Before stabilization it should likely be harmonized with command line syntax already used for native library modifiers, see item (3) in #93901 (comment).
Modifiers can be converted to /- form

I don't really see the relationship between #[link] and --extern flag:. The latter controls how a Rust dependency is handled/consumed by this particular rustc invocation, whereas the former controls how an external non-Rust dependency is finally linked.

(I also think the whole #[link] mechanism is a bad idea and should be deprecated, not extended. Controlling how things are ultimately linked is the responsibility of the build system, and so the functionality should be moved to Cargo.)

I don't think that is referring to #[link], just that it should use the same /- syntax of the -l flag, like --extern prev,-prelude:foo=libfoo.rlib or possibly rearranged to --extern foo: prev,-prelude=libfoo.rlib