Tag: lifetimes

As I've said before, I'm working on a book about lifetimes. Or maybe it's just a long series - I haven't decided the specifics yet. Like every one of my series/book things, it's long, and it starts you off way in the periphery of the subject, and takes a lot of detours to get there.

In other words - it's great if you want an adventure (which truly understanding Rust definitely is), but it's not the best if you are currently on the puzzled end of a conversation with your neighborhood lifetime enforcer, the Rust compiler.

So, let's try to tackle that the crux of the issue another way - hopefully a more direct one.

I've held all of these misconceptions at some point and I see many beginners struggle with these misconceptions today. In a nutshell: A variable's lifetime is how long the data it points to can be statically verified by the compiler to be valid at its current memory address. I'll now spend the next ~6000 words going into more detail about where people commonly get confused.

I’ve recently learned a new piece of Rust syntax related to specifying lifetimes with types that don’t have an explicit lifetime defined. To save time for anyone who might already know about this “trick” here’s the resulting function definition:

pub fn events(&self) -> impl Iterator<Item = Event> + '_;

The rest of this article is about explaining the + '_ part, why it’s needed and what it solves. Also, VSCode + RLS currently does not highlight this properly, which tells me this is a pretty niche feature. Before I begin explaining what the issue was and how it’s solved, I think it’s best to show the overall problem I was trying to “code my way around”.

I wanted to write an article about one aspect of Rust I really put off for a long while — lifetimes. They are one of the hardest parts about Rust to wrap one’s brain around. Many of us are simply not used to a compiler with a paradigm around memory ownership where such things are needed.

Lifetimes help the compiler make your code safer (i.e. less prone to crashing by using unexpected places in memory). Even if we don’t write them in our code, the compiler is smart enough to figure out your lifetimes without you under the covers. They are often times your secret allies, so let's learn a bit about them.