Fix CI: half requiring rust 1.81

[Vrixyz]

Criterion brings half crate which Minimum Supported Rust Version is 1.81, so CI is failing, let's discuss how to fix:

• I bumped rust version and kept a check on MSRV.
• I updated the expected error message for rust 1.81 ; Update errors for a more recent rust version #1504 may be relevant

I removed the warning to errors, let's see how it goes.

[Ralith]

We probably don't need to cover whatever targets pull in criterion in the MSRV test? 1.81 seems fine to me though.

[Vrixyz]

We probably don't need to cover whatever targets pull in criterion in the MSRV test? 1.81 seems fine to me though.

Not sure I understand, are you suggesting to keep a test on MSRV ?

• it's challenging because criterion is pulled from our dev-dependencies, which we can't make optional (Allow optional dev-dependencies rust-lang/cargo#1596), even though it's only used for benches,

[Ralith]

Not sure I understand, are you suggesting to keep a test on MSRV ?

No, I was referring to the CI job that checks MSRV.

it's challenging because criterion is pulled from our dev-dependencies, which we can't make optional (

We don't need to make it optional, we just need to not compile any targets that involve dev-dependencies for purposes of validating the MSRV, because they are not relevant to it.

[Ralith]

@Vrixyz do you intend to fix the MSRV test to only compile library targets?

[Vrixyz]

@Vrixyz do you intend to fix the MSRV test to only compile library targets?

I can make fixes, but I'm not 100% sure what you mean:

I believe Cargo check doesn't test for dev-dependencies?

Do you mean passing cargo check with specific packages ? Or do you mean changing the features set ?

Thanks for your patience (I have limited computer access for ~1 week)

[Ralith]

The purpose of the MSRV test is to ensure that downstream users on older rustc versions can still compile nalgebra. Criterion, being a dev dependency, is not relevant. We should configure the MSRV validation CI job to not attempt to build Criterion (or any other dev-dependency), and instead to only build library targets.

[Vrixyz]

🤔 That's kind of what this PR is doing then:

• There was not a dedicated "MSRV test", it was running cargo test, which inherently requires dev-dependencies.
• So I created a new cargo check, dedicated to MSRV test.
  • (cargo check doesn't bring in dev-dependencies if we do not provide --tests or --benches)

I could add a --lib to that new msrv check, but as we don't have a binary, this wouldn't change anything.

I changed the existing cargo test to use the lowest supported rust version for tests, but that can be changed to latest; I think it's interesting to know which MSRV we have for "dev-dependencies".

Sorry if I'm misunderstanding something, thx for the patience :)

[Ralith]

I think it's interesting to know which MSRV we have for "dev-dependencies".

I don't think this is useful, but since CI does check exact error messages it seems like we need to pin this version to something anyway.