msrv-policy regression in 1.79.0-nightly (nightly-2024-04-21)

[charles-r-earp]

Problem

Cargo no longer respects "package.rust-version" when generating a lockfile with -Zmsrv-policy.

1.78.0-nightly works correctly. The issue appears with nightly-2024-04-21 (80d5b60 2024-04-19).

Steps

Install 1.78.0-nightly, 1.79.0-nightly, 1.70.0:

rustup install nightly-2024-03-15 nightly-2024-04-21 1.70.0

Create a new package with manifest:

[package]
name = "msrv-issue"
version = "0.1.0"
edition = "2021"
rust-version = "1.70.0"
publish = false

[dependencies]
clap = "4"

Succeeds with 1.78.0-nightly:

cargo +nightly-2024-03-15 -Zmsrv-policy generate-lockfile # clap v4.4.18
cargo +1.70.0 check

Fails with 1.79.0-nightly:

cargo +nightly-2024-04-21 -Zmsrv-policy generate-lockfile # clap v4.5.4 (latest)
cargo +1.70.0 check

Error:

error: package `clap_builder v4.5.2` cannot be built because it requires rustc 1.74 or newer, while the currently active rustc version is 1.70.0
Either upgrade to rustc 1.74 or newer, or use
cargo update -p clap_builder@4.5.2 --precise ver
where `ver` is the latest version of `clap_builder` supporting rustc 1.70.0

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.79.0-nightly (80d5b607d 2024-04-19)
release: 1.79.0-nightly
commit-hash: 80d5b607dde6ef97dfff4e23923822c01d2bb036
commit-date: 2024-04-19
host: x86_64-unknown-linux-gnu
libgit2: 1.7.2 (sys:0.18.3 vendored)
libcurl: 8.6.0-DEV (sys:0.4.72+curl-8.6.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Ubuntu 22.4.0 (jammy) [64-bit]

[weihanglo]

Thanks for the report. This is expected behavior after the change in #13769, which prepares for the upcoming change in Edition 2024.

You should be able to use MSRV-aware resolver in the latest nightly with the following configuration (note that the config name and value are just placeholders for now):

$ cargo +nightly generate-lockfile -Zmsrv-policy --config "resolver.something-like-precedence='something-like-rust-version'"
    Updating crates.io index
     Locking 22 packages to latest Rust 1.70.0 compatible versions
      Adding clap v4.4.18 (latest: v4.5.4)
      Adding clap_builder v4.4.18 (latest: v4.5.2)
      Adding clap_lex v0.6.0 (latest: v0.7.0)
      Adding strsim v0.10.0 (latest: v0.11.1)

See the doc for more details.

[compenguy]

To be clear, this isn't even just about respecting rust-version in the Cargo.toml. If I set my rustup toolchain to 1.72 and use this Cargo.toml:

[package]
name = "msrv-resolve-cargo-platform"
version = "0.1.0"

[dependencies]
cargo_metadata = "0.14.2"

I get the following behavior:

$ cargo -V
cargo 1.72.1 (103a7ff2e 2023-08-15)
$ cargo update
    Updating crates.io index
$ cargo check
error: package `cargo-platform v0.1.8` cannot be built because it requires rustc 1.73 or newer, while the currently active rustc version is 1.72.1
Either upgrade to rustc 1.73 or newer, or use
cargo update -p cargo-platform@0.1.8 --precise ver
where `ver` is the latest version of `cargo-platform` supporting rustc 1.72.1

Basic development workflows are completely broken for people developing with non-latest compilers if the wrong component makes it into their dependency tree.

Yes, the msrv-aware resolver corrects this, but this requires mixed stable/nightly toolchains for development, and careful development workflows with complicated command arguments to ensure correct results.

[compenguy]

Apologies - the failure I'm reporting looks similar, but is not new - this has apparently always been broken.

[weihanglo]

@compenguy, Cargo cannot travel back and add msrv-aware resolver support for 1.72.1. You need to set package.rust-version = "1.72" and the nightly cargo generates a workable lockfile for you.

[weihanglo]

Going to close this. The behavior of -Zmsrv-policy is unstable and subject to change until stabilization (targeting at Edition 2024). Let us know if there is any other bug in the implementation. Thank you all.