Lint for workspaces using different editions among workspace members

[BatmanAoD]

What it does

In a workspace, determine if all crates have the same edition; if not, error.

Advantage

It's easy to forget to ensure that a new crate is at the same edition as the rest of the crates in a workspace, and it may be confusing if particular Rust features or idioms are not consistent across crates.

Drawbacks

There are plausible reasons to want different editions across a workspace. The lint should therefore be off by default.

Example

edition = "2021" in one workspace crate, and edition = "2024" in another

[BatmanAoD]

Migrated from rust-lang/rust-clippy#15383

[epage]

One way to avoid this generally is to use workspace inheritance. cargo new will even auto-inherit fields.

The question then is what inheritance lints might be appropriate.

We do have a (currently uncontrolled) warning when an edition is not set. This might encourage people to set the edition directly. Maybe we should check if the workspace edition has been set and also suggest that?

[BatmanAoD]

Ah, I didn't actually know that edition was one of the inheritable package keys. Thanks!

I definitely think it's appropriate to suggest workspace = true when the workspace edition is already set; and even if the workspace edition isn't set, it may be appropriate to suggest doing so.