Panic when crate not found

[theduke]

Just upgrade my nightly to (4b9b70c39 2018-04-09), and it seems that cargo-add does not handle non-existant crates gracefully anymore.

I mistyped a crate (serde_dervie) and got this error:

Command failed due to unhandled error: Invalid JSON (the crate may not exist)
Caused by: missing field `versions` at line 1 column 35
Backtrace: stack backtrace:
   0:     0x55b7225d188c - backtrace::backtrace::trace::heda8b332efd986ab
   1:     0x55b7225d0072 - backtrace::capture::Backtrace::new::h727f6efd73fa06c5
   2:     0x55b7225cfb2c - error_chain::make_backtrace::h4659a0010e2ff03a
   3:     0x55b722448398 - <core::result::Result<T, E> as cargo_edit::errors::ResultExt<T>>::chain_err::h49285f85800c0898
   4:     0x55b72245474b - cargo_edit::fetch::get_latest_dependency::h99ea9f82808078df
   5:     0x55b72243bfb7 - cargo_add::args::Args::parse_dependencies::h235fd639915b3939
   6:     0x55b7224421df - cargo_add::main::h8de47bb74222f147
   7:     0x55b72243ab42 - std::rt::lang_start::{{closure}}::h97d95e748c99d208
   8:     0x55b722688137 - std::rt::lang_start_internal::{{closure}}::h81dab1c80d2e9e5d
                        at libstd/rt.rs:59
                         - std::panicking::try::do_call::h7973a503106e370a
                        at libstd/panicking.rs:305
   9:     0x55b722699afe - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:102
  10:     0x55b72267dab5 - std::panicking::try::h21b7d973eaa3febb
                        at libstd/panicking.rs:284
                         - std::panic::catch_unwind::h07d8c36256237e04
                        at libstd/panic.rs:361
                         - std::rt::lang_start_internal::hde534450bd97bd5e
                        at libstd/rt.rs:58
  11:     0x55b722442ef3 - main
  12:     0x7fddf0444f49 - __libc_start_main
  13:     0x55b722438db9 - _start
  14:                0x0 - <unknown>

[ordian]

I wasn't able to reproduce the panic neither with 0.3.0-beta.1, nor with master branch ( 88b3e49). What version of cargo-edit are you using?

❯ rustc +nightly --version                                               
rustc 1.27.0-nightly (4b9b70c39 2018-04-09)
❯ cargo +nightly add serde_dervie
Command failed due to unhandled error: Invalid JSON (the crate may not exist)

Caused by: missing field `versions` at line 1 column 35

As for the unhelpful message, we should fix it (@bjgill I thought #200 was supposed to fix this?).

[bjgill]

Thanks for the report. It looks like what you're seeing is the expected output from attempting to add a crate that doesn't actually exist whilst having backtrace enabled.

error-chain claims to capture a backtrace if the RUST_BACKTRACE environment variable is set - which we then print out if available. @theduke - do you have RUST_BACKTRACE set?

If this is indeed the case, whilst expected, this is nonetheless a self-evidently poor user experience. I think there are probably a couple of things we could/should do:

1. Attempting to add a non-existent crate is probably not uncommon. We should ensure that we detect it reliably and then offer a better error message.
2. People who've set the RUST_BACKTRACE environment variable might not expect the effect this has on our error reporting. We could hide it better (possibly with another environment variable?), or remove the feature altogether?

(as I remember, #200 only prevented us from hitting that error in the specific case where you were trying to upgrade a path dependency. I don't think it did anything to improve handling of the general error case of attempting to add a non-existent crate)

[theduke]

@bjgill yes I indeed have RUST_BACKTRACE, so that explains it, although still the behaviour seems to have changed because I remember gettign nicer error messages.

I think it's a reasonable to assume that a lot of developers using cargo-edit have it set, so your PR is appreciated.

[epage]

I think this can now be closed.