Moving repository to a virtual manifest can cause surprising dependency resolution errors due to resolver fallback

[thomaseizinger]

I am not entirely sure this is a bug report or a feature request but I'll describe anyway what happened.

We have a fairly large workspace over at https://github.com/libp2p/rust-libp2p. Previously, the root Cargo.toml was both a crate and a workspace manifest. For various reasons, I wanted to move away from that.

That root crate is edition = 2021 which implies the new dependency resolver (version 2). Thus, we did not specify resolver = 2 explicitly as part of the [workspace] table. We need the new feature resolver because we have some target-specific features to activiate:

https://github.com/libp2p/rust-libp2p/blob/7069d78ee333c92a3b99bbad100d9c6ba2fe361a/transports/noise/Cargo.toml#L26-L30

What happened now is that by splitting the root manifest into a separate crate, the edition = 2021 disappeared and cargo was now falling back using v1 of the resolver, resulting in a failing build because the features of the snow dependency within the Cargo.toml of libp2p-noise were incorrectly merged.

This was especially surprising because I was running cargo build --package libp2p which does have edition = 2021. I guess this could be considered a bug? I am not sure.

The problem was fixed by explicitly specifying resolver = 2 in the [workspace] table.

Here is the PR: libp2p/rust-libp2p#3536

[weihanglo]

Thanks for the report. This is a known inconvenience we want to address, by either better documentation or hint or automation. See this comment for more #9996 (comment).

Also, see these related issues and pull requests below. Some of them has been stale for a while. I'll appreciate if anyone could continue driving them.

• Add lint to explicitly set resolver in a virtual workspace #10112
• Infer resolver version from workspace edition #10587
• Add "resolver = 2" option to workspace manifest examples #10625
• Warn when an edition 2021 crate is in a virtual workspace with default resolver #10910

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This was especially surprising because I was running cargo build --package libp2p which does have edition = 2021. I guess this could be considered a bug? I am not sure.

If you're under a workspace running that, it will respect workspace settings. That means without resolver = "2" it is still running with resolver v1.

[ehuss]

Closing as a duplicate of the above linked issues.