`cargo why <crate>`

[homersimpsons]

Problem

It is hard to know why a specific crate is required (and why it has a specific version).

Proposed Solution

There could be a sub command named why that could display the constraints applied to a crate and the dependency tree linked to it in order to understand why a specific crate is required.

Composer (PHP's package manager) exposes such sub command: https://getcomposer.org/doc/03-cli.md#depends-why-

Notes

This issue should be flagged with A-new-subcommand I guess.

A "similar" issue exists #1211 but it does not have the same intent

[weihanglo]

cargo-tree covers some usages here. For example. run cargo tree -i curl shows the reversed dependency tree:

curl v0.4.40
├── cargo v0.59.0 (/dev/cargo)
├── crates-io v0.33.0 (/dev/cargo/crates/crates-io)
│   └── cargo v0.59.0 (/dev/cargo)
└── git2-curl v0.14.1
    └── cargo v0.59.0 (/dev/cargo)

The tree command also supports showing enabled features for dependencies. But unfortunately, it doesn't cover "why it has a specific version" as you mentioned.

[homersimpsons]

Oh, I didn't notice that you could invert cargo-tree. Thank you for this information !

[epage]

Since we have cargo tree -i, do we still need this issue? Is there something we could do to make it easier to go from user problem ("why this dependency") to discovering cargo tree -i?

Granted, for myself, I just use cargo tree | less and just search the output because I forget about -i (speaking of, maybe we should have pager support for cargo tree).

[epage]

Closing this as covered by cargo tree. #12903 adds some more documentation around this.