Regression in nightly handling features + debug assertions

[richardwhiuk]

Problem

1.43.0-nightly (4ad624882 2020-03-03) has regressed handling of debug assertions with features.

Steps

1. Install 1.43.0-nightly (4ad624882 2020-03-03)

2. Build a crate with the following Cargo.toml

[package]
name = "test-bin"
version = "0.1.0"
edition = "2018"

[dependencies]
rocket_contrib = { version = "0.4.2", features = [ "templates", "tera_templates" ] }

3. Build fails with:

   Compiling rocket_contrib v0.4.3
error[E0463]: can't find crate for `notify`
  --> /opt/rust/registry/src/crates-io-27bb2a0041c1489d/rocket_contrib-0.4.3/src/templates/fairing.rs:35:5
   |
35 |     extern crate notify;
   |     ^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: could not compile `rocket_contrib`.

To learn more, run the command again with --verbose.

Cargo has not downloaded the required crate, so I assume a Cargo bug, instead of a rustc bug.

4. Build works with cargo 1.43.0-nightly (e57bd0299 2020-02-21)

Possible Solution(s)

Dependency is pulled in via a debug assertion:

https://github.com/SergioBenitez/Rocket/blob/v0.4.2/contrib/lib/Cargo.toml#L79-L80

notify is not downloaded by Cargo during a clean build.

Cargo.lock does reference notify:

[[package]]
name = "rocket_contrib"
version = "0.4.3"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "66926894ef94396204c53885f54a1b0e94a88ce6c9478c2fd403261fc0efb523"
dependencies = [
 "glob 0.3.0",
 "log 0.4.8",
 "notify",
 "rocket",
 "serde",
 "serde_json",
 "tera",
]

This is a recent regression - rust version 1.43.0-nightly (d3c79346a 2020-02-29) also works.

Suspect #7962 is at fault,.

[alexcrichton]

In general cfg(debug_assertions) can't be used to gate inclusion of crates in dependencies, and a warning was added for that in #7660. This seems likely due to #7820 (cc @ehuss) where we reorganized where this information was calculated.