Add target specific rustc_flags last to the command line (#2927)

This allows target specific flags to override toolchain specific flags, e.g: to disable LTO on examples.
bazelbuild · Oct 9, 2024 · 1da48bc · 1da48bc
1 parent 7890b42
commit 1da48bc
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Showing 2 changed files with 16 additions and 10 deletions.
diff --git a/rust/private/rustc.bzl b/rust/private/rustc.bzl
@@ -999,13 +999,6 @@ def construct_arguments(
     # Deduplicate data paths due to https://github.com/bazelbuild/bazel/issues/14681
     data_paths = depset(direct = getattr(attr, "data", []), transitive = [crate_info.compile_data_targets]).to_list()
 
-    rustc_flags.add_all(
-        expand_list_element_locations(
-            ctx,
-            getattr(attr, "rustc_flags", []),
-            data_paths,
-        ),
-    )
     add_edition_flags(rustc_flags, crate_info)
 
     # Link!
@@ -1105,6 +1098,15 @@ def construct_arguments(
     if _is_no_std(ctx, toolchain, crate_info):
         rustc_flags.add('--cfg=feature="no_std"')
 
+    # Add target specific flags last, so they can override previous flags
+    rustc_flags.add_all(
+        expand_list_element_locations(
+            ctx,
+            getattr(attr, "rustc_flags", []),
+            data_paths,
+        ),
+    )
+
     # Needed for bzlmod-aware runfiles resolution.
     env["REPOSITORY_NAME"] = ctx.label.workspace_name
 

diff --git a/test/toolchain/toolchain_test.bzl b/test/toolchain/toolchain_test.bzl
@@ -9,13 +9,15 @@ EXEC_TOOLCHAIN_FLAG = "missing"
 TOOLCHAIN_FLAG = "before"
 CONFIG_FLAG = "after"
 CRATE_FLAGS = {"cdylib": ["cdylib_flag"], "rlib": ["rlib_flag"]}
+TARGET_FLAG = "-Ccodegen-units=1"
 
 def _toolchain_adds_rustc_flags_impl(ctx, crate_type):
     """ Tests adding extra_rustc_flags on the toolchain, asserts that:
 
     - extra_rustc_flags added by the toolchain are applied BEFORE flags added by a config on the commandline
     - The exec flags from the toolchain don't go on the commandline for a non-exec target
     - crate type rustc flags are added
+    - target specific rustc flags are added AFTER the crate type rustc flags
     """
     env = analysistest.begin(ctx)
     target = analysistest.target_under_test(env)
@@ -25,16 +27,16 @@ def _toolchain_adds_rustc_flags_impl(ctx, crate_type):
 
     asserts.true(
         env,
-        action.argv[-2:] == [TOOLCHAIN_FLAG, CONFIG_FLAG],
+        action.argv[-3:] == [TOOLCHAIN_FLAG, CONFIG_FLAG, TARGET_FLAG],
         "Unexpected rustc flags: {}\nShould have ended with: {}".format(
             action.argv,
-            [TOOLCHAIN_FLAG, CONFIG_FLAG],
+            [TOOLCHAIN_FLAG, CONFIG_FLAG, TARGET_FLAG],
         ),
     )
 
     asserts.true(
         env,
-        action.argv[-3] == CRATE_FLAGS[crate_type][0],
+        action.argv[-4] == CRATE_FLAGS[crate_type][0],
         "Unexpected rustc flags: {}\nShould have contained: {}".format(
             action.argv,
             CRATE_FLAGS["rlib"],
@@ -140,12 +142,14 @@ def _define_targets():
         name = "lib",
         srcs = ["lib.rs"],
         edition = "2021",
+        rustc_flags = [TARGET_FLAG],
     )
 
     rust_shared_library(
         name = "shared_lib",
         srcs = ["lib.rs"],
         edition = "2021",
+        rustc_flags = [TARGET_FLAG],
     )
 
     native.filegroup(