More helpful error message for unsupported editions: display the required `rust-version`

[tarcieri]

Problem

When cargo encounters an unsupported edition, the error message it displays isn't actionable, where the action in this case is to upgrade cargo/rustc to 1.85 or newer:

Caused by:
  feature `edition2024` is required

  The package requires the Cargo feature called `edition2024`, but that feature is not stabilized in this version of Cargo (1.81.0 (2dbb1af80 2024-08-20)).

Proposed Solution

In the event an edition is unsupported, It would be more helpful if cargo printed the standard error for an out-of-date rust version by extracting package.rust-version when specified:

error: rustc 1.81.0 is not supported by the following package:
  mypackage@0.1.0 requires rustc 1.85

Notes

No response

[epage]

This would be a great improvement!

I'm unsure if we should completely mask the current error or extend it. This would be especially important for people passing --ignore-rust-versions who would then feel like its not being ignored.

On the implementation side, is it safe to assume that if we hit the feature gate and a package.rust-version is present, that the package is newer than the current toolchain? I've not dealt much with nightly-only packages. The challenge with checking if package.rust-version is newer than rustc -V is that we want to cache that call and we currently only cache that call in the workspace, so we need to know the workspace. However, the manifest parsing code is the same before we've discovered the workspace and after and trying to tunnel in a maybe-workspace feels weird. Maybe we could cache it in GlobalContext once we first discover it and then manifest parsing could use it if its present.

Or we could just add a

help: mypackage@0.1.0 requires rustc 1.85

and not state one way or the other whether that is the problem, just additional context.

[weihanglo]

How do old toolchains know a new Rust Edition requires a certain toolchain version?

[epage]

We wouldn't but we may know a package's MSRV which can work just as well.