Because of the Crater runs which Mara mentions, the Rust teams are actually in a good place to judge what the right call here is.
In practice, for a programming language, even something relatively simple like C, the specification is too woolly to be enough, and so the fact of what compilers actually do matters. So Rust having a specification would not magically prevent this problem.
Rust's Crater runs allow sunlight from the real world to intrude into decision making. Suppose it is discovered that although the docs say X doesn't happen, actually the compiler does X. Instead of "I bet lots of people rely on X, we must change the specs to match" versus "I bet everybody trusted our spec, which says X doesn't happen, fix the compiler" we have real facts. "Popular crates cool_stuff and jims_other_stuff both rely on X in versions from the last 18 months. Their authors both said they tried this and assumed the documentation was wrong. We should update the documentation to match reality" vs "There's only one crate which uses X, it's a test case in a sub-component of obscure_doodad 0.0.1 from 2017, it appears to receive no usage, and two of the other test cases in obscure_doodad fail on current Rust, so perhaps it is just broken anyway? We should just fix the compiler"
The C++ people couldn't have done this twenty years ago. Today they probably could attempt this with various Free Software C++ codebases out there (e.g. GitHub's C++ projects), but as well as considerable technical effort to attempt this, it would be a political burden because lots of non-Free stuff believes it has secret sauce and so vague and unsourced claims about what does or doesn't exist in proprietary C++ are a big problem for compatibility. If you're on team "Don't change X" of course you'll claim huge industrial partners you've seen rely on X in code you can't show anybody. If you're on team "X is obsolete garbage no-uses" you'll claim it's apparent nobody relies on X.
This is much less true for Rust. There obviously is proprietary Rust, including in-house projects and stuff that's deliberately proprietary rather than just nobody cared, but Crate covers a lot of what's out there.
In practice, for a programming language, even something relatively simple like C, the specification is too woolly to be enough, and so the fact of what compilers actually do matters. So Rust having a specification would not magically prevent this problem.
Rust's Crater runs allow sunlight from the real world to intrude into decision making. Suppose it is discovered that although the docs say X doesn't happen, actually the compiler does X. Instead of "I bet lots of people rely on X, we must change the specs to match" versus "I bet everybody trusted our spec, which says X doesn't happen, fix the compiler" we have real facts. "Popular crates cool_stuff and jims_other_stuff both rely on X in versions from the last 18 months. Their authors both said they tried this and assumed the documentation was wrong. We should update the documentation to match reality" vs "There's only one crate which uses X, it's a test case in a sub-component of obscure_doodad 0.0.1 from 2017, it appears to receive no usage, and two of the other test cases in obscure_doodad fail on current Rust, so perhaps it is just broken anyway? We should just fix the compiler"
The C++ people couldn't have done this twenty years ago. Today they probably could attempt this with various Free Software C++ codebases out there (e.g. GitHub's C++ projects), but as well as considerable technical effort to attempt this, it would be a political burden because lots of non-Free stuff believes it has secret sauce and so vague and unsourced claims about what does or doesn't exist in proprietary C++ are a big problem for compatibility. If you're on team "Don't change X" of course you'll claim huge industrial partners you've seen rely on X in code you can't show anybody. If you're on team "X is obsolete garbage no-uses" you'll claim it's apparent nobody relies on X.
This is much less true for Rust. There obviously is proprietary Rust, including in-house projects and stuff that's deliberately proprietary rather than just nobody cared, but Crate covers a lot of what's out there.