`cargo add` takes `package.rust-version` into account when no version-req is specified

[epage]

• Implementation: Consider rust-version when selecting packages for cargo add #12078
• Unstable feature: -Zmsrv-policy

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Problem

When running cargo add foo, we create a version-req out of the latest version in the index. If this has a higher rust-version, then that can break for people.

Proposed Solution

Constrain the candidate versions in the index by their rust-version being lesser or equal to the current crate's rust-version.

The main question is what to do when there are no matches due to rust-version.

• Add anyways
• Error with newest version's rust-version

Notes

No response

[epage]

See also killercup/cargo-edit#587

[epage]

When rust-version gets added to the registry, we should inform Dependabot people on dependabot/dependabot-core#5423

[cassaundra]

@rustbot claim