[swift-build-dev] [SwiftPM] Add option to dump a package’s dependency tree

[Honza Dvorsky]

Hi All,

In the last couple of weeks I’ve been building small Swift servers and
interacting with the young ecosystem of public SwiftPM packages. One thing
that quickly came up is the need to track down which of my dependencies (or
any of its parents) is pulling in this one broken dependency, so that I can
fix or work around it.

This quickly becomes a very tedious task, because the SwiftPM packages I’ve
encountered are usually small (npm style), which leads to relatively large
dependency trees even for moderate-size frameworks.

To assist developers in easily looking up a misbehaving package or quickly
understanding how SwiftPM arrived at a resolved version, I propose we add a
new mode to swift-build. This mode would simply dump the dependency tree of
the root package into the console.

Usage could look like this:
```
swift build --dump-dependency-tree
```
which would print the dependency tree, having the root package as the root
of the tree, growing down from there.

The metadata I suggest we include in each node:
- Package name
- Package version range
- Package remote URL (to easily detect conflicting forks)
- Package dependencies as the children of this node

This could be used for:
- detecting conflicting forks in the dependency tree
- understanding conflicting version ranges of dependencies
- graphing the dependency tree in a graphing app, into an image (think
centralized SwiftPM index which can show you a clickable dependency tree)
- debugging/testing of SwiftPM

There are a couple of formats we could output the tree in
- .gv (https://en.wikipedia.org/wiki/DOT_(graph_description_language))
- Pretty text-based tree for quick CLI viewing
- JSON

Personally, I think we should print in the text-based form by default and
add an option to output the tree in the .gv format, which can be consumed
by many apps e.g. OmniGraffle for further processing.

Overall I think it’d be a nice feature to have for all the reasons outlined
above, but primarily it’s a good way to make the version-resolution magic
more transparent.

I appreciate all feedback. If you think this should be turned into a formal
proposal, I can also do that.

Honza Dvorsky
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