Cargo fails to run when an *optional* dependency is missing

#!/usr/bin/env bash

set -e

cd /tmp
[ -d cargo-optional-deps ] && rm -rf cargo-optional-deps
cargo new cargo-optional-deps
cd cargo-optional-deps

cargo build
cat >> Cargo.toml <<EOF
optional_dep_does_not_exist = { version = "1.0", optional = true }
EOF
cargo build

Fails with

error: no matching package named optional_dep_does_not_exist found (required by cargo-optional-deps)

I did not expect this because the dependency is not required as per http://doc.crates.io/manifest.html#the-features-section:

optional dependencies, which enhance a package, but are not required; and

I'm running into this behavior while integrating Cargo into a private build system that wants control over dependencies. As such, I need to elide optional dependencies by default in this system's dependency list.

I'm opening this issue to get some input on:

1. Would ignoring missing optional dependencies for inactive features be a problem for Cargo?
2. I tried to look through existing issues - has something like this been discussed before?
3. An idea about resourcing/priority of the feature and how it fits into the roadmap for Cargo's integration with existing systems.

Run the shell reproduction from the issue to confirm Cargo's behavior when an inactive optional dependency cannot be found. Investigate Cargo's dependency resolution and feature handling, then determine whether missing optional dependencies should be ignored and what the resulting dependency list should contain.