Rust Lifetimes Finally Explained!

[@letsgetrusty]

📝 Get your *FREE Rust cheat sheet* : www.letsgetrusty.com/cheatsheet

[@ChumX100]

I really like how Rust doesn't hide complexity from developers behind trade-offs, but rather offers helpful tools to manage complexity as well as sensible defaults.

[@HermanWillems]

This transparancy i like. I dislike magical garbage collectors and other stuff. This is why rust is so great.

[@kevinmcfarlane2752]

There are arguments for both, depending on your application domain. There is never a one size fits all.

[@jay_browncomedy]

biggest draw for me as well

[@alinmgheorghe]

Well put. I'm still wrangling my brain around some of the stuff but slowly it's getting there and with each step you realise how awesome it really is.

[@thingsiplay]

I had trouble understanding the lifetime annotations system, until this video. The key information I needed was "It does not change the lifetime, but specifies relationships" explanation. Your videos and explanation are clear and easy to follow. Can't wait to see more material. Thanks for going through this and doing all the work.

[@letsgetrusty]

My pleasure!

[@piranhi]

Came here to say exactly the same thing

[@NathanHedglin]

Dido

[@TheOracleFile]

@@NathanHedglin Definitely this in combination with the fact that the borrow checker now uses the passed params to compare the lifetime with result

[@egonzalez4294]

Funny how people understand things differently, for me that was meaningless, what made sense to me was like, "the compiler needs to know the lifetime of the output" I thought what would it be like to be the borrow checker, and try to figure out the code. These annotations just help the checker figure things out.

Frustrated beginner Rust programmer: "Just let me write bad code!!" The Rust compiler: No. Here's some super helpful tips instead.

[@joelimbergamo639]

That's exactly how I feel writing rust, I want to write code that I know it's wrong but I want to only try it and rust forces me to write good code, it's a love hate relationship 😂

[@garlicat878]

Exactly. We should make a meme like: Me after learning other languages for hours: Wow Let's make a app/game/website. Me after learning Rust for weeks: Why doesn't it compile? What lifetime annotation should I put here? Should I borrow that thing?

[@HermanWillems]

I love Rust is forcing me to do the right thing. For example it forces me to handle the options, use the match. But also with using the match statement it forces me to use all posibilities. No null. I love it doesn't have null. (Well in safe space though) but i like how its the default. The defaults are chosen perfectly.

[@vorrnth8734]

@@joelimbergamo639 Having had to debug muuuch bad code I really love this behaviour. Dangling pointers can be a real pita.

[@conradmbugua9098]

🤣🤣🔥

[@Hardzinho_Yay]

As a Java/Node/python programmer and learning Rust I never understood lifetimes in Rust, you`re the first to make this `click` in my brain. Thank you.

[@richasha670]

I totally understand lifetime by your video! Thank you so much as a Rust beginner, you made me understand Rust Book 200%!

[@kevinmcfarlane2752]

Yeah he complements the book nicely. I find with technical topics that I need a combination of written material, presentations like this and experimentation/practice to get the richest understanding.

[@AlanDarkworld]

Whoever decided to use single quotes in a non-pairwise fashion in the Rust syntax just wants to see the world burn.

[@Lexikon00]

I just wanted to address something that a lot of new rust programmers get wrong, and you also did explain it wrongly. At least it's rarely the case someone wants to do this. I'm going to copy the code you used for the illustration: struct ImportantExcerpt ImportantExcerpt

[@letsgetrusty]

Hey Alexander, thank you for the correction and taking the time to write this out.

[@johnegan2484]

To clarify, because there are two input lifetimes both &self and announcement:&str are given their own lifetimes.(Rule #1) Because one of the parameters is &self, the return type gets the lifetime of &self. (Rule #3)

[@bertolucci1985]

Thanks for taking the time to produce these videos mate. Learning Rust concepts alone isn’t easy. I appreciate it.

[@chsblue2]

Perfect explanation. You just gave me that little extra information that I was missing to finally get it into my head. Thx man! 👏🏻

[@kumatmebrah9044]

Watching this after going through the chapter helps drive the point home. I'm glad I found this.

[@GolangDojo]

Just let me live life dangerously

[@ajesh-mishra]

Rust Compiler: Nope! Not on my watch ⌚ safety first 🎈

[@vorrnth8734]

Then do C or assembler.

