Conversation

I've been working on a Rust project with a friend and I swear he's the best tester for compiler error messages. Today, when he wanted to add a generic type to an enum that already had a lifetime, he wrote: enum Foo<'a><T> { ... } The compiler was not ready for this case! 😂 /1

Sunjay

@Sunjay03

Feb 16, 2020

If you think about it, this is a perfectly logical thing to do. Lifetimes and generics are separate things, so why not put them in separate angle-bracketed sections? Sure you get it once you see <'a, T> for the first time, but I don't blame my friend at all for trying this out /2

ᵉᵈᵈʸᵇ

@eddyb_r

Feb 18, 2020

I've been trying to call out this teaching failure mode for years now lifetime parameters *are* generic parameters! that's why they are in the same angle brackets! you add them to data types *expressly* to generalize over any lifetime (otherwise you're stuck with only 'static)

ᵉᵈᵈʸᵇ

@eddyb_r

Feb 18, 2020

teaching lifetimes like they're different leads to many unnecessary wrong assumptions like when people think you add lifetimes to a struct to relate the "lifetime of the struct" with references inside: that's not a thing, and it's a sign that something went wrong in teaching!