> The implicit update surface is somewhat limited by the fact that versions in Cargo.toml implicitly assume the `^` operator on versions that don't specify a different operator, so "1.2.3" means "1.2.x, where x >= 3". For reasons that have never been clear to me, people also seem to really like not putting the patch version in though and just putting stuff like "1.2", meaning that anything other than a major version bump will get pulled in.

Not quite: "1.2.3" = "^1.2.3" = ">=1.2.3, <2.0.0" in Cargo [0], and "1.2" = "^1.2.0" = ">=1.2.0, <2.0.0", so you get the "1.x.x" behavior either way. If you actually want the "1.2.x" behavior (e.g., I've sometimes used that behavior for gmp-mpfr-sys), you should write "~1.2.3" = ">=1.2.3, <1.3.0".

[0] https://doc.rust-lang.org/cargo/reference/specifying-depende...