> I've not had much pain with semver violations in the Rust ecosystem
The Rust ecosystem is pretty good about crate authors not knowingly violating semver. This tool is mostly to keep crate authors from unknowingly violating semver. It's possible that the reason that you haven't noticed is because even if a crate accidentally violates semver by making an unanticipated breaking change to a given API, either you're not using that particular API (who uses 100% of any given library?) or else that the breakage doesn't affect your specific usage (e.g. maybe some type no longer implements a trait, but you weren't using that trait to begin with).
Exactly right. Most accidental semver violations aren't noticed by anyone, let alone most people. But every so often, an accidental semver violation causes ecosystem-wide breakage and lots of pain for both maintainers and users.
The goal of this tool is to aid in preventing regrettable accidents like that, by informing maintainers about semver issues before they are published.
The Rust ecosystem is pretty good about crate authors not knowingly violating semver. This tool is mostly to keep crate authors from unknowingly violating semver. It's possible that the reason that you haven't noticed is because even if a crate accidentally violates semver by making an unanticipated breaking change to a given API, either you're not using that particular API (who uses 100% of any given library?) or else that the breakage doesn't affect your specific usage (e.g. maybe some type no longer implements a trait, but you weren't using that trait to begin with).