Error message when dependent library version conflicts

[driftluo]

version:

$ cargo -V
cargo 1.39.0 (1c6ec66d5 2019-09-30)

dependence

[dependencies]
secp256k1="0.16.0"
backtrace="0.3.40"
git2 = "0.10.2"

error message:

error: failed to select a version for `cc`.
    ... required by package `secp256k1 v0.16.0`
    ... which is depended on by `test-lib v0.1.0 (/home/luoc/rust-work/ckb-work/test-lib)`
versions that meet the requirements `>= 1.0.28, < 1.0.42` are: 1.0.41, 1.0.40, 1.0.39, 1.0.38, 1.0.37, 1.0.36, 1.0.35, 1.0.34, 1.0.33, 1.0.32, 1.0.31, 1.0.30, 1.0.29, 1.0.28

all possible versions conflict with previously selected packages.

  previously selected package `cc v1.0.42`
    ... which is depended on by `backtrace-sys v0.1.32`
    ... which is depended on by `backtrace v0.3.40`
    ... which is depended on by `test-lib v0.1.0 (/home/luoc/rust-work/ckb-work/test-lib)`

failed to select a version for `cc` which could resolve this conflict

However, the real error should be that secp256k1 v0.16.0 conflicts with git-sys v0.9.2 which required cc v1.0.42, backtrace does not specify cc version requirements

[alexcrichton]

cc @Eh2406

an interesting error message conundrum! Currently we only render one path from the root crate to the crate in question (test-lib to cc). This happens to include backtrace-sys. It's true though that here the more relevant path is through libgit2-sys which has a higher requirement for cc.

[Eh2406]

We could list all the paths, but I think it will get overwhelming. ( open to suggestions. ) But we currently do not have any logic to decide which path to show. We can straightforwardly add some heuristics. Perhaps we can like parent that has the fewest alternatives?