Updating Cargo.lock can result in dependency cycle

[mati865]

Bumping few packages can result in dependency cycle.

Single command reproducer:

$ cargo update --aggressive -p cargo            
    Updating crates.io index
    Updating git repository `https://github.com/rust-lang/cargo`
error: cyclic package dependency: package `compiler_builtins v0.1.19` depends on itself. Cycle:
package `compiler_builtins v0.1.19`
    ... which is depended on by `alloc v0.0.0 (/home/mateusz/Projects/rust/rust/src/liballoc)`
    ... which is depended on by `panic_unwind v0.0.0 (/home/mateusz/Projects/rust/rust/src/libpanic_unwind)`
    ... which is depended on by `std v0.0.0 (/home/mateusz/Projects/rust/rust/src/libstd)`
    ... which is depended on by `proc_macro v0.0.0 (/home/mateusz/Projects/rust/rust/src/libproc_macro)`
    ... which is depended on by `test v0.0.0 (/home/mateusz/Projects/rust/rust/src/libtest)`

cc @alexcrichton

[alexcrichton]

Looks like Cargo is correct here but it's a bad error message. I'll work on the error message independently, but the real error is:

error: cyclic package dependency: package `compiler_builtins v0.1.19` depends on itself. Cycle:
package `compiler_builtins v0.1.19`
    ... which is depended on by `cfg-if v0.1.10`
    ... which is depended on by `getrandom v0.1.12`
    ... which is depended on by `jobserver v0.1.17`
    ... which is depended on by `cc v1.0.45`
    ... which is depended on by `compiler_builtins v0.1.19`
    ... which is depended on by `alloc v0.0.0 (/home/alex/code/rust2/src/liballoc)`

This happens because cc now has the parallel feature enabled when updating libgit2-sys. I'm gonna close this since I'm about to fix the error message in Cargo and otherwise I believe this is a legitimate error.

[alexcrichton]

Note that this is an instance of rust-lang/cargo#4866 where the host cfg-if shouldn't have rustc-dep-of-std enabled, but it does.

[mati865]

Oh it's nasty one. Thanks for the explanation, it's clear now.