Is Rust compiler version dependency update a breaking change?

Consider a crate X that compiled well with Rust 1.40, but now needs Rust 1.55 (version numbers are an arbirtrary example). Users who still use Rust 1.40 will not be able to use X. So is updating a Rust compiler dependency considered a breaking change (needing a major version increment) or is it assumed that people always use the latest stable release at least?