Test cargo build being resilient to filesystem loop

Author: weihanglo

tests/testsuite/build.rs

@@ -1668,7 +1668,7 @@ package `test v0.0.0 ([CWD])`
 }
 
 #[cargo_test]
-/// Make sure broken symlinks don't break the build
+/// Make sure broken and loop symlinks don't break the build
 ///
 /// This test requires you to be able to make symlinks.
 /// For windows, this may require you to enable developer mode.
@@ -1681,6 +1681,10 @@ fn ignore_broken_symlinks() {
         .file("Cargo.toml", &basic_bin_manifest("foo"))
         .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
         .symlink("Notafile", "bar")
+        // To hit the symlink directory, we need a build script
+        // to trigger a full scan of package files.
+        .file("build.rs", &main_file(r#""build script""#, &[]))
+        .symlink_dir("a/b", "a/b/c/d/foo")
         .build();
 
     p.cargo("build").run();