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Can someone explain to me why the PhantomData<fn() -> T> behavior cannot be the default implied behavior for unused types in Rust? (aka: pretend T is not stored on the type ... because it isn't). PhantomData comes up constantly for me in safe code and it feels SO clunky to me.

Carter Anderson

@cart.work

In a language where you can define some Foo trait for behaviors that don't take "self" as a parameter (ex: Foo::do_thing()), it feels pretty wild to me that being generic on that behavior requires anything more than specifying that you want MyType<T: Foo>.

4/12/25, 1:31 am0 reposts4 likes

Carter Anderson

@cart.work

4 Dec 2025

I've read doc.rust-lang.org/nomicon/phan... and rust-lang.github.io/rfcs/0738-va... Some of the rationale behind not defaulting feels wild to me, given how clunky and common it is in practice. "Phantom type parameters occur relatively rarely anyhow" does not ring true to me.