[@embedded_software]

I was watching this video on the treadmill, and I was enjoying it so much that I actually worked out longer than I was planning without even realizing it until the video was over! Thanks, Bogdan!

[@dmitriidemenev5258]

Hi, Bogdan! I would be really cool to see a video about structs where some fields reference others. More precisely, why it is or is not bad and what are the alternatives.

[@zemja]

You cannot do this in Rust.

[@joehsiao6224]

Resolving a longtime mystery in my mind feels so good. I'd spam the like button if I could.

[@johnegan2484]

You state at ~13:15 that the lifetime annotation is saying our struct cannot outlive the reference passed into part:first sentence so if we try to use our variable i (agreed) which is an instance of our struct after part:first sentence has gone out of scope we would get an error. However, would note that the annotation means that an instance of ImportantExcerpt cannot outlive the reference it holds in its part:first_sentence field and in that regard the reference in the ImportantExcerpt instance is valid, the owning variable novel does not go out of scope prior to ImportantExcerpt going out of scope.

[@sahilverma4077]

Damn, we have a lot of restrictions, but I wish if I get through all these problems I will have a safer, low level, and fast code. Btw, I recently saw in google news that google is rewriting some part of android with Rust, wish more companies start adopting it.

[@letsgetrusty]

It's good to catch the wave early :)

[@ajesh-mishra]

Also the new modules in Linux Kernel may be written in Rust and that would be big!

[@darshanv1748]

at 16:50 if we return announcement from the function , then according to lifetime ellisions it'll be assigned lifetime of self, but don't you think that it might be wrong in some cases

[@proloycodes]

yes it will be wrong. im sure it will give some "announcement does not live long enough." type of error

[@klaymen0]

Thanks for the video series, it's a great addition to the book as it gives a different perspective to a lot of things. One thing I wonder about lifetimes (and I'm learning, so maybe I did not grasp the concept fully yet): It seems to me that assigning the same lifetime 'a to ALL parameters of a function plus all returned parameters is the most restrictive way possible in terms of lifetimes. If an actual use of a function fulfills these "strictest" rule, why is it actually required to write these lifetimes out when defining it? I think, in the longest-string example, no different lifetime declaration than this most restrictive one is even possible - if you try it, a bug occurs where you try to return the "other" parameter. I understand it's probably a complex problem for the compiler, maybe NP-complete, to find the optimal setting, but in cases where the most restrictive remaining one works, why not defaulting to this? One possible explanation (and maybe this is the reason) is when you write a library that exports functions, so the compiler does not know how it is actually used, so lifetime declarations must be exported as well. But this would not apply for private functions. Another way to make it easier would be to add a mode (maybe compiler flag) to automatically fill up lacking lifetimes with the most restrictive setting still possible? Or what also might be interesting (maybe this exists) were external tools, a bit like lint, or optimally included in an IDE, that could auto-suggest lifetimes?

[@fixpoint]

Thank you for explaining lifetimes in a clear and easy to understand manner. I just discovered your series and can’t wait to go through the rest of them! Thank you for the time and effort you have put in to produce these.

[@sulfurasmr6365]

this channel has been hugely helpful for me trying to learn rust, especially since I've never heard of a language doing 'borrowing' before. thank you so much!

[@oliverhunter1847]

Hands down, the best explanation of lifetimes I've ever seen

[@web3987]

Thank you for such a comprehensive lifetimes lesson. The learnings from this video will last a lifetime.

[@gzoechi]

Good explanation of a bunch of things I had a really hard time understanding from the Rust book, when I started learning Rust. This could have spared me a lot of pain 😬

[@abhinavchavali1443]

This is the best youtube video I have ever watched.

[@oyunovacom]

Bogdan, this video is even clear for me as a code illiterate! Thank you for this and PLEASEEEE keep your lectures this minimal! Cheers!

[@SansDev]

This series is outstanding tbh, thank you for your hard work. Understanding each of Rust's features makes me love it so much more!

[@2002budokan]

You know what? This is currently the best video resource to learn Rust. Thank you Bogdan.

[@tobi9648]

wow, I saw these 'a-ticks and couldnt find a good/easy explanation for it. You just explained it so easy and perfect. Thanks!!

