recommended-bin-packages field in Cargo.toml

text/3383-recommended-bin-crates.md

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+- Feature Name: recommended-bin-crates
+- Start Date: 2023-01-04
+- RFC PR: [rust-lang/rfcs#3383](https://github.com/rust-lang/rfcs/pull/3383)
+
+# Summary
+[summary]: #summary
+
+Add an optional `recommended-bin-crates` field to the `[package]`
+section of `Cargo.toml`, to enable crate authors to point out related
+binary crates in the error message Cargo users get when attempting to
+`cargo install` a crate without binaries.
+
+# Motivation
+[motivation]: #motivation
+
+Command-line tools written in Rust are often published in crates named
+different than the command, since that name is already occupied by a
+related library crate, for instance:
+
+* the `diesel` command is provided by the `diesel_cli` binary crate,
+  that depends on the `diesel` library crate
+
+* the `wasm-bindgen` command is provided by the `wasm-bindgen-cli`
+  binary crate, which is different from the `wasm-bindgen` library crate
+
+While such a setup has several benefits, it currently leads to a
+user experience problem with Cargo: To obtain a command, users will be
+tempted to run `cargo install <command>`, which will however inevitably fail:
+
+```
+$ cargo install diesel
+error: there is nothing to install in `diesel v2.0.3`, because it has no binaries
+`cargo install` is only for installing programs, and can't be used with libraries.
+To use a library crate, add it as a dependency in a Cargo project instead.
+```
+
+The idea of this RFC is that the `Cargo.toml` of such
+a library-only crate could specify for instance:
+
+```toml
+[package]
+name = "diesel"
+# ...
+recommended-bin-crates = ["diesel-cli"]
+```
+
+which could be picked up by Cargo in order to additionally include
+a note such as the following in the above error message:
+
+> The developers of `diesel` suggest you may want to install `diesel-cli` instead.
+
+resulting in a more seamless user experience.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The following is written as if it was part of the [manifest page] of the Cargo Book.
+
+## The `recommended-bin-crates` field
+
+The `recommended-bin-crates` field is an array of names of related binary crates.
+
+```toml
+[package]
+name = "foobar"
+# ...
+recommended-bin-crates = ["foobar-cli"]
+```
+
+Specifying this field for a library-only crate, enables Cargo to print
+a more user-friendly error message for `cargo install`, for example:
+
+```
+$ cargo install foobar
+error: there is nothing to install in `foobar v1.2.3`, because it has no binaries
+`cargo install` is only for installing programs, and can't be used with libraries.
+To use a library crate, add it as a dependency in a Cargo project instead.
+
+The developers of `foobar` suggest you may want to install `foobar-cli` instead.
+```
+
+(Notice the last line in the above output, which is enabled by the
+`recommended-bin-crates` field.)
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `cargo install` command has already parsed the `Cargo.toml` manifest
+file when it prints this error message, so it would simply have to
+additionally check for this new field when printing the error message.
+
+`cargo publish` should assert that the referenced
+crates already exist and have not been yanked.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+* It introduces yet another manifest `field`.
+
+* The crates referenced by this field could become abandoned, out-of-date or yanked
+  (although the referencing and referenced crate will presumably often be published
+  by the same person/group, in which case this can be simply avoided).
+
+* Like any other manifest field, updating the field requires you to publish
+  a new version of the crate (a patch version bump suffices).
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The problem addressed by this RFC can be sidestepped by publishing the
+library along with the binary in a single crate. This does however come
+with two disadvantages:
+
+* Cargo currently doesn't support artifact-specific dependencies
+  (although that may change, see [RFC 2887] & [RFC 3374]).
+
+* You would have to bump the library version each time you want to
+  publish a new version of the binary. If you want independently
+  incrementable versions for your library and your binary, you have to
+  publish them in separate crates.
+
+The problem could also be sidestepped by publishing the command in a
+crate with the same name as the command and using a different name for
+the library crate. (While `cargo add` currently does not fail for
+binary-only crates that could very well be addressed without an RFC.)
+A disparity between crate name and library name again comes with its
+own disadvantages, e.g. Rust users often expect libraries published
+on crates.io to be documented at docs.rs/{lib_name}.
+
+So while these two alternatives have their own disadvantages, they are
+also generally not an option for projects that have already been published
+using the described naming convention, since converting a library-only
+crate to a binary-only crate is bound to result in confusion.
+
+The proposed field provides a simple solution to a rather annoying
+problem and could be easily applied to existing projects using this
+common naming convention to solve the described user experience problem.
+
+# Prior art
+[prior-art]: #prior-art
+
+Most other package managers do not have such a problem since their
+install command does not mandate that the package contains binaries.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+* Is `recommended-bin-crates` a good name for the field?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+* crates.io and/or lib.rs could additionally link crates referenced via
+  this new field on their library web pages
+
+* Clippy could gain a lint to check that the referenced crates actually
+  exist, have not been yanked and are actually binary crates.
+
+
+[manifest page]: https://doc.rust-lang.org/cargo/reference/manifest.html
+[RFC 2887]: https://github.com/rust-lang/rfcs/pull/2887
+[RFC 3374]: https://github.com/rust-lang/rfcs/pull/3374