cargo update with `resolver.incompatible-rust-versions = "fallback"` should respect supported editions even if no compatible version is found

[tyilo]

Problem

I have a project where I'm stuck on Rust 1.67. To update the dependencies I use cargo +stable update with the following .cargo/config.toml:

[resolver]
incompatible-rust-versions = "fallback"

This works very well, except recently when updating the dependencies of getrandom@0.3.4. This version of getrandom depends on wasip2 on some wasm target, which I don't use: https://docs.rs/crate/getrandom/0.3.4/source/Cargo.toml#64-66

All versions of wasip2 requires Rust 1.82+ and for the relevant wit-bindgen versions that wasip2 depends on it is the same.

This is not a problem for my project as I don't compile for that target and thus cargo build still works. However, cargo +stable update with the above config will update the version of wit-bindgen to a version using the 2024 edition. This doesn't affect cargo build, but this breaks cargo metadata.

---

I don't know if I should create another issue for the case where cargo build works, but cargo metadata does not. I had a hard time reproducing this on stable as there isn't any crates using edition 2027+, so I don't know if it has been fixed since 1.67.

Steps

Setup:

$ mkdir /tmp/foo
$ cd /tmp/foo
$ rustup override set 1.67.1
$ cargo init
$ cat > Cargo.toml
[package]
name = "foo"
version = "0.1.0"
edition = "2021"
rust-version = "1.67.1"

[dependencies]
^D
$ cargo add getrandom@=0.3.4

cargo run works, but cargo metadata doesn't:

$ cargo run
...
Hello, world!
$ cargo metadata
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
error: failed to download `wit-bindgen v0.51.0`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/tyilo/.cargo/registry/src/github.com-1ecc6299db9ec823/wit-bindgen-0.51.0/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2024` edition, and only supports `2015`, `2018`, and `2021` editions.

Manually downgrading wasip2 can get cargo metadata to work:

$ cargo update -p wasip2 --precise 1.0.1+wasi-0.2.4
    Updating crates.io index
    Updating wasip2 v1.0.2+wasi-0.2.9 -> v1.0.1+wasi-0.2.4
    Updating wit-bindgen v0.51.0 -> v0.46.0
$ cargo run
...
Hello, world!
$ cargo metadata
... (Now works!)

However, running cargo +stable update with incompatible-rust-versions = "fallback" results in cargo metadata breaking:

$ mkdir .cargo
$ cat > config.toml
[resolver]
incompatible-rust-versions = "fallback"
^D
$ cargo +stable update -v
    Updating crates.io index
     Locking 2 packages to latest Rust 1.67.1 compatible versions
   Unchanged getrandom v0.3.4 (available: v0.4.1, requires Rust 1.85)
   Unchanged r-efi v5.3.0 (requires Rust 1.68)
    Updating wasip2 v1.0.1+wasi-0.2.4 -> v1.0.2+wasi-0.2.9 (requires Rust 1.87.0)
    Updating wit-bindgen v0.46.0 -> v0.51.0 (requires Rust 1.87.0)
note: to see how you depend on a package, run `cargo tree --invert <dep>@<ver>`
$ cargo run
Hello, world!
$ cargo metadata
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
error: failed to download `wit-bindgen v0.51.0`

Caused by:
  unable to get packages from source

Caused by:
  failed to parse manifest at `/home/tyilo/.cargo/registry/src/github.com-1ecc6299db9ec823/wit-bindgen-0.51.0/Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  this version of Cargo is older than the `2024` edition, and only supports `2015`, `2018`, and `2021` editions.

Possible Solution(s)

I think that the update logic should be updated to be:

• Prefer versions that are compatible with rust-version in Cargo.toml
• New: If no such version can be found, prefer versions with an edition that existed in the Rust version specified by rust-version
• If no such version can be found, use the newest

Notes

No response

Version

cargo 1.93.0 (083ac5135 2025-12-15)

[epage]

I don't know if I should create another issue for the case where cargo build works, but cargo metadata does not. I had a hard time reproducing this on stable as there isn't any crates using edition 2027+, so I don't know if it has been fixed since 1.67.

This is expected. cargo build analyzes everything using the Index which we try to keep compatible between Rust versions (and have improved the error messages when it can't). Once it makes a decision about what is to be built, then the manifests get parsed for what will be built. On the otherhand, cargo metadata loads all dependencies and loads information only available in Cargo.toml.

Since the dependencies are for irrelevant platforms, you may be able to make use of --filter-platform <TRIPLE> to skip them.

We have new plumbing commands in the works which will hopefully not have this problem.

[epage]

In the RFC, we discussed inferring package.rust-version from package.edition but we did not evaluate adding package.edition to the index and using it as another signal.

If we did this, we'd need to gracefully be able to handle unknown editions.

I'm hesitant but can't fully articulate the reason and will likely need some time to contemplate this.

[weihanglo]

I'm hesitant but can't fully articulate the reason and will likely need some time to contemplate this.

Here is one: Having edition = "2024" doesn't always mean MSRV is 1.85. It could be used when testing unstable editions.

[epage]

While it doesn't mean "MSRV is 1.85" it is a strong signal that it won't work with your MSRV of 1.67. Granted, we could say the same thing about a lot of other new features in a manifest and that gets into an alternative/future possibility that was in a previous iteration of the RFC but not seeing it now which is a second MSRV field in the index that is a "minimum bound on the possible MSRV, inferred from features used".