impl const wrongly accepts impl with non-const provided methods

[jonas-schievink]

This prints no error, but prov is not const-safe, so the impl const Tr should not be accepted:

#![feature(const_trait_impl)]

trait Tr {
    fn req(&self);
    
    fn prov(&self) {
        println!("lul");
        self.req();
    }
}

struct S;

impl const Tr for S {
    fn req(&self) {}
}

[oli-obk]

I guess for impl const blocks we need to enforce that no defaulted fallbacks are used at all. This is discussed a bit in the RFC, but for now we should just disallow this entirely

[kadiwa4]

This issue applies again, it seems that the part of the test that checked this was removed in #96964.

Or is this intentionally accepted without an error now?

[kadiwa4]

This has been fixed again (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=aea283ceefe384376052b1cde86373cb)