cargo fails when `patch` overrides only a git branch

[olix0r]

I have two crates: conduit-proxy and tower-h2. conduit-proxy depends on tower-h2 via a git dependency:

tower-h2              = { git = "https://github.com/tower-rs/tower-h2" }

I add another crate into the tower-h2 repo, tower-h2-balance. Before I merge tower-h2-balance to master, I want to test it in conduit-proxy, so I add a dependency:

tower-h2-balance      = { git = "https://github.com/tower-rs/tower-h2" }

Then, I'd like to patch these dependencies to use my working branch in the tower-h2 repo:

[patch."https://github.com/tower-rs/tower-h2"]
tower-h2         = { git = "https://github.com/tower-rs/tower-h2", branch = "ver/tower-h2-balance" }
tower-h2-balance = { git = "https://github.com/tower-rs/tower-h2", branch = "ver/tower-h2-balance" }

This is especially necessary because tower-h2 is a diamond dependency (i.e. it is also depended on by another dependency of conduit-proxy), and so it's important that these middle dependencies are patched to use the proper version of tower-h2.

After making this change, cargo check emits the following error:

error: failed to resolve patches for `https://github.com/tower-rs/tower-h2`

Caused by:
  patch for `tower-h2` in `https://github.com/tower-rs/tower-h2` points to the same source, but patches must point to different sources

Is this expected to work? If not, is there a suggested workaround?

For now, I've used .cargo/config to override the dependency, but this is brittle (and I get a big ugly warning about it).

---

active toolchain
----------------

stable-x86_64-apple-darwin (default)
rustc 1.26.2 (594fb253c 2018-06-01)

:; cargo version
cargo 1.26.0 (0e7c5a931 2018-04-06)

[alexcrichton]

This is unfortunately a bug in Cargo right now, and I think it's a duplicate of #5478 so closing in favor of that