Add proper support for custom Swift toolchains (#1025)

Previously we did some amount of handling if you passed a custom
toolchain identifier. This didn't handle the case that the toolchain
required new compatibility libraries to actually link your binary. To
handle this we have to pass a new -L to the path in the root of the
toolchain. We do this with a custom env var that we replace, similar to
the DEVELOPER_DIR variable. Ideally this would be cached in memory, but
we can't do that without editing bazel, luckily xcrun has a cache that
makes this likely fast enough (about 0.01s for a cached invocation).

README.md

@@ -52,15 +52,15 @@ uses `clang`.
 
 ## Building with Custom Toolchains
 
-**macOS hosts:** You can build with a custom toolchain installed in
-`/Library/Developer/Toolchains` instead of Xcode's default. To do so, pass the
-following flag to Bazel:
+**macOS hosts:** You can build with a custom Swift toolchain (downloaded
+from https://swift.org/download) instead of Xcode's default. To do so,
+pass the following flag to Bazel:
 
 ```lang-none
---define=SWIFT_CUSTOM_TOOLCHAIN=toolchain.id
+--action_env=TOOLCHAINS=toolchain.id
 ```
 
-where `toolchain.id` is the value of the `CFBundleIdentifier` key in the
+Where `toolchain.id` is the value of the `CFBundleIdentifier` key in the
 toolchain's Info.plist file.
 
 To list the available toolchains and their bundle identifiers, you can run:

swift/internal/xcode_swift_toolchain.bzl

@@ -195,7 +195,8 @@ def _sdk_developer_framework_dir(apple_toolchain, target_triple, xcode_config):
 def _swift_linkopts_providers(
         apple_toolchain,
         target_triple,
-        toolchain_label):
+        toolchain_label,
+        toolchain_root):
     """Returns providers containing flags that should be passed to the linker.
 
     The providers returned by this function will be used as implicit
@@ -207,6 +208,8 @@ def _swift_linkopts_providers(
         target_triple: The target triple `struct`.
         toolchain_label: The label of the Swift toolchain that will act as the
             owner of the linker input propagating the flags.
+        toolchain_root: The path to a custom Swift toolchain that could contain
+            libraries required to link the binary
 
     Returns:
         A `struct` containing the following fields:
@@ -216,14 +219,22 @@ def _swift_linkopts_providers(
         *   `objc_info`: An `apple_common.Objc` provider that will provide
             linker flags to binaries that depend on Swift targets.
     """
+    linkopts = []
+    if toolchain_root:
+        # This -L has to come before Xcode's to make sure libraries are
+        # overridden when applicable
+        linkopts.append("-L{}/usr/lib/swift/{}".format(
+            toolchain_root,
+            target_triples.platform_name_for_swift(target_triple),
+        ))
+
     swift_lib_dir = paths.join(
         apple_toolchain.developer_dir(),
         "Toolchains/XcodeDefault.xctoolchain/usr/lib/swift",
         target_triples.platform_name_for_swift(target_triple),
     )
 
-    linkopts = [
-        "-Wl,-rpath,/usr/lib/swift",
+    linkopts.extend([
         "-L{}".format(swift_lib_dir),
         "-L/usr/lib/swift",
         # TODO(b/112000244): These should get added by the C++ Starlark API,
@@ -232,7 +243,8 @@ def _swift_linkopts_providers(
         # variables not provided by cc_common. Figure out how to handle this
         # correctly.
         "-Wl,-objc_abi_version,2",
-    ]
+        "-Wl,-rpath,/usr/lib/swift",
+    ])
 
     return struct(
         cc_info = CcInfo(
@@ -571,12 +583,6 @@ def _xcode_swift_toolchain_impl(ctx):
 
     xcode_config = ctx.attr._xcode_config[apple_common.XcodeVersionConfig]
 
-    swift_linkopts_providers = _swift_linkopts_providers(
-        apple_toolchain = apple_toolchain,
-        target_triple = target_triple,
-        toolchain_label = ctx.label,
-    )
-
     # `--define=SWIFT_USE_TOOLCHAIN_ROOT=<path>` is a rapid development feature
     # that lets you build *just* a custom `swift` driver (and `swiftc`
     # symlink), rather than a full toolchain, and point compilation actions at
@@ -591,10 +597,22 @@ def _xcode_swift_toolchain_impl(ctx):
     # script, use `--define=SWIFT_CUSTOM_TOOLCHAIN=<id>` as shown below.
     swift_executable = get_swift_executable_for_toolchain(ctx)
     toolchain_root = ctx.var.get("SWIFT_USE_TOOLCHAIN_ROOT")
-    custom_toolchain = ctx.var.get("SWIFT_CUSTOM_TOOLCHAIN")
+
+    # TODO: Remove SWIFT_CUSTOM_TOOLCHAIN for the next major release
+    custom_toolchain = ctx.var.get("SWIFT_CUSTOM_TOOLCHAIN") or ctx.configuration.default_shell_env.get("TOOLCHAINS")
+    custom_xcode_toolchain_root = None
     if toolchain_root and custom_toolchain:
         fail("Do not use SWIFT_USE_TOOLCHAIN_ROOT and SWIFT_CUSTOM_TOOLCHAIN" +
              "in the same build.")
+    elif custom_toolchain:
+        custom_xcode_toolchain_root = "__BAZEL_CUSTOM_XCODE_TOOLCHAIN_PATH__"
+
+    swift_linkopts_providers = _swift_linkopts_providers(
+        apple_toolchain = apple_toolchain,
+        target_triple = target_triple,
+        toolchain_label = ctx.label,
+        toolchain_root = toolchain_root or custom_xcode_toolchain_root,
+    )
 
