Add test for regression after #4040.

test/unit/cargo_build_script_data_runfiles/BUILD

@@ -0,0 +1,21 @@
+load("@rules_rust//rust:defs.bzl", "rust_binary")
+load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+cargo_build_script(
+    name = "build_script",
+    srcs = ["build.rs"],
+    data = ["data.txt"],
+    deps = [
+        "@rules_rust//tools/runfiles",
+    ],
+)
+
+rust_binary(
+    name = "reproduce_bin",
+    srcs = ["main.rs"],
+    deps = [":build_script"],
+)

test/unit/cargo_build_script_data_runfiles/build.rs

@@ -0,0 +1,22 @@
+use std::env;
+use std::path::Path;
+
+fn main() {
+    let r = runfiles::Runfiles::create().unwrap();
+    // The rlocation path is relative to the workspace name
+    let rlocation_path = "rules_rust/test/unit/cargo_build_script_data_runfiles/data.txt";
+    let path = runfiles::rlocation!(r, rlocation_path);
+
+    match path {
+        Some(p) => {
+            if p.exists() {
+                println!("cargo:warning=Found data.txt at {}", p.display());
+            } else {
+                panic!("data.txt path returned but does not exist: {}", p.display());
+            }
+        }
+        None => {
+            panic!("Failed to resolve runfile path: {}", rlocation_path);
+        }
+    }
+}

test/unit/cargo_build_script_data_runfiles/data.txt

@@ -0,0 +1 @@
+hello world

test/unit/cargo_build_script_data_runfiles/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello from main");
+}