Merge cc toolchain flags into build script env (bazelbuild#1675)

Right now, setting build_script_env will _overwrite_ the values from the
cc_toolchain, which is almost certainly ~always wrong.

Instead, allow _appending_ values, which means that if overrides are
necessary people can add them (e.g. disabling specific warnings).

Note: This is technically a breaking change, but one which should
improve faithfulness of the build.

cargo/private/cargo_build_script.bzl

@@ -175,7 +175,7 @@ def _cargo_build_script_impl(ctx):
     # Add environment variables from the Rust toolchain.
     env.update(toolchain.env)
 
-    env.update(expand_dict_value_locations(
+    _merge_env_dict(env, expand_dict_value_locations(
         ctx,
         ctx.attr.build_script_env,
         getattr(ctx.attr, "data", []) +
@@ -316,6 +316,13 @@ cargo_build_script = rule(
     incompatible_use_toolchain_transition = True,
 )
 
+def _merge_env_dict(prefix_dict, suffix_dict):
+    """Merges suffix_dict into prefix_dict, appending rather than replacing certain env vars."""
+    for key in ["CFLAGS", "CXXFLAGS", "LDFLAGS"]:
+        if key in prefix_dict and key in suffix_dict and prefix_dict[key]:
+            prefix_dict[key] += " " + suffix_dict.pop(key)
+    prefix_dict.update(suffix_dict)
+
 def name_to_pkg_name(name):
     """Sanitize the name of cargo_build_script targets.
 

examples/zig_cross_compiling/BUILD.bazel

@@ -1,5 +1,6 @@
 load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
 load("@crate_index//:defs.bzl", "aliases", "all_crate_deps")
+load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
 load("@rules_rust//rust:defs.bzl", "rust_binary")
 
 rust_binary(
@@ -8,7 +9,17 @@ rust_binary(
     aliases = aliases(),
     # We don't care about building this for our own platform, this just exists for the transition
     tags = ["manual"],
-    deps = all_crate_deps(normal = True),
+    deps = all_crate_deps(normal = True) + [":check_merged_flags"],
+)
+
+cargo_build_script(
+    name = "check_merged_flags",
+    srcs = ["src/check_merged_flags.rs"],
+    build_script_env = {
+        "CFLAGS": "cbeep",
+        "CXXFLAGS": "cxxbeep",
+        "LDFLAGS": "ldbeep",
+    },
 )
 
 platform(

examples/zig_cross_compiling/src/check_merged_flags.rs

@@ -0,0 +1,13 @@
+fn main() {
+    let cflags = std::env::var("CFLAGS").unwrap();
+    assert!(cflags.contains("-target aarch64-linux-gnu.2.28"), "Expected CFLAGS to contain `-target aarch64-linux-gnu.2.28` because of zig toolchain but was `{}`", cflags);
+    assert!(cflags.contains(" cbeep"), "Expected CFLAGS to contain ` cbeep` because of build_script_env but was `{}`", cflags);
+
+    let cxxflags = std::env::var("CXXFLAGS").unwrap();
+    assert!(cxxflags.contains("-target aarch64-linux-gnu.2.28"), "Expected CXXFLAGS to contain `-target aarch64-linux-gnu.2.28` because of zig toolchain but was `{}`", cxxflags);
+    assert!(cxxflags.contains(" cxxbeep"), "Expected CXXFLAGS to contain ` cxxbeep` because of build_script_env but was `{}`", cxxflags);
+
+    let ldflags = std::env::var("LDFLAGS").unwrap();
+    assert!(ldflags.contains("-target aarch64-linux-gnu.2.28"), "Expected LDFLAGS to contain `-target aarch64-linux-gnu.2.28` because of zig toolchain but was `{}`", ldflags);
+    assert!(ldflags.contains(" ldbeep"), "Expected LDFLAGS to contain ` ldbeep` because of build_script_env but was `{}`", ldflags);
+}