     # Compute the default requested features and conditional ones based on Xcode
     # version.

tools/common/BUILD

@@ -12,9 +12,18 @@ cc_library(
         "//tools:clang-cl": [
             "-Xclang",
             "-fno-split-cold-code",
+            "/std:c++17",
+        ],
+        "//tools:msvc": [
+            "/std:c++17",
+        ],
+        "//conditions:default": [
+            "-std=c++17",
         ],
-        "//conditions:default": [],
     }),
+    deps = [
+        ":process",
+    ],
 )
 
 cc_library(

tools/common/bazel_substitutions.cc

@@ -15,10 +15,14 @@
 #include "tools/common/bazel_substitutions.h"
 
 #include <cstdlib>
+#include <filesystem>
 #include <iostream>
 #include <map>
+#include <sstream>
 #include <string>
 
+#include "tools/common/process.h"
+
 namespace bazel_rules_swift {
 namespace {
 
@@ -30,6 +34,11 @@ static const char kBazelXcodeDeveloperDir[] = "__BAZEL_XCODE_DEVELOPER_DIR__";
 // at runtime.
 static const char kBazelXcodeSdkRoot[] = "__BAZEL_XCODE_SDKROOT__";
 
+// The placeholder string used by the Apple and Swift rules to be replaced with
+// the absolute path to the custom toolchain being used
+static const char kBazelToolchainPath[] =
+    "__BAZEL_CUSTOM_XCODE_TOOLCHAIN_PATH__";
+
 // Returns the value of the given environment variable, or the empty string if
 // it wasn't set.
 std::string GetAppleEnvironmentVariable(const char *name) {
@@ -45,6 +54,48 @@ std::string GetAppleEnvironmentVariable(const char *name) {
   return env_value;
 }
 
+std::string GetToolchainPath() {
+#if !defined(__APPLE__)
+  return "";
+#endif
+
+  char *toolchain_id = getenv("TOOLCHAINS");
+  if (toolchain_id == nullptr) {
+    return "";
+  }
+
+  std::ostringstream output_stream;
+  int exit_code =
+      RunSubProcess({"xcrun", "--find", "clang", "--toolchain", toolchain_id},
+                    &output_stream, /*stdout_to_stderr=*/true);
+  if (exit_code != 0) {
+    std::cerr << output_stream.str() << "Error: TOOLCHAINS was set to '"
+              << toolchain_id << "' but xcrun failed when searching for that ID"
+              << std::endl;
+    exit(EXIT_FAILURE);
+  }
+
+  if (output_stream.str().empty()) {
+    std::cerr << "Error: TOOLCHAINS was set to '" << toolchain_id
+              << "' but no toolchain with that ID was found" << std::endl;
+    exit(EXIT_FAILURE);
+  } else if (output_stream.str().find("XcodeDefault.xctoolchain") !=
+             std::string::npos) {
+    // NOTE: Ideally xcrun would fail if the toolchain we asked for didn't exist
+    // but it falls back to the DEVELOPER_DIR instead, so we have to check the
+    // output ourselves.
+    std::cerr << "Error: TOOLCHAINS was set to '" << toolchain_id
+              << "' but the default toolchain was found, that likely means a "
+                 "matching "
+              << "toolchain isn't installed" << std::endl;
+    exit(EXIT_FAILURE);
+  }
+
+  std::filesystem::path toolchain_path(output_stream.str());
+  // Remove usr/bin/clang components to get the root of the custom toolchain
+  return toolchain_path.parent_path().parent_path().parent_path().string();
+}
+
 }  // namespace
 
 BazelPlaceholderSubstitutions::BazelPlaceholderSubstitutions() {
@@ -56,8 +107,11 @@ BazelPlaceholderSubstitutions::BazelPlaceholderSubstitutions() {
       {kBazelXcodeDeveloperDir, PlaceholderResolver([]() {
          return GetAppleEnvironmentVariable("DEVELOPER_DIR");
        })},
-      {kBazelXcodeSdkRoot,
-       PlaceholderResolver([]() { return GetAppleEnvironmentVariable("SDKROOT"); })},
+      {kBazelXcodeSdkRoot, PlaceholderResolver([]() {
+         return GetAppleEnvironmentVariable("SDKROOT");
+       })},
+      {kBazelToolchainPath,
+       PlaceholderResolver([]() { return GetToolchainPath(); })},
   };
 }