Rust is notoriously hard to learn, with lifetimes and the borrow checker being a big part of it. A friend of mine took Rust for a spin, and these are some of the things that he found made lifetimes particularly tough to grok. ntietz.com/blog/rust-li... - ThreadSky

ntietz.com • 3 days ago

Rust is notoriously hard to learn, with lifetimes and the borrow checker being a big part of it. A friend of mine took Rust for a spin, and these are some of the things that he found made lifetimes particularly tough to grok.

https://ntietz.com/blog/rust-lifetimes-learning/

Comments

barretts.club•2 days ago

always love to see the difficulties of beginners documented. please consider sticking this on twir! https://github.com/rust-lang/this-week-in-rust

barretts.club•2 days ago

also, i do wanna note that lifetimes could power some optimizations, particularly if you consider when `Drop::drop` is called!

in MIR, they become labelled 'lifetime blocks', which can drastically change performance in some cases

barretts.club•2 days ago

for example, promoting a reference to 'static can result in its `Drop::drop` being unnecessary - it'll be optimized away! https://github.com/rust-lang/rust/blob/81d8edc2000aa38b08ad09fce22d90f1990b6459/compiler/rustc_mir_transform/src/promote_consts.rs#L1049

https://github.com/rust-lang/rust/blob/81d8edc2000aa38b08ad09fce22d90f1990b6459/compiler/rustc_mir_transform/src/promote_consts.rs#L110

barretts.club•2 days ago

it can also result in refs being treated as const within `const` functions, meaning they're completely eval'd at comptime :) https://github.com/rust-lang/rust/blob/81d8edc2000aa38b08ad09fce22d90f1990b6459/compiler/rustc_mir_transform/src/promote_consts.rs#L390

barretts.club•2 days ago

(note that these have requirements, like not using certain `unsafe` features)

ntietz.com•2 days ago

I submitted it, thanks for suggesting that! And thank you for all the additional knowledge you dropped in this thread :)

alilleybrinker.com•14 hours ago

Good article! It may help to say that lifetimes are for a familiar kind of polymorphism: subtype polymorphism! While regular "generics" in Rust are parametric polymorphism (substitution of types, with constraints checked eagerly), lifetimes have a subtype relationship based on "outliving" a bound.

alilleybrinker.com•14 hours ago

This is also how "variance" (the type theory term for "how do you deal with having both parametric and subtype polymorphism?") shows up in Rust; there are rules for how lifetimes and generics combine.

https://doc.rust-lang.org/nomicon/subtyping.html

alilleybrinker.com•14 hours ago

Learning to think of lifetimes as subtypes based on "outliving" a bound helps improve intuition and makes them seem less magical!

nekotachi.bsky.social•11 hours ago

there's so many different types of polymorphism, i'm curious if anyone has compiled a definitive list.

wikipedia has a lot, but is notably missing the duck-type polymorphism found in languages like zig (although i suppose you could say this is a form of subtype polymorphism)

alilleybrinker.com•10 hours ago

Parametric, subtype, and ad hoc are the three main types as I recall. They all have different flavors but those are the major categories

nekotachi.bsky.social•10 hours ago

what is it when you just coerce everything to void* ?

i guess technically that's subtyping, with the type being "normal sized pointer"

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