test: isolate local rules and reduce repeated fixture setup

crates/api-macros/tests/renamed_dependency.rs

@@ -1,13 +1,38 @@
+//! Compile the real public macro through a renamed dependency in an isolated consumer.
+//! Only Cargo artifacts are shared; Cargo checks the fresh consumer on every invocation.
+
 use std::{fs, path::PathBuf, process::Command};
 
+struct Fixture(PathBuf);
+
+impl Fixture {
+    fn new() -> Self {
+        for attempt in 0..1000 {
+            let path = std::env::temp_dir().join(format!(
+                "api-macros-renamed-dependency-{}-{attempt}",
+                std::process::id()
+            ));
+            match fs::create_dir(&path) {
+                Ok(()) => return Self(path),
+                Err(error) if error.kind() == std::io::ErrorKind::AlreadyExists => continue,
+                Err(error) => panic!("create renamed dependency fixture: {error}"),
+            }
+        }
+        panic!("could not allocate a renamed dependency fixture directory");
+    }
+}
+
+impl Drop for Fixture {
+    fn drop(&mut self) {
+        let _ = fs::remove_dir_all(&self.0);
+    }
+}
+
 #[test]
 fn macro_expansion_resolves_a_renamed_support_dependency() {
-    let fixture = std::env::temp_dir().join(format!(
-        "api-macros-renamed-dependency-{}",
-        std::process::id()
-    ));
-    let _ = fs::remove_dir_all(&fixture);
-    fs::create_dir_all(fixture.join("src")).expect("create fixture source directory");
+    let fixture = Fixture::new();
+    let fixture = &fixture.0;
+    fs::create_dir(fixture.join("src")).expect("create fixture source directory");
 
     let package_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
     let package_path = package_path
@@ -49,13 +74,21 @@ pub fn operation_count() -> usize {
     )
     .expect("write fixture source");
 
+    // Keep nested Cargo checks off the outer build's target lock. Cargo owns cache
+    // invalidation and locking between concurrent consumers, not a saved
+    // test-success marker.
+    let executable = std::env::current_exe().expect("current test executable");
+    let target_dir = executable
+        .parent()
+        .and_then(|deps| deps.parent())
+        .expect("test executable has a profile directory")
+        .join("compile-tests/api-macros-renamed");
     let output = Command::new(env!("CARGO"))
         .args(["check", "--quiet", "--offline"])
-        .current_dir(&fixture)
-        .env("CARGO_TARGET_DIR", fixture.join("target"))
+        .current_dir(fixture)
+        .env("CARGO_TARGET_DIR", target_dir)
         .output()
         .expect("run cargo check for renamed dependency fixture");
-    let _ = fs::remove_dir_all(&fixture);
 
     assert!(
         output.status.success(),