Unless you have to interface with a lot of mostly-header/header-only libraries written in C or C++, Rust is just so much more productive to work with than C or C++, even if you don't have to worry about memory unsafety.
I find C++ more productive. In C++ I mainly fight with template error messages. In Rust, I fight with typechecked generics, inserting & and '_ here or there, adding and deleting imports, refactoring to add Some and Ok - tons of nonsense housekeeping. These features have value but are quite a slog when writing code.
Some of the kinds of bugs that are much easier to accidentally write in C++ than in Rust are: use-after-free, use-after-move, out-of-bounds array accesses, data races and other synchronization errors. I want my code to be correct, even if it runs in a sandbox.
But the great strengths of Rust, like memory and thread safety, are blunted in WASM, which is already memory-safe and thread-crippled. So Rust's success in WASM must be due to other factors.
I'm confused by that statement. Do you not care whether your code works correctly? Do you consider finding and fixing bugs to be separate from writing code?
> "upstream crates may add a new impl" I have satisfied the compiler but no bugs were prevented.
Trial-and-error debugging which version causes a bug is busywork, preventing it right when someone introduces the potential problem is at best annoying, but very fast compared to the alternative.
https://play.rust-lang.org/?gist=84883cb7cdd09acdcd919888cef...
It's especially bad if your type is an enum, since there's no obvious place to put the PhantomData:
https://github.com/rust-lang/rust/issues/32739
> Trial-and-error debugging which version causes a bug is busywork, preventing it right when someone introduces the potential problem is at best annoying
It's a silly limitation. For example, u64 and u128 are not From<usize> because...well I have no idea. But you can't make them From, because Rust wants to reserve the right to make them From in the future. And you can't write a function that assumes they are NOT From, for the same reason.
So it's pointlessly hard to write generics over integers. I encounter lots of weird holes like this.
https://play.rust-lang.org/?gist=600a1ca784ee7df02351e58df43...
> For example, u64 and u128 are not From<usize> because...well I have no idea.
Because usize is not known statically, it's target arch dependent. Yes, it's silly, but that's how technical safety works. (Also, just as a silly technical counter example the AS/400 virtual instruction set has 128 bit sized pointers.)
There's no question about the need for more ergonomics. That's what this whole post is about after all. For example in some cases where currently PhantomData is needed the intent of the programmer can be easily and unambiguously figured out, but for this folks need to be at least sufficiently certain that this makes things easier, makes code readable, doesn't constraint later language evolution, etc. (If I understand correctly some associated trait bound enhancements will lead to more readable code, less PhantomData.)