Add support to the compiler for musl and the LINUX_STATIC SDK.

This is really just about setting appropriate defaults (such as
making sure that the static Linux triple causes us to use lld).

rdar://123506306

Author: al45tair

lib/Basic/Platform.cpp

@@ -231,7 +231,17 @@ StringRef swift::getPlatformNameForTriple(const llvm::Triple &triple) {
   case llvm::Triple::XROS:
     return getPlatformNameForDarwin(getDarwinPlatformKind(triple));
   case llvm::Triple::Linux:
-    return triple.isAndroid() ? "android" : "linux";
+    if (triple.isAndroid())
+      return "android";
+    else if (triple.isMusl()) {
+      // The triple for linux-static is <arch>-swift-linux-musl, to distinguish
+      // it from a "normal" musl set-up (ala Alpine).
+      if (triple.getVendor() == llvm::Triple::Swift)
+        return "linux-static";
+      else
+        return "musl";
+    } else
+      return "linux";
   case llvm::Triple::FreeBSD:
     return "freebsd";
   case llvm::Triple::OpenBSD:

lib/ClangImporter/ClangImporter.cpp

@@ -643,7 +643,7 @@ importer::getNormalInvocationArguments(
     // using Glibc or a libc that respects that flag. This will cause some
     // source breakage however (specifically with strerror_r()) on Linux
     // without a workaround.
-    if (triple.isOSFuchsia() || triple.isAndroid()) {
+    if (triple.isOSFuchsia() || triple.isAndroid() || triple.isMusl()) {
       // Many of the modern libc features are hidden behind feature macros like
       // _GNU_SOURCE or _XOPEN_SOURCE.
       invocationArgStrs.insert(invocationArgStrs.end(), {

lib/ClangImporter/ClangIncludePaths.cpp

@@ -180,7 +180,7 @@ createClangArgs(const ASTContext &ctx, clang::driver::Driver &clangDriver) {
 
 static bool shouldInjectLibcModulemap(const llvm::Triple &triple) {
   return triple.isOSGlibc() || triple.isOSOpenBSD() || triple.isOSFreeBSD() ||
-         triple.isAndroid() || triple.isOSWASI();
+         triple.isAndroid() || triple.isMusl() || triple.isOSWASI();
 }
 
 static SmallVector<std::pair<std::string, std::string>, 2>
@@ -254,8 +254,9 @@ static void getLibStdCxxFileMapping(
   // We currently only need this when building for Linux.
   if (!triple.isOSLinux())
     return;
-  // Android uses libc++.
-  if (triple.isAndroid())
+  // Android uses libc++, as does our fully static Linux config.
+  if (triple.isAndroid()
+      || (triple.isMusl() && triple.getVendor() == llvm::Triple::Swift))
     return;
 
   // Extract the libstdc++ installation path from Clang driver.
@@ -534,6 +535,9 @@ ClangInvocationFileMapping swift::getClangInvocationFileMapping(
     // WASI Mappings
     libcFileMapping =
         getLibcFileMapping(ctx, "wasi-libc.modulemap", std::nullopt, vfs);
+  } else if (triple.isMusl()) {
+    libcFileMapping =
+        getLibcFileMapping(ctx, "musl.modulemap", StringRef("SwiftMusl.h"), vfs);
   } else {
     // Android/BSD/Linux Mappings
     libcFileMapping = getLibcFileMapping(ctx, "glibc.modulemap",

lib/Driver/UnixToolChains.cpp

@@ -110,7 +110,9 @@ ToolChain::InvocationInfo toolchains::GenericUnix::constructInvocation(
 }
 
 std::string toolchains::GenericUnix::getDefaultLinker() const {
-  if (getTriple().isAndroid())
+  if (getTriple().isAndroid()
+      || (getTriple().isMusl()
+          && getTriple().getVendor() == llvm::Triple::Swift))
     return "lld";
 
   switch (getTriple().getArch()) {
@@ -268,7 +270,8 @@ toolchains::GenericUnix::constructInvocation(const DynamicLinkJobAction &job,
   }
 
   SmallString<128> SharedResourceDirPath;
-  getResourceDirPath(SharedResourceDirPath, context.Args, /*Shared=*/true);
+  getResourceDirPath(SharedResourceDirPath, context.Args,
+                     /*Shared=*/!(staticExecutable || staticStdlib));
 
   SmallString<128> swiftrtPath = SharedResourceDirPath;
   llvm::sys::path::append(swiftrtPath,