A project can not depend on itself as a path dependency

src/cargo/core/resolver/mod.rs

@@ -1009,6 +1009,15 @@ fn check_cycles(resolve: &Resolve) -> CargoResult<()> {
     for id in resolve.iter() {
         let map = graph.entry(id).or_insert_with(BTreeMap::new);
         for (dep_id, listings) in resolve.deps_not_replaced(id) {
+            // We don't want to allow a package to depend on itself as a path.
+            // See https://github.com/rust-lang/cargo/issues/9518 for more details.
+            if id == dep_id && dep_id.source_id().is_path() {
+                anyhow::bail!(
+                    "cyclic package dependency: package `{}` depends on itself.",
+                    id,
+                );
+            }
+
             let transitive_dep = listings.iter().find(|d| d.is_transitive());
 
             if let Some(transitive_dep) = transitive_dep.cloned() {

tests/testsuite/build.rs

@@ -1660,9 +1660,34 @@ fn self_dependency() {
         .with_status(101)
         .with_stderr(
             "\
-[ERROR] cyclic package dependency: package `test v0.0.0 ([CWD])` depends on itself. Cycle:
-package `test v0.0.0 ([CWD])`
-    ... which satisfies path dependency `test` of package `test v0.0.0 ([..])`",
+[ERROR] cyclic package dependency: package `test v0.0.0 ([CWD])` depends on itself.",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn dev_self_dependency() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.1.0"
+                authors = []
+
+                [dev-dependencies]
+                foo = { path = "." }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] cyclic package dependency: package `foo v0.1.0 ([CWD])` depends on itself.",
         )
         .run();
 }

tests/testsuite/features2.rs

@@ -695,18 +695,14 @@ fn cyclical_dev_dep() {
         )
         .build();
 
-    // Old way unifies features.
-    p.cargo("run true").run();
-    // dev feature should always be enabled in tests.
-    p.cargo("test").run();
-
-    // New behavior.
-    switch_to_resolver_2(&p);
-    // Should decouple main.
-    p.cargo("run false").run();
-
-    // And this should be no different.
-    p.cargo("test").run();
+    // TODO: Figure out how to properly fix this test case.
+    p.cargo("run true")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] cyclic package dependency: package `foo v0.1.0 ([CWD])` depends on itself.",
+        )
+        .run();
 }
 
 #[cargo_test]

tests/testsuite/test.rs

@@ -3366,7 +3366,13 @@ fn cyclic_dev() {
         .file("tests/foo.rs", "extern crate foo;")
         .build();
 
-    p.cargo("test --workspace").run();
+    p.cargo("test --workspace")
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] cyclic package dependency: package `foo v0.1.0 ([CWD])` depends on itself.",
+        )
+        .run();
 }
 
 #[cargo_test]