cargo tree -d with --target can be confusing

[ehuss]

Problem
cargo tree -d can show duplicates when used with the --target flag that, although the packages will be built twice, it is unavoidable when cross-compiling. This can be confusing because the output shows the same package twice with the same version, but no other explanation of why they are shown.

Steps
Create a manifest where a dependency shows up as both a normal dependency and a build-dependency (or proc-macro). Example:

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

[dependencies]
anyhow = "1.0.40"

[build-dependencies]
anyhow = "1.0.40"

Running cargo tree -d --target x86_64-unknown-linux-gnu will show:

anyhow v1.0.40
[build-dependencies]
└── foo v0.1.0 (/Users/eric/Temp/foo)

anyhow v1.0.40
└── foo v0.1.0 (/Users/eric/Temp/foo)

Possible Solution(s)
I can think of two possible solutions:

1. Don't treat host/target duplicates as duplicates. In essence, behave the same as without --target (but still filtering for the given target).
2. Display some additional information (like host/target on each line).

Notes

Output of cargo version:
cargo 1.54.0-nightly (070e459 2021-05-11)

[Rustin170506]

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