Improve the error message when running `cargo add cubing @v0.5.0`

[lgarron]

This has now been rescoped to providing a good error message if a v prefix is present, see #12331 (comment)

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Problem

I keep doing something the following:

cargo add cubing@v0.5.0

This causes the following error:

error: invalid version requirement `v0.5.0`

Caused by:
  unexpected character 'v' while parsing major version number

I do this because I have a strong habit of prefixing my version numbers with v where possible, as vX.Y.Z tends to be a more unmistakable way to specify a version. And although it is not universal, it is also very common to prefix project release tags with v — to the point that the canonical cargo test for for a tag dependency even uses a value like this: https://github.com/rust-lang/cargo/blob/5b377cece0e0dd0af686cf53ce4637d5d85c2a10/tests/testsuite/cargo_add/git_tag/mod.rs#L22C1-L22C1
(I know that doesn't have a semantic meaning in this case, but it illustrates how widespread it is to indicate a version using a prefix of v.)

Further, other tools like npm allow a prefix of v (e.g. npm install cubing@v0.38.1), which is an inconsistency that causes a small stumbling block when working with multiple web technologies.

Proposed Solution

In order of preference:

• Accept an extra v before a semantic version number.
• Recognize the extra v and provide a helpful error message that includes something like: If you want to install a specific version, please run: cargo add cubing@0.5.0

Notes

No response

[epage]

From the angle of prior art (see #10472)

• npm and go support a leading v
• poetry, yarn, pnpm, bundle, and pub do not

[epage]

If we supported v here, we should also evaluate whether to support it in cargo install and all commands that accept pkgid.

[epage]

Personally, I think a v being supported for cargo add would be incorrect. cargo add accepts a version requirement and not a version and we would not want to confuse that further by having a "sigil" that implies its a version. For the other commands, there would be more room for consideration with the drawback that it would make the commands more consistent in accepted syntax while adding support for @ in all commands was a drive for consistency (#10582).

[weihanglo]

cargo install --version <ver-req> also falls into the same category where cargo add is. That mean if cargo is going to support v, there will be a inconsistentcy between cargo add/cargo install and other commands. I lean towards not support such thing for consistency.

[lgarron]

It sounds like there are good reasons not to support this, as it could produce other inconsistencies.

In that case, I would advocate for the "friendly error message" in the vein that Rust is known for:

• Recognize the extra v and provide a helpful error message that includes something like: If you want to install a specific version, please run: cargo add cubing@0.5.0

[lgarron]

Personally, I think a v being supported for cargo add would be incorrect. cargo add accepts a version requirement and not a version and we would not want to confuse that further by having a "sigil" that implies its a version.

Just to note: npm also supports e.g. npm add cubing@latest. It would be similarly nice to apply the same solution as a v prefix (either support it or show a helpful message suggesting how to force an install of the latest version — particularly, a command that will pull a version even if it was just published moments ago).

[epage]

@latest is being discussed at #10741

[weihanglo]

• Recognize the extra v and provide a helpful error message that includes something like: If you want to install a specific version, please run: cargo add cubing@0.5.0

The tricky part of this error message is that the suggestion “please run: cargo add cubing@0.5.0” might not be the correct one. People run with toolchain override cargo +nightly add … could also get the same error.

If we want this, suggesting cubing@0.5.0 instead of the full command line args is preferable I guess?