Allow passing env files to build script runtime env (#3632)

Fixes #3625, see that
issue for explanation

Author: dzbarsky

cargo/private/cargo_build_script.bzl

@@ -536,6 +536,9 @@ def _cargo_build_script_impl(ctx):
 
     build_script_inputs = []
 
+    build_script_inputs.extend(ctx.files.build_script_env_files)
+    args.add_all(ctx.files.build_script_env_files, format_each = "--input_dep_env_path=%s")
+
     for dep in ctx.attr.link_deps:
         if rust_common.dep_info in dep and dep[rust_common.dep_info].dep_env:
             dep_env_file = dep[rust_common.dep_info].dep_env
@@ -614,6 +617,25 @@ cargo_build_script = rule(
         "build_script_env": attr.string_dict(
             doc = "Environment variables for build scripts.",
         ),
+        "build_script_env_files": attr.label_list(
+            doc = dedent("""\
+                Files containing additional environment variables to set for rustc.
+
+                These files should  contain a single variable per line, of format
+                `NAME=value`, and newlines may be included in a value by ending a
+                line with a trailing back-slash (`\\\\`).
+
+                The order that these files will be processed is unspecified, so
+                multiple definitions of a particular variable are discouraged.
+
+                Note that the variables here are subject to
+                [workspace status](https://docs.bazel.build/versions/main/user-manual.html#workspace_status)
+                stamping should the `stamp` attribute be enabled. Stamp variables
+                should be wrapped in brackets in order to be resolved. E.g.
+                `NAME={WORKSPACE_STATUS_VARIABLE}`.
+            """),
+            allow_files = True,
+        ),
         "crate_features": attr.string_list(
             doc = "The list of rust features that the build script should consider activated.",
         ),

cargo/private/cargo_build_script_wrapper.bzl

@@ -22,6 +22,7 @@ def cargo_build_script(
         link_deps = [],
         proc_macro_deps = [],
         build_script_env = {},
+        build_script_env_files = [],
         use_default_shell_env = None,
         data = [],
         compile_data = [],
@@ -109,6 +110,8 @@ def cargo_build_script(
             links attribute and therefore provide environment variables to this build script.
         proc_macro_deps (list of label, optional): List of rust_proc_macro targets used to build the script.
         build_script_env (dict, optional): Environment variables for build scripts.
+        build_script_env_files (list of label, optional): Files containing additional environment variables to set
+            when running the build script.
         use_default_shell_env (bool, optional): Whether or not to include the default shell environment for the build script action. If unset the global
             setting `@rules_rust//cargo/settings:use_default_shell_env` will be used to determine this value.
         data (list, optional): Files needed by the build script.
@@ -209,6 +212,7 @@ def cargo_build_script(
         crate_features = crate_features,
         version = version,
         build_script_env = build_script_env,
+        build_script_env_files = build_script_env_files,
         use_default_shell_env = sanitized_use_default_shell_env,
         links = links,
         deps = deps,

crate_universe/src/context/crate_context.rs

@@ -229,6 +229,9 @@ pub(crate) struct BuildScriptAttributes {
     #[serde(skip_serializing_if = "Select::is_empty")]
     pub(crate) build_script_env: Select<BTreeMap<String, String>>,
 
+    #[serde(skip_serializing_if = "Select::is_empty")]
+    pub(crate) build_script_env_files: Select<BTreeSet<String>>,
+
     #[serde(skip_serializing_if = "Select::is_empty")]
     pub(crate) rundir: Select<String>,
 
@@ -276,6 +279,7 @@ impl Default for BuildScriptAttributes {
             link_deps: Default::default(),
             extra_link_deps: Default::default(),
             build_script_env: Default::default(),
+            build_script_env_files: Default::default(),
             rundir: Default::default(),
             extra_proc_macro_deps: Default::default(),
             proc_macro_deps: Default::default(),

crate_universe/src/rendering.rs

@@ -386,6 +386,9 @@ impl Renderer {
                 build_script_attrs
                     .rustc_env_files
                     .insert(":cargo_toml_env_vars".to_owned(), None);
+                build_script_attrs
+                    .build_script_env_files
+                    .insert(":cargo_toml_env_vars".to_owned(), None);
             }
         }
 

test/cargo_build_script/build_script_env_files/BUILD.bazel

@@ -0,0 +1,31 @@
+load("@bazel_skylib//rules:write_file.bzl", "write_file")
+load("//cargo:defs.bzl", "cargo_build_script")
+load("//rust:defs.bzl", "rust_library", "rust_test")
+
+write_file(
+    name = "generate_build_script_env_file",
+    out = "build_script_env_file",
+    content = [
+        "GREETING=Howdy",
+        "",
+    ],
+)
+
+cargo_build_script(
+    name = "cargo_build_script",
+    srcs = ["build.rs"],
+    build_script_env_files = [":generate_build_script_env_file"],
+    edition = "2018",
+)
+
+rust_library(
+    name = "test_lib",
+    srcs = ["test_lib.rs"],
+    edition = "2018",
+    deps = [":cargo_build_script"],
+)
+
+rust_test(
+    name = "consume_build_script",
+    crate = ":test_lib",
+)

test/cargo_build_script/build_script_env_files/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-env=FROM_BUILD_SCRIPT={}", std::env::var("GREETING").unwrap());
+}

test/cargo_build_script/build_script_env_files/test_lib.rs

@@ -0,0 +1,4 @@
+#[test]
+fn check_env_set() {
+    assert_eq!("Howdy", env!("FROM_BUILD_SCRIPT"));
+}