C++20 is adding a module system to C++. That's a major change to the compilation model.
It is also adding concepts, a major change in the way templates are to be written. The very article we're commenting on is discussing how exceptions should possibly be replaced by another error report mechanism, several proposals are in flight for this. There's also the "destructive moves" proposals that have the potential to change a lot how we write types.
A telltale sign that these changes are major is that the entire std has to be "modularized" to support modules, and to be modified to support concepts. Similarly if exceptions are to be revised a large chunk of exception using functions from the std would need to be modified.
On the Rust side, I think the only change that has even a comparable impact is const generics (and maybe specialization).
Existential types and GAT will change how to express some traits (allowing a LendingIterator for example), but I don't expect they will affect a large portion of Rust's std.
Also of note is that the Rust changes come to add new systems orthogonal to the existing features (const generics fill an obvious void compared to C++, same with GAT and existential types where Rust's expressivity is limited in comparison with C++ atm). By contrast in C++, the module system comes to replace the headers, and a change to exceptions would replace the current exception system, creating churn.
That's probably correct, I did not use concepts at the moment (stuck in C++14 right now). I certainly have my beefs with both features (modules that are orthogonal to namespace, no standard way to find modules, new and exciting ways of committing ODR violations, generally a complicated module system with quirks when there is so much prior art on modules in other languages, concepts being structural and not nominal, concepts being a "lower bound" on behavior and not an "upper bound" (thus not eliminating duck typing)), but my larger point is the scope of these changes to the language, not their (purported) benefit that I'm by and large unable to assess right now.
C++20 is adding a module system to C++. That's a major change to the compilation model. It is also adding concepts, a major change in the way templates are to be written. The very article we're commenting on is discussing how exceptions should possibly be replaced by another error report mechanism, several proposals are in flight for this. There's also the "destructive moves" proposals that have the potential to change a lot how we write types.
A telltale sign that these changes are major is that the entire std has to be "modularized" to support modules, and to be modified to support concepts. Similarly if exceptions are to be revised a large chunk of exception using functions from the std would need to be modified.
On the Rust side, I think the only change that has even a comparable impact is const generics (and maybe specialization).
Existential types and GAT will change how to express some traits (allowing a LendingIterator for example), but I don't expect they will affect a large portion of Rust's std.
Also of note is that the Rust changes come to add new systems orthogonal to the existing features (const generics fill an obvious void compared to C++, same with GAT and existential types where Rust's expressivity is limited in comparison with C++ atm). By contrast in C++, the module system comes to replace the headers, and a change to exceptions would replace the current exception system, creating churn.