Some "as" cast replacements about curve25519-dalek HOT 3 CLOSED

Hi @leonardo-m!

My understanding is that the idea that

"as" casts are bug-prone

is due to the fact that bugs result from developers not expecting truncating behaviour when casting downwards, e.g. u128::MAX as u64.

from curve25519-dalek.

You're welcome, happy to help your project :-)

"as" casts are bug-prone for various reasons, one reason is that they specify the destination type only, so if the start type changes because of code refactorings, the cast keeps happily working, sometimes losing information. So wise programmers should minimize the number of "as" casts in their code. from() was invented to tell apart lossless casts from lossy ones.

from curve25519-dalek.

In the case we're discussing (low-level arithmetic), the starting types are pretty much fixed. In the specific example given, they're fixed in the function signature, which is type-checked, so using from() adds no additional type safety.

from curve25519-dalek.