I think what author is saying is that Rust won't let you have any type-specific behaviors in generic code, on principle. Generic Rust code is meant to rely only on the trait bounds, and nothing else. The compiler will eventually know concrete types, but it won't let you depend on that.
This is in contrast with Rust macros and C++ templates where the template code can perform operations that work only on some concrete types and not others, without declaring that upfront with trait bounds.
In other words, Rust's generic code is type-checked at the definition time (while the type T is still a truly abstract concept), rather than type-checked at use/instantiation/monomorphisation time when the type T can be substituted for something specific.
It helps with the overall goal of optimizing code for types, but I wouldn't say it changes the principles. The principle remains that generics are type-checked at definition time, and Rust tries to avoid having monomorphisation-time type errors.
• When you have `impl<T> Trait<T> for MyType` you still can't make any type-specific assumptions about T in this impl, even with specialization.
• When you have `impl Trait<i32> for MyType` then you can rely on the specific type, even without specialization.
So rules for individual implementations remain unchanged, specialization merely allows both implementations to coexist.
I think we agree that Rust allows generic code to be constrained, which means it can be eagerly type checked at the definition site. Contrast to C++ templates which are unconstrained meaning the compiler can't type check until they're expanded (barring the use of concepts).
What I was trying to say is that whether or not concrete type of a generic argument can be used as a constraint is orthogonal to those semantics, it's an implementation detail.
How does this work? Monomorphization? Does the rust compiler do wvole-program analysis to create each possible an instantiation following the types that implement a given trait?
The compiler does analyse the entire codebase and creates a a copy of the function for every type you use the generic with. This is why Rust compiles libraries with your binary rather than using precompiled ones like you would in C.
This is in contrast with Rust macros and C++ templates where the template code can perform operations that work only on some concrete types and not others, without declaring that upfront with trait bounds.
In other words, Rust's generic code is type-checked at the definition time (while the type T is still a truly abstract concept), rather than type-checked at use/instantiation/monomorphisation time when the type T can be substituted for something specific.