Enable code coverage in Xcode for swift-built targets (#1623)

This is the second in a stack of PRs across three repositories.

- bazelbuild/apple_support#491
- #1623 <-- you are here
- MobileNativeFoundation/rules_xcodeproj#3250

It takes the approach suggested in
MobileNativeFoundation/rules_xcodeproj#1119 and applies it to our swift
toolchain. By breaking sandboxing rules and remapping a derivable path
to the source root, we can to allow Xcode, which bookkeeps all code
coverage representations in the IDE as absolute paths, to present code
coverage information in the gutters and test results.

Author: aaronsky

swift/internal/feature_names.bzl

@@ -69,6 +69,18 @@ SWIFT_FEATURE_DEBUG_PREFIX_MAP = "swift.debug_prefix_map"
 # of remote builds.
 SWIFT_FEATURE_COVERAGE_PREFIX_MAP = "swift.coverage_prefix_map"
 
+# A private feature that is used to embed absolute source paths in coverage builds.
+#
+# If enabled, coverage builds will use a `-coverage-prefix-map` that remaps
+# the current working directory to a canonical location, which permits
+# coverage representation in non-sandboxed builds with tools that expect
+# absolute paths such as Xcode.
+#
+# This feature should only be used with non-sandboxed builds inside tools such as
+# Xcode, and enabling it effectively breaks Bazel's ability to rely on the
+# remote cache those builds. It should not be enabled by users of the toolchain.
+SWIFT_FEATURE__COVERAGE_PREFIX_MAP_ABSOLUTE_SOURCES_NON_HERMETIC = "swift._coverage_prefix_map_absolute_sources_non_hermetic"
+
 # If enabled, C and Objective-C libraries that are direct or transitive
 # dependencies of a Swift library will emit explicit precompiled modules that
 # are compatible with Swift's ClangImporter and propagate them up the build

swift/toolchains/config/compile_config.bzl

@@ -90,6 +90,7 @@ load(
     "SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE",
     "SWIFT_FEATURE_USE_PCH_OUTPUT_DIR",
     "SWIFT_FEATURE_VFSOVERLAY",
+    "SWIFT_FEATURE__COVERAGE_PREFIX_MAP_ABSOLUTE_SOURCES_NON_HERMETIC",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__SUPPORTS_UPCOMING_FEATURES",
     "SWIFT_FEATURE__SUPPORTS_V6",
@@ -558,6 +559,17 @@ def compile_action_configs(
             features = [
                 [SWIFT_FEATURE_COVERAGE_PREFIX_MAP, SWIFT_FEATURE_COVERAGE],
             ],
+            not_features = [SWIFT_FEATURE__COVERAGE_PREFIX_MAP_ABSOLUTE_SOURCES_NON_HERMETIC],
+        ),
+        ActionConfigInfo(
+            actions = all_compile_action_names(),
+            configurators = [
+                add_arg("-Xwrapped-swift=-coverage-prefix-pwd-is-canonical"),
+            ],
+            features = [
+                [SWIFT_FEATURE__COVERAGE_PREFIX_MAP_ABSOLUTE_SOURCES_NON_HERMETIC, SWIFT_FEATURE_COVERAGE],
+            ],
+            not_features = [SWIFT_FEATURE_COVERAGE_PREFIX_MAP, SWIFT_FEATURE_FILE_PREFIX_MAP],
         ),
 
         # Ensure that .swiftsourceinfo files are tracked and not deleted by the worker

tools/worker/swift_runner.cc

@@ -288,11 +288,12 @@ bool SwiftRunner::ProcessArgument(
 
   // Helper function for adding path remapping flags that depend on information
   // only known at execution time.
-  auto add_prefix_map_flags = [&](const std::string &flag) {
+  auto add_prefix_map_flags = [&](const std::string &flag,
+                                  const std::string &new_path = ".") {
     // Get the actual current working directory (the execution root), which
     // we didn't know at analysis time.
     consumer(flag);
-    consumer(std::filesystem::current_path().string() + "=.");
+    consumer(std::filesystem::current_path().string() + "=" + new_path);
 
 #if __APPLE__
     std::string developer_dir = "__BAZEL_XCODE_DEVELOPER_DIR__";
@@ -316,7 +317,18 @@ bool SwiftRunner::ProcessArgument(
       } else if (new_arg == "-coverage-prefix-pwd-is-dot") {
         // Replace the $PWD with . to make the paths relative to the workspace
         // without breaking hermiticity.
-        add_prefix_map_flags("-file-prefix-map");
+        add_prefix_map_flags("-coverage-prefix-map");
+        changed = true;
+      } else if (new_arg == "-coverage-prefix-pwd-is-canonical") {
+        // Replace the $PWD with . to make the paths relative to the workspace
+        // without breaking hermiticity.
+        auto cwd = std::filesystem::current_path();
+        // The bazel execroot is a normal directory, but inside of it there are
+        // symlinks to our source tree. This fetches the true path of a known
+        // directory in order to get the actual source root of the project. This
+        // should only work with sandboxing disabled.
+        auto target_path = std::filesystem::canonical(cwd / "BUILD.bazel").parent_path();
+        add_prefix_map_flags("-coverage-prefix-map", target_path.string());
         changed = true;
       } else if (new_arg == "-file-prefix-pwd-is-dot") {
         // Replace the $PWD with . to make the paths relative to the workspace