This commit adds a new key to the `Cargo.toml` manifest, `workspace.exclude`.
This new key is a list of strings which is an array of directories that are
excluded from the workspace explicitly. This is intended for use cases such as
vendoring where path dependencies into a vendored directory don't want to pull
in the workspace dependencies.

There's a number of use cases mentioned on #3192 which I believe should all be
covered with this strategy. At a bare minimum it should suffice to `exclude`
every directory and then just explicitly whitelist crates through `members`
through inclusion, and that should give precise control over the structure of a
workspace.

Closes #3192

Previously Cargo had a bug where if a crate *without* a default feature was
overridden with one that did indeed have a default feature then the default may
not end up getting activated by accident. The fix was to avoid returning too
quickly hen activating dependencies until after we've inspected and learned
about replacements.

Closes #3812

…atklad

Fix overriding mixing with default features

Previously Cargo had a bug where if a crate *without* a default feature was
overridden with one that did indeed have a default feature then the default may
not end up getting activated by accident. The fix was to avoid returning too
quickly hen activating dependencies until after we've inspected and learned
about replacements.

Closes #3812

Previously this'd cause a stack overflow in Cargo later in the compilation
graph, but this is intended to get caught during resolution.

Closes #3831

Detect cyclic dependencies through [replace]

Previously this'd cause a stack overflow in Cargo later in the compilation
graph, but this is intended to get caught during resolution.

Closes #3831

Report the name of the test that failed (fix #2529)

Fix #2529

Encourage tools writers to explicitly pin metadata version

We do support versioning of metadata, but let's encourage tool's writers to actually use it.

They might not realize that this flag exists at all, or they can be too lazy (like myself :( ) to use it.

We can also make this flag mandatory, but I think that's a little bit to far.

Travis looks to have switched to rustup.rs

Attempt to fix CI

Travis looks to have switched to rustup.rs

Add a workspace.exclude key

This commit adds a new key to the `Cargo.toml` manifest, `workspace.exclude`.
This new key is a list of strings which is an array of directories that are
excluded from the workspace explicitly. This is intended for use cases such as
vendoring where path dependencies into a vendored directory don't want to pull
in the workspace dependencies.

There's a number of use cases mentioned on #3192 which I believe should all be
covered with this strategy. At a bare minimum it should suffice to `exclude`
every directory and then just explicitly whitelist crates through `members`
through inclusion, and that should give precise control over the structure of a
workspace.

Closes #3192