`cargo install` when only pre-releases are available

[eopb]

Problem

It's possible for crates to have only published pre-release versions to crates.io. This was the case for early versions of my crate, cargo-override.

My reason for doing this was due to the subtle difference between what I felt each version communicated.

• 0.0.x series, communicates that the API is unstable and may change in every release.
• 0.1.0-alpha.x series. communicates that the API is unstable, may change every release, and the behaviour may be broken or buggy.

Other than this difference I perceive in what these versions communicate, their SemVer obligations are effectively the same.

This test, eopb/cargo/pull/1/files, displays the current behaviour. This behaviour is confusing for the following reasons:

• The error returned does not recommend cargo install crate@version but instead may suggest to users that no crate with that name is in the registry
• The behaviour does not match what crates.io displays. crates.io is happy to display pre-release versions by default if they are the only versions available

Proposed Solution

When only pre-release versions are available, select the pre-release with the greatest version, as specified by ordering in the semver crate

Notes

No response

[eopb]

Similarly cargo install will avoid pre-releases unless explicitly asked to install one.

https://doc.rust-lang.org/cargo/reference/resolver.html#pre-releases

This documentation is relevant and will likely need to be updated if this is changed.

[eopb]

@rustbot label Command-install

[samueltardieu]

It should at least warn about the existence of a pre-release version, and tell how to use '@' to select it. However, I don't think it should install a pre-release version by default without the user explicitly asking for it.

[Eh2406]

The fact that the error message fitters out pre-release versions is a long known bug, but I can't find the issues now.

[epage]

In general, Cargo's story around pre-releases is not in a good place and we're working to improve that a step at a time with precise-prerelease being a first step.

cargo add is a similar case and already has some pre-release support. In that case, if no version is specified, we do our own lookup scheme. In this, we prioritize the regular releases over pre-releases and then prefer maximal versions

cargo/src/cargo/ops/cargo_add/mod.rs

Lines 678 to 683 in 7984efa

	possibilities.sort_by_key(|s| {
	// Fallback to a pre-release if no official release is available by sorting them as
	// less.
	let stable = s.version().pre.is_empty();
	(stable, s.version().clone())
	});