Bad error message when a weak dep features's depedency is an unused optional dependency

[epage]

Problem

If nothing can activate an optional dependency, cargo acts as if the dependency doesn't exist which creates poor error messages.

Also, with how things are arranged, the unused optional dependency lint doesn't get reported which could at least reduce the burden on the error message.

Steps

Baseline Cargo.toml:

cargo-features = ["edition2024"]
[package]
name = "cargo-14010"
version = "0.1.0"
edition = "2024"

[dependencies]
serde = { version = "1.0.203", optional = true }

$ cargo +nightly check -Zcargo-lints
warning: unused optional dependency
 --> Cargo.toml:8:1
  |
8 | serde = { version = "1.0.203", optional = true }
  | -----
  |
  = note: `cargo::unused_optional_dependency` is set to `warn` by default
  = help: remove the dependency or activate it in a feature with `dep:serde`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

Add the following to Cargo.toml:

[features]
serde = ["serde?/derive"]

$ cargo +nightly check -Zcargo-lints
error: failed to parse manifest at `/home/epage/src/personal/dump/cargo-14010-1/Cargo.toml`

Caused by:
  feature `serde` includes `serde?/derive`, but `serde` is not a dependency

Possible Solution(s)

• Improve the error message
• Show the lint (which might be more difficult)

Notes

This came up in discussion in #14010

#14016 covers strong dep features as we likely will want a different solution.

Version

No response

[linyihai]

@rustbot claim
I have a unpolish improvement need to try.