Historically, the criteria for inclusion in the standard library is, does it fall under one of these three categories:
* Used in the vast majority of Rust programs.
* Reasonably common things that require a lot of unsafe to implement.
* Traits for interoperation purposes.
HashMap and mutexes fall under #2. The idea is that they'll receive significantly more scrutiny, and this is a good thing for the ecosystem.
(These are guidelines, not hard and fast rules, so not literally everything fits here, but that's the framework that was used to think about what should be included or not, generally.)
To be honest I think #2 was applied a bit too broadly. Sure rustc gets scrutiny but the std isn't the easiest library to contribute to. Not only is it large (by rust crate standards) but it's also tied to the compiler which adds another barrier to contributions.
Mind you, increasingly parts of the std are more like wrappers around third party crates e.g. hashbrown and parking_lot.
* Used in the vast majority of Rust programs.
* Reasonably common things that require a lot of unsafe to implement.
* Traits for interoperation purposes.
HashMap and mutexes fall under #2. The idea is that they'll receive significantly more scrutiny, and this is a good thing for the ecosystem.
(These are guidelines, not hard and fast rules, so not literally everything fits here, but that's the framework that was used to think about what should be included or not, generally.)