Fixed incorrect locking logic when artifact-dir == build-dir (#16385)

### What does this PR try to resolve?

This PR fixes incorrect locking behavior in `layout.rs` that forgets to
lock the `build-dir` when `build-dir == artifact-dir`.
This is problematic is the default for both are `$WORKSPACE/target` and
I overlooked this :(

Issue reported in #16384

### How to test and review this PR?

See the tests. You can also run `cargo check` and check for the presence
of `.cargo-lock`

r? @weihanglo

Author: weihanglo

src/cargo/core/compiler/layout.rs

@@ -151,7 +151,12 @@ impl Layout {
         // actual destination (sub)subdirectory.
         paths::create_dir_all(dest.as_path_unlocked())?;
 
-        let build_dir_lock = if root == build_root || is_on_nfs_mount(build_root.as_path_unlocked())
+        // We always need to take the build-dir lock but if the build-dir == artifact-dir then we
+        // only take the artifact-dir. (locking both as they are the same dir)
+        // However we need to take into account that for some builds like `cargo check` we avoid
+        // locking the artifact-dir. We still need to lock the build-dir to avoid file corruption.
+        let build_dir_lock = if (must_take_artifact_dir_lock && root == build_root)
+            || is_on_nfs_mount(build_root.as_path_unlocked())
         {
             None
         } else {

tests/testsuite/check.rs

@@ -1710,13 +1710,35 @@ fn check_build_should_not_output_files_to_artifact_dir() {
 }
 
 #[cargo_test]
-fn check_build_should_not_lock_artifact_dir() {
+fn check_build_should_lock_target_dir_when_artifact_dir_is_same_as_build_dir() {
     let p = project()
         .file("src/main.rs", r#"fn main() { println!("Hello, World!") }"#)
         .build();
 
     p.cargo("check").enable_mac_dsym().run();
-    assert!(!p.root().join("target/debug/.cargo-lock").exists());
+    assert!(p.root().join("target/debug/.cargo-lock").exists());
+}
+
+#[cargo_test]
+fn check_build_should_not_lock_artifact_dir_when_build_dir_is_not_same_dir() {
+    let p = project()
+        .file("src/main.rs", r#"fn main() { println!("Hello, World!") }"#)
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [build]
+            target-dir = "target-dir"
+            build-dir = "build-dir"
+            "#,
+        )
+        .build();
+
+    p.cargo("check").enable_mac_dsym().run();
+
+    // Verify we did NOT take the build-dir lock
+    assert!(!p.root().join("target-dir/debug/.cargo-lock").exists());
+    // Verify we did take the build-dir lock
+    assert!(p.root().join("build-dir/debug/.cargo-lock").exists());
 }
 
 // Regression test for #16305