const Path initialisation

[conradludgate]

Path::new cannot be made const currently, due to its usage of AsRef<OsStr>.

Since Rust does not yet have any concept of const traits, this PR adds a couple new methods in order to construct const Paths.

The expected usage is

const PATH: &Path = Path::from_os_str(OsStr::from_str("/my/path"));

Alternatives

In some future, we may get const traits, such that we could do

impl const From<&str> for &Path {
    // ...
}

const PATH: &Path = Path::from("/my/path");

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @joshtriplett (or someone else) soon.

Please see the contribution instructions for more information.

[joshtriplett]

I think it may sometimes make sense to have methods like this regardless, both for self-documenting code and to usefully constrain inference.

To evaluate whether these methods carry enough weight to add, it would help to know roughly when enough of const traits will become available to make these methods unnecessary for that purpose.

cc @oli-obk @rust-lang/wg-const-eval; do you have a rough idea of the timeline upon which we can support const traits well enough to not need these methods? That would help determine whether we want to add workarounds like this.

[fee1-dead]

Perhaps a sane way is to stabilize a subset of const_trait_impl that only allows calling functions from concrete const implementations of a trait. We could save defining a impl const, calling/defining a function that uses const trait bounds for stabilization later and enable uses like this from the standard library.

[bors]

☔ The latest upstream changes (presumably #91673) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

@conradludgate if you can resolve the conflicts we can push this forward

[joshtriplett]

This looks reasonable to me.

It seems like the common case will be making a const path from a str, rather than an OsStr. I think it would make sense to add a method for doing that directly. Such a method probably shouldn't use the name from_str since that would conflict with FromStr::from_str, but I think a method should exist for that to simplify the common case.

cc @rust-lang/libs-api: name suggestions?

Also, could you please file a tracking issue and use it in the stability attributes?

[the8472]

osstr.as_ref() -> Path already exists. Imo it's better to wait until that can be made const before introducing additional API surface for the sole purpose of supporting constification.

[pitaj]

Ping from triage. @conradludgate you still need to reference the tracking issue in your stability attributes.

@joshtriplett It appears there are mixed reactions, are you still interested in these additions?

[JohnCSimon]

@conradludgate

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive

[ferdonline]

I find this really useful. Without it we sometimes have to resort to lazy_static. If the community is interested I can help pushing this forward.