Can't use unstable feature in ~/.cargo/config without enabling it in Cargo.toml

[Nemo157]

Problem
Trying to use an unstable feature in my global config requires enabling that feature in every Cargo.toml I interact with.

Steps

# ~/.cargo/config
cargo-features = ["named-profiles"]
[profile.release-lto]
lto = true

> cargo build --profile release-lto -Z unstable-options
error: config profile `release-lto` is not valid (defined in `/home/nemo157/.cargo/config`)

Caused by:
  feature `named-profiles` is required

  consider adding `cargo-features = ["named-profiles"]` to the manifest

Possible Solution(s)
Allow the config to enable unstable features too (preferably only applying to features used within it, not features used in Cargo.toml).

Notes

Output of cargo version: cargo 1.55.0-nightly (3ebb5f15a 2021-07-02) (and tested on cargo 1.57.0-nightly (9a28ac83c 2021-09-18) which fails in the same way with a slightly different message).

[weihanglo]

IMHO this is convenient for developers to enable features globally. Since we are already baking patch-in-config, hope that the config-merge rules and other implementations wouldn't be too hard.

BTW, labeling this as C-feature-request may be better 😆

[weihanglo]

Close in favor of #11552, as features mentioned there are still unstable, which makes the intent a bit more clear.