provide correct paths to build scripts

As build scripts are built in a build script subfolder, the paths
need to be one level up.

Author: dae

examples/env_locations/BUILD

@@ -2,6 +2,7 @@ load(
     "@io_bazel_rules_rust//rust:rust.bzl",
     "rust_test",
 )
+load("@io_bazel_rules_rust//cargo:cargo_build_script.bzl", "cargo_build_script")
 
 # generate a file
 genrule(
@@ -10,22 +11,35 @@ genrule(
     cmd = "echo hello > $@",
 )
 
+_env = {
+    "GENERATED_DATA": "$(rootpath generated.data)",
+    "SOME_TOOL": "$(rootpath @com_google_protobuf//:protoc)",
+}
+
+_data = [
+    # provide the generated file as a dep
+    "generated.data",
+    # we should also be able to access external binaries
+    # such as protoc.
+    "@com_google_protobuf//:protoc",
+]
+
+cargo_build_script(
+    name = "build",
+    srcs = ["build.rs"],
+    data = _data,
+    rustc_env = _env,
+)
+
 rust_test(
     name = "test",
     srcs = [
         "main.rs",
     ],
-    data = [
-        # provide the generated file as a dep
-        "generated.data",
-        # we should also be able to access external binaries
-        # such as protoc.
-        "@com_google_protobuf//:protoc",
-    ],
+    data = _data,
     edition = "2018",
-    rustc_env = {
-        # and pass in the locations to rustc
-        "GENERATED_DATA": "$(rootpath generated.data)",
-        "SOME_TOOL": "$(rootpath @com_google_protobuf//:protoc)",
-    },
+    rustc_env = _env,
+    deps = [
+        ":build",
+    ],
 )

examples/env_locations/build.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let generated_data = std::fs::read_to_string(env!("GENERATED_DATA")).unwrap();
+    // our generated data file should be readable
+    assert_eq!(generated_data, "hello\n");
+    // and we should be able to read (and thus execute) our tool
+    assert_eq!(std::fs::read(env!("SOME_TOOL")).unwrap().is_empty(), false);
+}

rust/private/rustc.bzl

@@ -311,13 +311,23 @@ def get_linker_and_args(ctx, cc_toolchain, feature_configuration, rpaths):
 
     return ld, link_args, link_env
 
+def _expand_location(ctx, data, is_build_script, env_string):
+    expanded = ctx.expand_location(env_string, data)
+    if env_string != expanded and is_build_script:
+        # locations are relative to parent folder when building
+        # a build script
+        expanded = "../" + expanded
+    return expanded
+
 def _expand_locations(ctx, env, aspect):
     "Expand $(location ...) references in user-provided env vars."
     if aspect:
         data = getattr(ctx.rule.attr, "data", [])
     else:
         data = getattr(ctx.attr, "data", [])
-    return dict([(k, ctx.expand_location(v, data)) for (k, v) in env.items()])
+    is_build_script = ctx.label.name.endswith("_script_")
+
+    return dict([(k, _expand_location(ctx, data, is_build_script, v)) for (k, v) in env.items()])
 
 def _process_build_scripts(
         ctx,