Use llvm-ar as default librarian on Unix platforms

Resolves #5761

Author: finagolfin

Sources/PackageModel/UserToolchain.swift

@@ -172,9 +172,7 @@ public final class UserToolchain: Toolchain {
                 }
                 return "link"
             }
-            // TODO(compnerd) consider defaulting to `llvm-ar` universally with
-            // a fallback to `ar`.
-            return triple.isAndroid() ? "llvm-ar" : "ar"
+            return "llvm-ar"
         }()
 
         if let librarian = UserToolchain.lookup(
@@ -190,7 +188,18 @@ public final class UserToolchain: Toolchain {
         if let librarian = try? UserToolchain.getTool(tool, binDirectories: binDirectories) {
             return librarian
         }
-        return try UserToolchain.findTool(tool, envSearchPaths: searchPaths, useXcrun: useXcrun)
+        if triple.isApple() || triple.isWindows() {
+            return try UserToolchain.findTool(tool, envSearchPaths: searchPaths, useXcrun: useXcrun)
+        } else {
+            if let librarian = try? UserToolchain.findTool(tool, envSearchPaths: searchPaths, useXcrun: false) {
+                return librarian
+            }
+            // Fall back to looking for binutils `ar` if `llvm-ar` can't be found.
+            if let librarian = try? UserToolchain.getTool("ar", binDirectories: binDirectories) {
+                return librarian
+            }
+            return try UserToolchain.findTool("ar", envSearchPaths: searchPaths, useXcrun: false)
+        }
     }
 
     /// Determines the Swift compiler paths for compilation and manifest parsing.

Tests/BuildTests/BuildPlanTests.swift

@@ -4244,7 +4244,7 @@ final class BuildPlanTests: XCTestCase {
                     description: "Archiving \(buildPath.appending(components: "library.a").escapedPathString())"
                     args: ["\(result.plan.buildParameters.toolchain.librarianPath.escapedPathString())","-static","-o","\(buildPath.appending(components: "library.a").escapedPathString())","@\(buildPath.appending(components: "rary.product", "Objects.LinkFileList").escapedPathString())"]
                 """))
-            } else {    // assume Unix `ar` is the librarian
+            } else {    // assume `llvm-ar` is the librarian
                 XCTAssertMatch(contents, .contains("""
                   "C.rary-debug.a":
                     tool: shell

Tests/BuildTests/MockBuildTestHelper.swift

@@ -22,10 +22,8 @@ struct MockToolchain: PackageModel.Toolchain {
     let librarianPath = AbsolutePath("/fake/path/to/link.exe")
 #elseif os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
     let librarianPath = AbsolutePath("/fake/path/to/libtool")
-#elseif os(Android)
-    let librarianPath = AbsolutePath("/fake/path/to/llvm-ar")
 #else
-    let librarianPath = AbsolutePath("/fake/path/to/ar")
+    let librarianPath = AbsolutePath("/fake/path/to/llvm-ar")
 #endif
     let swiftCompilerPath = AbsolutePath("/fake/path/to/swiftc")
     let includeSearchPaths = [AbsolutePath]()