Driver: allow static executables when using Musl (#1383)

This error diagnostic doesn't make sense when linking on Linux with Musl, which allows fully statically linked executables.

Author: MaxDesiatov

Sources/SwiftDriver/Driver/Driver.swift

@@ -660,7 +660,9 @@ public struct Driver {
 
     self.lto = Self.ltoKind(&parsedOptions, diagnosticsEngine: diagnosticsEngine)
     // Figure out the primary outputs from the driver.
-    (self.compilerOutputType, self.linkerOutputType) = Self.determinePrimaryOutputs(&parsedOptions, driverKind: driverKind, diagnosticsEngine: diagnosticEngine)
+    (self.compilerOutputType, self.linkerOutputType) = 
+      Self.determinePrimaryOutputs(&parsedOptions, targetTriple: self.frontendTargetInfo.target.triple, 
+                                   driverKind: driverKind, diagnosticsEngine: diagnosticEngine)
 
     // Multithreading.
     self.numThreads = Self.determineNumThreads(&parsedOptions, compilerMode: compilerMode, diagnosticsEngine: diagnosticEngine)
@@ -1994,6 +1996,7 @@ extension Driver {
   /// Determine the primary compiler and linker output kinds.
   private static func determinePrimaryOutputs(
     _ parsedOptions: inout ParsedOptions,
+    targetTriple: Triple,
     driverKind: DriverKind,
     diagnosticsEngine: DiagnosticsEngine
   ) -> (FileType?, LinkOutputType?) {
@@ -2005,7 +2008,7 @@ extension Driver {
     if let outputOption = parsedOptions.getLast(in: .modes) {
       switch outputOption.option {
       case .emitExecutable:
-        if parsedOptions.contains(.static) {
+        if parsedOptions.contains(.static) && !targetTriple.supportsStaticExecutables {
           diagnosticsEngine.emit(.error_static_emit_executable_disallowed)
         }
         linkerOutputType = .executable

Sources/SwiftDriver/Utilities/Triple.swift

@@ -1705,3 +1705,14 @@ fileprivate extension Array {
     }
   }
 }
+
+// MARK: - Linker support
+
+extension Triple {
+    /// Returns `true` if a given triple supports producing fully statically linked executables by providing `-static`
+    /// flag to the linker. This implies statically linking platform's libc, and of those that Swift supports currently
+    /// only Musl allows that reliably.
+    var supportsStaticExecutables: Bool {
+        self.environment == .musl
+    }
+}

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2100,10 +2100,42 @@ final class SwiftDriverTests: XCTestCase {
       XCTAssertFalse(cmd.contains(.flag("-dylib")))
       XCTAssertFalse(cmd.contains(.flag("-shared")))
     }
+
+    do {
+      // executable linking linux static stdlib with musl
+      var driver = try Driver(args: commonArgs + [
+        "-emit-executable", "-Osize", "-static-stdlib", "-static-executable", "-target", "x86_64-unknown-linux-musl"
+      ], env: env)
+      let plannedJobs = try driver.planBuild()
+
+      XCTAssertEqual(plannedJobs.count, 4)
+
+      let autolinkExtractJob = plannedJobs[2]
+      XCTAssertEqual(autolinkExtractJob.kind, .autolinkExtract)
+
+      let autolinkCmd = autolinkExtractJob.commandLine
+      XCTAssertTrue(commandContainsTemporaryPath(autolinkCmd, "foo.o"))
+      XCTAssertTrue(commandContainsTemporaryPath(autolinkCmd, "bar.o"))
+      XCTAssertTrue(commandContainsTemporaryPath(autolinkCmd, "Test.autolink"))
+
+      let linkJob = plannedJobs[3]
+      let cmd = linkJob.commandLine
+      XCTAssertTrue(cmd.contains(.flag("-o")))
+      XCTAssertTrue(commandContainsTemporaryPath(cmd, "foo.o"))
+      XCTAssertTrue(commandContainsTemporaryPath(cmd, "bar.o"))
+      XCTAssertTrue(cmd.contains(.flag("--start-group")))
+      XCTAssertTrue(cmd.contains(.flag("--end-group")))
+      XCTAssertTrue(cmd.contains(.flag("-Os")))
+      XCTAssertTrue(cmd.contains(.flag("-static")))
+      XCTAssertEqual(linkJob.outputs[0].file, try VirtualPath(path: "Test"))
+
+      XCTAssertFalse(cmd.contains(.flag("-dylib")))
+      XCTAssertFalse(cmd.contains(.flag("-shared")))
+    }
     #endif
 
     do {
-      // static WASM linking
+      // static Wasm linking
       var driver = try Driver(args: commonArgs + ["-emit-library", "-static", "-target", "wasm32-unknown-wasi"], env: env)
       let plannedJobs = try driver.planBuild()
 
@@ -2135,7 +2167,7 @@ final class SwiftDriverTests: XCTestCase {
       try withTemporaryDirectory { path in
         try localFileSystem.writeFileContents(
           path.appending(components: "wasi", "static-executable-args.lnk")) { $0 <<< "garbage" }
-        // WASM executable linking
+        // Wasm executable linking
         var driver = try Driver(args: commonArgs + ["-emit-executable", "-Ounchecked",
                                                     "-target", "wasm32-unknown-wasi",
                                                     "-resource-dir", path.pathString,