Rustdoc flag --enable-index-page no longer works consistently

[jplatte]

For two project I maintain, the docs generated by CI no longer includes an index file despite the appropriate nightly flags being used. Here is a reproduction:

• Clone https://github.com/ruma/ruma/tree/rustdoc-bug
• With the environment variable RUSTDOCFLAGS set to '--enable-index-page -Zunstable-options', run cargo +nightly doc --no-deps
• Observe no index.html file in target/doc

Meta

• Reproducible with the latest rustdoc as of right now (1.63.0-nightly (5435ed691 2022-06-07))
• Reproducible with rustdoc 1.61.0-nightly (f0c4da499 2022-03-01) (rustup nightly-2022-03-02)
• Not reproducible with rustdoc 1.61.0-nightly (4ce374923 2022-02-28) (rustup nightly-2022-03-01).

@rustbot labels: +T-rustdoc

[jplatte]

To be clear, the title says "consistently" at the end because I have tried to reproduce this in a Cargo project with just one package as well as a smaller Cargo workspace, but couldn't.

[jplatte]

This is a cargo bug, closing in favor of rust-lang/cargo#10744.