Missing documentation for registry-index in dependency specifying

[oxalica]

Problem

registry-index can be used to directly specify a registry index url for dependencies in Cargo.toml, without need to touch user's config .cargo/config.toml like registry. It's quite useful since everyone can clone and build your crate without the need to pollute their own user config.

Here's a internal test for this feature

cargo/tests/testsuite/package.rs

Line 971 in 9bc7a37

registry-index = "{}"

The field is fully functional on stable cargo 1.54.0 but NOT mentioned in cargo reference. Only registry is documented.
https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html
https://doc.rust-lang.org/cargo/reference/registries.html

Possible Solution(s)

Add the documentation for the registry-index feature.

Notes

Output of cargo version: cargo 1.54.0 (5ae8d74 2021-06-22)

I actually found this field in the crate cargo_toml, which defines structures of Cargo.toml. And I surprisingly found this undocumented but working-on-stable feature.

[ehuss]

FWIW, it was intentionally not documented. There is more context at #4880, but generally that field is an internal detail of Cargo that we don't intend for users to access. Changing that may require a bit more consideration than just changing the documentation.