Panic when adding cyclic dev-dependency

[dbrgn]

When adding a library as its own dependency, Cargo detects it:

$ cargo test
error: cyclic package dependency: package `foo v0.1.0 (file:///tmp/foo)` depends on itself

But when creating an integration test and adding the current library as dev-dependency, it panics on cargo test. Steps to reproduce:

$ cargo new foo
$ cd foo
$ rustup override add nightly
$ echo -e '[dev-dependencies]\nfoo = { path = "." }' >> Cargo.toml
$ mkdir tests
$ echo "extern crate foo;" > tests/integration.rs
$ cargo test
thread 'main' panicked at 'assertion failed: my_dependencies.insert(dep.clone())', /checkout/cargo/src/cargo/util/dependency_queue.rs:86
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Backtrace:

   0: cargo::ops::cargo_rustc::compile
             at /checkout/cargo/src/cargo/util/dependency_queue.rs:86
             at /checkout/cargo/src/cargo/ops/cargo_rustc/job_queue.rs:103
             at /checkout/cargo/src/cargo/ops/cargo_rustc/mod.rs:237
   1: cargo::ops::cargo_rustc::compile_targets
             at /checkout/cargo/src/cargo/ops/cargo_rustc/mod.rs:135
   2: cargo::ops::cargo_compile::compile_ws
             at /checkout/cargo/src/cargo/ops/cargo_compile.rs:269
   3: cargo::ops::cargo_compile::compile_with_exec
             at /checkout/cargo/src/cargo/ops/cargo_compile.rs:153
   4: cargo::ops::cargo_test::compile_tests
             at /checkout/cargo/src/cargo/ops/cargo_compile.rs:141
             at /checkout/cargo/src/cargo/ops/cargo_test.rs:71
   5: cargo::ops::cargo_test::run_tests
             at /checkout/cargo/src/cargo/ops/cargo_test.rs:17
   6: cargo::try_execute_builtin_command
             at /checkout/cargo/src/bin/test.rs:147
             at /checkout/cargo/src/cargo/lib.rs:120
             at /checkout/cargo/src/bin/cargo.rs:259
   7: cargo::execute
             at /checkout/cargo/src/bin/cargo.rs:223
   8: cargo::call_main_without_stdin
             at /checkout/cargo/src/cargo/lib.rs:120
   9: cargo::main
             at /checkout/cargo/src/bin/cargo.rs:91
             at /checkout/cargo/src/bin/cargo.rs:84

(Fun fact: When running this with RUST_BACKTRACE=1, the backtrace takes reeeeally long to generate.)

[dbrgn]

As a sidenote, I tried adding the library as its own dev-dependency to see whether that could be a way to pass feature flags to integration tests. I wasn't trying to break Cargo :)

[alexcrichton]

Wow I don't think I've ever seen an assertion get tripped in that module!