Path dependencies leaving the workspace get hashed.

While trying to achieve full determinism builds in a complicated real-world setup, I noticed that the crate hashes (which show up in filenames and are passed to rustc in -C metadata=...) would change for crates which were referred to by relative paths, but which left the workspace.

Is this intentional? If so, I'm curious what the reasoning behind that is.

I could imagine a situation where two packages with the name and the same version could show up in the same dependency tree, but isn't that an error anyway?

Minimal example (using the eddyb/cargo-issue-7645-example repo):

# Initialize issue7645-{1,2}
git clone https://github.com/eddyb/cargo-issue-7645-example issue7645-1
cp -a issue7645-1 issue7645-2

# Build issue7645-1/{,nested/}
cargo build --manifest-path=issue7645-1/Cargo.toml
cargo build --manifest-path=issue7645-1/nested/Cargo.toml

# Build issue7645-2/{,nested/}
cargo build --manifest-path=issue7645-2/Cargo.toml
cargo build --manifest-path=issue7645-2/nested/Cargo.toml

# List all the copies of libfoo's that were built
find issue7645-{1,2}/{,nested/}target/debug/deps/libfoo-*.rmeta

On the current nightly, I get:

issue7645-1/target/debug/deps/libfoo-fd649d53ec9bad59.rmeta
issue7645-2/target/debug/deps/libfoo-fd649d53ec9bad59.rmeta
issue7645-1/nested/target/debug/deps/libfoo-1eae900464f3a413.rmeta
issue7645-2/nested/target/debug/deps/libfoo-467e958a8a02d00e.rmeta

As you can see, the top-level workspace builds have the same hash, but the nested workspace builds have a different hash, because foo is outside of that workspace (and has a different path).

cc @ehuss (with whom I've previously discussed this on Discord)

Start with the linked eddyb/cargo-issue-7645-example repository and reproduce the four cargo build commands and libfoo hash comparison. Trace how Cargo handles path dependencies inside versus outside a workspace and how that affects rustc metadata. Done means determining whether the differing hashes are intentional and reaching a documented resolution or agreed behavior.