fix: prefix pwd for imacros flag (bazelbuild#4116)

toolchains_llvm
[now](bazel-contrib/toolchains_llvm@1194923)
force-includes a header via `-imacros`, which surfaced this bug.

Author: gl3nnleblanc

cargo/private/cargo_build_script.bzl

@@ -225,6 +225,10 @@ def _pwd_flags_resource_dir(args):
     """Prefix execroot-relative paths in -resource-dir arguments with ${pwd}."""
     return _prefix_pwd_to_flag(args, ["-resource-dir=", "-resource-dir"])
 
+def _pwd_flags_imacros(args):
+    """Prefix execroot-relative paths in -imacros arguments with ${pwd}."""
+    return _prefix_pwd_to_flag(args, ["-imacros"])
+
 _DIRECT_LIB_EXTENSIONS = (".a", ".o", ".so", ".dylib")
 
 def _pwd_flags_direct_libs(args):
@@ -257,7 +261,7 @@ def _pwd_paths(args):
     return _prefix_pwd_to_paths(args)
 
 def _pwd_flags(args):
-    return _pwd_flags_direct_libs(_pwd_flags_fsanitize_ignorelist(_pwd_flags_isystem(_pwd_flags_L(_pwd_flags_B(_pwd_flags_resource_dir(_pwd_flags_sysroot(args)))))))
+    return _pwd_flags_direct_libs(_pwd_flags_imacros(_pwd_flags_fsanitize_ignorelist(_pwd_flags_isystem(_pwd_flags_L(_pwd_flags_B(_pwd_flags_resource_dir(_pwd_flags_sysroot(args))))))))
 
 def _feature_enabled(ctx, feature_name, default = False):
     """Check if a feature is enabled.

cargo/tests/cargo_build_script/cc_args_and_env/BUILD.bazel

@@ -6,6 +6,8 @@ load(
     "direct_libs_relative_test",
     "fsanitize_ignorelist_absolute_test",
     "fsanitize_ignorelist_relative_test",
+    "imacros_absolute_test",
+    "imacros_relative_test",
     "include_absolute_test",
     "include_mixed_test",
     "include_relative_test",
@@ -48,6 +50,10 @@ fsanitize_ignorelist_absolute_test(name = "fsanitize_ignorelist_absolute_test")
 
 fsanitize_ignorelist_relative_test(name = "fsanitize_ignorelist_relative_test")
 
+imacros_absolute_test(name = "imacros_absolute_test")
+
+imacros_relative_test(name = "imacros_relative_test")
+
 legacy_cc_toolchain_test(name = "legacy_cc_toolchain_test")
 
 libpath_absolute_test(name = "libpath_absolute_test")

cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl

@@ -613,6 +613,28 @@ def include_mixed_test(name):
         }),
     )
 
+def imacros_relative_test(name):
+    cargo_build_script_with_extra_cc_compile_flags(
+        name = "%s/cargo_build_script" % name,
+        extra_cc_compile_flags = ["-imacros", "test/relative/redacted_dates.h"],
+    )
+    cc_args_and_env_analysis_test(
+        name = name,
+        target_under_test = "%s/cargo_build_script" % name,
+        expected_cflags = ["-imacros", "${pwd}/test/relative/redacted_dates.h"],
+    )
+
+def imacros_absolute_test(name):
+    cargo_build_script_with_extra_cc_compile_flags(
+        name = "%s/cargo_build_script" % name,
+        extra_cc_compile_flags = ["-imacros", "/test/absolute/redacted_dates.h"],
+    )
+    cc_args_and_env_analysis_test(
+        name = name,
+        target_under_test = "%s/cargo_build_script" % name,
+        expected_cflags = ["-imacros", "/test/absolute/redacted_dates.h"],
+    )
+
 def direct_libs_relative_test(name):
     cargo_build_script_with_extra_cc_compile_flags(
         name = "%s/cargo_build_script" % name,