Limitations of `min_specialization` when specializing a generic implementation?

Дата публикации: 04-10-2026 15:30:59

I'm trying to understand #![feature(specialization)] vs. #![feature(min_specialization)] to inform the design of my types, but I haven't been able to find any novice-accessible documentation. How do the two differ?
Specifically, I expected the following (playground) to be straightforward specialization, but it does not work with min_specialization. Why not?
#![feature(min_specialization)]
trait A {}
trait B: A {}
struct Sa;
impl A for Sa {}
struct Sb;
impl A for Sb {}
impl B for Sb {}
trait X {
fn method(&self);
}
impl<T: A> X for T {
default fn method(&self) {
println!("default");
}
}
// error: cannot specialize on trait `B`
// --> src/main.rs:29:13
// |
// 29 | impl<T: A + B> X for T {
// | ^
impl<T: A + B> X for T {
fn method(&self) {
println!("specialized");
}
}
fn main() {
Sa.method();
Sb.method();
}
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