`RUSTDOCFLAGS=--show-coverage cargo doc` deletes any existing docs

[jyn514]

I tried this code: cargo doc && RUSTDOCFLAGS='-Z unstable-options '--show-coverage cargo doc

I expected to see this happen: There is documentation in target/doc/crate_name

Instead, this happened: The docs get deleted.

Meta

searched nightlies: from nightly-2021-04-28 to nightly-2021-04-29
regressed nightly: nightly-2021-04-29
searched commits: from 727d101 to ca075d2
regressed commit: 27bd3f5

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --script ./docs-exist.sh --preserve --start 2021-04-28 --end 2021-04-29 

[jyn514]

@ehuss looks like this is related to the recent rustdoc changes (rust-lang/cargo#9419 or rust-lang/cargo#9404).

[jyn514]

For context, this causes problems for docs.rs because we always run coverage after the docs are generated. We can work around it for now by running coverage before instead, but it would be nice to fix this. In the meantime I've pinned docs.rs to an older toolchain.

[jyn514]

Is there a reason cargo deletes things between each invocation? It should only need to delete things if the toolchain or source files change.

[jyn514]

Closing this in favor of the cargo issue tracker: rust-lang/cargo#9447