`cargo add --optional` overwrites previously declared feature

[ginnyTheCat]

Problem

When having a feature called x and running cargo add x --optional the x feature gets overwritten with x = ["dep:x"]

Steps

1. Have the following Cargo.toml:

[package]
name = "tmp"
version = "0.1.0"
edition = "2024"

[dependencies]
cookie_store = { version = "0.22", optional = true }

[features]
cookie = ["dep:cookie_store"]

2. Run cargo add cookie --optional
3. Now the Cargo.toml looks like this, with dep:cookie_store nowhere to be seen.

[package]
name = "tmp"
version = "0.1.0"
edition = "2024"

[dependencies]
cookie = { version = "0.18.1", optional = true }
cookie_store = { version = "0.22", optional = true }

[features]
cookie = ["dep:cookie"]

Possible Solution(s)

Theres three solutions I could think of:

1. The safe option is to just not generate the feature in this case, so just leave cookie = ["dep:cookie_store"] alone.
2. The other option would be to add it to the feature cookie = ["dep:cookie_store", "dep:cookie"]. In this case that's what I would want but I think it's quite dangerous as users would not expect it and in bigger Cargo.tomls seeing that change would be hard.
3. Add it under another name _cookie = ["dep:cookie"]? But idk what to do when that name is taken already as well.

Personally I like option 1 the most.

Notes

No response

Version

cargo 1.92.0-nightly (81c3f77a4 2025-10-10)
release: 1.92.0-nightly
commit-hash: 81c3f77a467359c8be6bc747dc93ec66a6e4ce11
commit-date: 2025-10-10
host: x86_64-unknown-linux-gnu
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.15.0-DEV (sys:0.4.83+curl-8.15.0 vendored ssl:OpenSSL/3.5.4)
ssl: OpenSSL 3.5.4 30 Sep 2025
os: Fedora 42.0.0 (Adams) [64-bit]

[epage]

Part of the intent in creating cookie = ["dep:cookie"] is to avoid leaving behind an implicit feature.

The safe option is to just not generate the feature in this case, so just leave cookie = ["dep:cookie_store"] alone.

In this case, the implicit feature is being overwritten, so maybe that is fine.

In this case that's what I would want but I think it's quite dangerous as users would not expect it and in bigger Cargo.tomls seeing that change would be hard.

This i what I would lean towards. What do you mean it is hard? Hard to notice?

[ginnyTheCat]

Part of the intent in creating cookie = ["dep:cookie"] is to avoid leaving behind an implicit feature.

I do like that cargo does this, just in this case I think it's not great.

Hard to notice?

Yes, especially since it's not something you would look for as it's quite unexpected for Cargo to just overwrite your changes like that. I could even imagine it seeming "intentional" enough to slip through a not so thorough code review.

I'm not as convinced that "user intent" is 100% clear in this case. If you think it's noticeable enough I trust your judgement more than mine hahahhaha.