Cargo Tree should be able to show dependency platforms

[TroyKomodo]

Problem

I am working on some features for rules_rust (bazel build rules for rust) and we depend on cargo tree to help with per feature target resolution

See:

• cargo metadata doesn't indicate that feature-enabled transitive platform-specific deps are platform-specific #9863
• crate_universe support for resolver = "2" target-specific features bazelbuild/rules_rust#1662

The problem is cargo tree's output sometimes is very hard to read. For example:

troy@scuf ➜  cargo-tree git:(main) ✗ cargo tree --target aarch64-pc-windows-msvc                                                                         
a v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/a)
└── b v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/b)
    └── c v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/c)
        └── windows-sys v0.60.2
            └── windows-targets v0.53.2
                └── windows_aarch64_msvc v0.53.0
[build-dependencies]
└── b v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/b)
    └── c v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/c)
        └── libc v0.2.174

b v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/b) (*)

b v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/b) (*)

c v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/c) (*)

c v0.1.0 (/home/troy/github/scuffle/cargo-tree/crates/c) (*)
troy@scuf ➜  cargo-tree git:(main) ✗ 

If we have a dependency which appears in both the build or underneath a proc-macro it will be repeated and it becomes unclear which (*) the star is referring to (the host version or the target version).

This isnt a problem in the above case because it happens that cargo-tree rendered a and its sub dependencies before rendering b and c however if that isnt the case it sometimes can render b before a in which case we would render 2 copies of b and be unsure which target it is for.

Proposed Solution

I think cargo tree should also have a {cfg} format option which outputs the {cfg} flags for a given dependency to be included. This would simply the output and show something like the following when expanding.

troy@scuf ➜  cargo-tree git:(main) ✗ cargo tree --target aarch64-pc-windows-msvc --format ';{p};{cfg};'
;a v0.1.0;aarch64-pc-windows-msvc; (/home/troy/github/scuffle/cargo-tree/crates/a)
└── ;b v0.1.0;aarch64-pc-windows-msvc; (/home/troy/github/scuffle/cargo-tree/crates/b)
    └── ;c v0.1.0;aarch64-pc-windows-msvc; (/home/troy/github/scuffle/cargo-tree/crates/c)
        └── ;windows-sys;'cfg(windows)'; v0.60.2
            └── windows-targets v0.53.2
                └── windows_aarch64_msvc v0.53.0
[build-dependencies]
└── ;b v0.1.0;x86_64-unknown-linux-gnu; (/home/troy/github/scuffle/cargo-tree/crates/b)
    └── ;c v0.1.0;x86_64-unknown-linux-gnu; (/home/troy/github/scuffle/cargo-tree/crates/c)
        └── ;libc v0.2.174;'cfg(unix)';

;b v0.1.0;x86_64-unknown-linux-gnu; (/home/troy/github/scuffle/cargo-tree/crates/b) (*)

;b v0.1.0;aarch64-pc-windows-msvc; (/home/troy/github/scuffle/cargo-tree/crates/b) (*)

;c v0.1.0;x86_64-unknown-linux-gnu; (/home/troy/github/scuffle/cargo-tree/crates/c) (*)

;c v0.1.0;aarch64-pc-windows-msvc; (/home/troy/github/scuffle/cargo-tree/crates/c) (*)
troy@scuf ➜  cargo-tree git:(main) ✗ 

Notes

# Cargo.toml

[workspace]
members = [
    "crates/a",
    "crates/b",
    "crates/c",
]

resolver = "3"

# crates/a/Cargo.toml

[package]
name = "a"
version = "0.1.0"
edition = "2024"

[dependencies]
b = { path = "../b" }

[build-dependencies]
b = { path = "../b" }

# crates/b/Cargo.toml

[package]
name = "b"
version = "0.1.0"
edition = "2024"

[dependencies]
c = { path = "../c" }

# crates/c/Cargo.toml

[package]
name = "c"
version = "0.1.0"
edition = "2024"

[dependencies]

[target.'cfg(windows)'.dependencies]
windows-sys = "0.60"

[target.'cfg(unix)'.dependencies]
libc = "0.2"

[TroyKomodo]

Alternatively just knowing if a top level package is part of a build / proc macro vs a normal dep would be enough for my case.