That's not how it works for most language standards, though. Most language standards are prescriptive, while Rust is descriptive.
Usually the standard comes first, compiler vendors implement it, and between releases of the spec the language is fixed. Using Ada as an example, there was Ada 95 and Ada 2003, but between 95 and 2003 there was only Ada 95. There was no in-progress version, the compiler vendors weren't making changes to the language, and an Ada95 compiler today compiles the same language as an Ada95 compiler 30 years ago.
Looking at the changelog for the Rust spec (https://rust-lang.github.io/fls/changelog.html), it's just the changelog of the language as each compiler verion is released, and there doesn't seem to be any intention of supporting previous versions. Would there be any point in an alternative compiler implementing "1.77.0" of the Rust spec?
And the alternative compiler implementation can't start implementing a compiler for version n+1 of the spec until that version of rustc is released because "the spec" is just "whatever rustc does", making the spec kind of pointless.
> Usually the standard comes first, compiler vendors implement it, and between releases of the spec the language is fixed.
This is not how C or C++ were standardized, nor most computer standards in the first place. Usually, vendors implement something, and then they come together to agree upon a standard second.
When updating standards, sometimes things are put in the standard before any implementations, but that's generally considered an antipattern for larger designs. You want real-world evaluation of the usefulness of something before it's been standardized.