Add `default` features for each [[bin]]

[Icelk]

Problem

When adding [[bin]] entries to a Cargo.toml, you can specify required dependencies, but not additional "recommended", default features.

Proposed Solution

Add a key, namely default-features to the [[bin]] sections in Cargo.toml.
Both the dependencies.default and this new key would be applied. If --no-default-features is passed, both declarations are ignored.

Notes

No response

[weihanglo]

Sorry, I don't get it. Where can I specify required dependencies for a [[bin]] target? Cannot find any relevant doc in 1.58.1 Cargo Guide. Only found an RFC still in discussion.

Presumably you want to build the binary without specifying features in CLI, there is a similar feature request you might be interested in, which proposes to enable required features automatically: #4663

[Icelk]

That issue is about enabling required dependencies. I’m talking about “recommended”, default dependencies, as those in dependencies.default. You can currently supply required dependencies per bin in <bin name>.required.features.
See the guide on the Cargo.toml format.

[weihanglo]

Sorry @Icelk, I've searched the key dependencies.default and still couldn't find such key in either 1.58.1 Cargo Guide or the link you provided. I might miss something important. Can you provide a screenshot or else to point out where it is?

Also, I am a bit confused. Is this issue concerning features or dependencies? They are not interchangeable terms in Cargo.toml. AFAIK, a target like [[bin]] only has a required-features field at this moment.

---

Here is my bold guess about what you mean by "recommended". Correct me if I am wrong.

# ...
[[bin]]
name = "foo"
default-features = ["serde", "log"]

In Cargo.toml, there is a bin target foo with the proposed default-features field, so when one tries to cargo build --bin foo, the serde and log features are on by default unless --no-default-features is also passed.

Is this what in your mind?

[Icelk]

Precisely!
I’m sorry, I of course mean features.default.

As you mentioned, one could specify both dependencies and other features in the <bin>.default-features field, just as is possible in features.default.

[weihanglo]

Discussion in #1982 cover some of scenarios here. rust-lang/rfcs#3374 is an attempt to resolve this as well.

[weihanglo]

Triage: after a re-read. This is kinda a duplicate of #4663, and is covered by #1982 and rust-lang/rfcs#3374, especially the latter that automatically enabling features for target requested. Let's continue discussing there. Thank you for your report :)