let toolchainLibDir = buildParameters.toolchain.toolchainLibDir
+        if fs.isDirectory(toolchainLibDir) {
+            args += ["-L", toolchainLibDir.pathString]
+        }
+
         return args
     }
 
@@ -921,7 +934,9 @@ public class BuildPlan {
         // for automatic libraries because they don't produce any output.
         for product in graph.allProducts where product.type != .library(.automatic) {
             productMap[product] = ProductBuildDescription(
-                product: product, buildParameters: buildParameters)
+                product: product, buildParameters: buildParameters,
+                fs: fileSystem
+            )
         }
 
         self.productMap = productMap

Sources/Build/Toolchain.swift

@@ -44,4 +44,9 @@ extension Toolchain {
     public var macosSwiftStdlib: AbsolutePath { 
         return resolveSymlinks(swiftCompiler).appending(RelativePath("../../lib/swift/macosx"))
     }
+
+    var toolchainLibDir: AbsolutePath {
+        // FIXME: Not sure if it's better to base this off of Swift compiler or our own binary.
+        return resolveSymlinks(swiftCompiler).appending(RelativePath("../../lib"))
+    }
 }

Tests/BuildTests/BuildPlanTests.swift

@@ -1360,6 +1360,7 @@ final class BuildPlanTests: XCTestCase {
     func testExtraBuildFlags() throws {
         let fs = InMemoryFileSystem(emptyFiles:
             "/A/Sources/exe/main.swift",
+            "/fake/path/lib/libSomething.dylib",
             "<end>"
         )
 
@@ -1388,7 +1389,7 @@ final class BuildPlanTests: XCTestCase {
         )
 
         let exe = try result.buildProduct(for: "exe").linkArguments()
-        XCTAssertMatch(exe, [.anySequence, "-L", "/path/to/foo", "-L/path/to/foo", "-Xlinker", "-rpath=foo", "-Xlinker", "-rpath", "-Xlinker", "foo", .anySequence])
+        XCTAssertMatch(exe, [.anySequence, "-L", "/path/to/foo", "-L/path/to/foo", "-Xlinker", "-rpath=foo", "-Xlinker", "-rpath", "-Xlinker", "foo", "-L", "/fake/path/lib"])
     }
 }