--locked doesn't prevent removing crates from workspace

Using --locked doesn't prevent removing a crate from a workspace. I would expect cargo to fail when invoked with --locked on a removed crate, but instead it succeeds and instead doesn't modify Cargo.lock.

To reproduce

In an empty directory, create Cargo.toml with the following contents

[workspace]
members = ["foo", "bar"]

run

cargo new foo
cargo new bar
cargo build

Keep track of the current state of Cargo.lock (e.g. by checking everything into git.)

Now run

rm -rf bar

and edit Cargo.toml to

[workspace]
members = ["foo"]

run cargo build --locked

Behavior

This command will successfully build foo and will not modify Cargo.lock, i.e. Cargo.lock will still contain a reference to the bar crate. (Running cargo build leads to Cargo.lock being updated to remove references to bar, as expected.)

Expected

The expected behavior would be for the last command to fail with an error message saying Cargo.lock needs to be updated.

Version

cargo 1.44.1 (88ba85757 2020-06-11)

(on OS X)

Reproduce the workspace setup with foo and bar, remove bar, edit Cargo.toml, and run cargo build --locked. Compare the behavior with an unlocked build and inspect how Cargo handles the workspace and Cargo.lock; done means the locked build fails when the lockfile still references the removed crate.