Needless rebuilds when run with or without --workspace

[ijackson]

Problem

cargo rebuilds things if I switch between cargo build and cargo build --workspace.

Steps

1. git clone https://salsa.debian.org/iwj/otter
2. cd otter
3. git checkout c9b5a88d7a33a6e1eac22736be0a38c6815e1706
4. cargo build --workspace
5. cargo build

To repeat, restarting from after step 4 or 5:

3. touch src/updates.rs
4. as above
5. as above

Expected behaviour

The whole workspace including the toplevel package otter was built in step 4, so step 5 should not build anything and should be very quick.

Actual behaviour

cargo rebuilds otter (including library and binaries) in step 5 (about six seconds on my laptop in a repeated test)

Notes

Output of cargo version:

cargo 1.51.0-nightly (783bc43c6 2021-01-20)

[ijackson]

@rustbot modify labels +A-rebuild-detection +A-workspaces

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[ijackson]

I have been attempting to work around this and the behaviours I'm seeing make me think it may be related to #4463.

[ijackson]

A workaround in my environment is to always pass --workspace (plus, usually, suitable -p options) to cargo.

[ehuss]

Yea, this is expected behavior in that the features chosen depend on which packages are being built. Closing as a duplicate of #4463.

There are other workarounds such as a common package that forces the same features (see rustc-workspace-hack as an example).