Warn if dependency doesn't have library target kind

[jethrogb]

When specifying a crate without a library target kind as a dependency, cargo currently just silently ignores it. For example:

[package]
name = "deptest"
version = "0.1.0"
edition = "2018"

[dependencies]
ripgrep = "*"

$ cargo run
   Compiling deptest v0.1.0 (/tmp/deptest)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/deptest`
Hello, world!

Note that the dependency isn't even compiled. Clearly, Cargo has already decided to elide this crate in the dependency graph, but the cause is not obvious to the user. All this can be confusing if the user is expecting to be able to use the dependency. I think a warning message is in order.

[lukaslueg]

I concur that this is confusing and should result in a warning (especially since there usually is ripgrep and libripgrep and such). Two use cases that are probably legitimate:

• An "empty" dependency still means "A solution to my dependency graph exists. Prove me wrong!"
• Docs are pulled in unless --no-deps is supplied.

[MonliH]

I'd be interested in implementing this.

@rustbot claim

[MonliH]

Unfortunately, I no longer have the time to work on this.

@rustbot release-assignment

[0xPoe]

@rustbot claim

[0xPoe]

@ehuss I'm trying to add this warning, but I can't find anywhere to add it when resolving dependencies. Could you help with some hints?

[ehuss]

Hm, I'm not sure exactly where would be the best place.

Essentially, it needs to iterate over the workspace member IDs, call resolve.deps(member_id), iterate over those dependencies and fetch the package object for each one (pkg_set.get_one(dep_id)), and call if !dep_pkg.targets().iter().any(|t| t.is_lib()) { /*warn*/ }.

There are a few places where it could do that. I think one place would be the bottom of resolve_ws_with_opts. That is a common place where resolution is done. Another place would be in create_bcx. That is where all the build commands create their build graph. A drawback of create_bcx is that would only warn on build commands, and not others. I'm not sure if that matters much.