Warn on missing `edition` field in Cargo.toml

[nyurik]

What it does

In 2026, it should be a mistake not to explicitly set language edition

Advantage

• avoid accidentally removing edition field from the Cargo.toml
• prevent ambiguity

Drawbacks

Will warn on ancient code

Example

[package]
# no edition field

Comparison with existing lints

See also #6431 which discussed it 6 years ago as part of a bigger "cargo.toml" lint set up

Additional Context

No response