[@RoamingAdhocrat]

and you may ask yourself, why did I struggle with this? Thanks for the explanation. I'd found lifetimes confusing up to now, but the way you demonstrated it, it's quite straightforward!

[@LagDaemonProgramming]

Your explanations of these topics are concise, complete, detailed and you move right along with them. Outstanding job, all of your videos make these ideas very clear. I tip my hat to you sir! :)

[@gitgudsec]

your videos are just so incredible man, extremely grateful for these beautifully articulated insights you put out

[@Tanger68]

You explain this very succinctly, thank you very much! In my mind, lifetimes are almost intuitive once you think about it. I think I just double guess myself because I expect it to be more complex than it is!

[@mmmbug]

You're a great teacher. Thank you for making this video! Do you know why lifetimes in Rust are rarely given descriptive names? Almost every example I find uses single-letter names like "a".

[@TAT4guitar]

I have been learning Rust from youtube and Rust by Example, and your videos always do the best job when it comes to tricky concepts such as ownership and lifestime. Thanks a lot!

[@meka4996]

Your explanation is super clear! I know lifetime now! Thank you

[@oussamasethoum2755]

If the block doesn't end until line 230 but the variable is not used after line 100, does rust deallocate the variable in line 101 or until the block ends? what is the case for the garbage collector?

[@absarrahman9177]

THIS IS BY FAR THE BEST EXPLANATION. Keep it up man

[@b_mathemagical]

I rarely comment on videos, but you did a great job with this. Thanks for sharing!

[@twentyeightO1]

This channel is a blessing!

[@dimiutube]

Awesome! Good explanation of the lifetime concept. I'm looking forward to deal with different lifetimes for variables and convince the compiler that all I'm doing is well planned 🙈 Thank you very much!!

[@user-dg9cr6wu9i]

Awesome! These examples are very clear for rust lifetime.

[@strawz1988]

Thanks for such a neat explanation. Just a request the audio is little low

[@vijaysinghbisht3016]

Why so less comments? This channel deserves far more attention than this... Sadness

[@hhhuhhh5692]

Thank you so much! This video and the others in this playlist really helps me understand Rust better! ❤

[@kch9001]

Best lifetime explanation on youtube

[@filipslezak5152]

I don't know why but rust is giving me some feeling of completion when binary is complete and running as expected.

[@PremalUpadhyay]

Hi, can you make series on poem framework of rust? Lack of resource availability is major concern and first obstacle to get started with it. Thanks :)

[@sebastianospino4861]

That is, the lifetimes only inform the compiler of the lifetimes that these references must have, and it checks if these conditions are fulfilled in the code, and if not, an error is raised.

[@Mortesins]

Thanks for the comment, it was exactly what I was missing.

[@michaelkastner3611]

Again, another great episode.

[@MameMedouneDiopXpert]

Lifetimes are clear now! Thank's a lot Bogdan

[@jayleejw1801]

My first time learning about lifetime in rust. I didn’t think it’s that difficult. In fact, it’s common sense. It’s about preventing dangling reference, meaning a reference to something that doesn’t exist anymore, which is a memory issue. Explicitly writing the tick generic lifetime annotation actually makes programmer think smarter about avoiding referencing something that may not live long enough in the program. It’s all about lifetime and it’s really that simple. Anything to do with reference, be careful with its lifetime, because you might reference to invalid memory. And some rules to follow, if only one reference parameter, there’s no need to write the lifetime annotation because definitely your output reference depends on it. If more than one, then you have to write the annotation and borrow checker will be able to determine, such that output reference lifetime is the smallest of the two parameters lifetime. Simple.

[@kevincarvalhodejesus4473]

Very well explained, thank you!

[@CountJamesMcGuigan]

Thank you for your tutorial. As a small piece of feedback for future videos, the faint music in the background was slightly distracting.

[@meowsqueak]

It seems to be a trend in programming videos lately, and it is VERY distracting. I often find myself pausing the video thinking it’s from elsewhere in my house. I really wish these otherwise talented content creators would ditch the pointless and distracting background audio - it almost ruins what would otherwise be an excellent resource.

[@OggerFN]

Great video. Only thing I miss here would be showing how different lifetimes interact with each other inside the fn scope to further illustrate the concept.

[@NeotenicApe]

Your videos are amazing! Thank you so much ❤️

[@PoetNoPoems]

These videos have been fantastic, thank you

[@fhdiaz87]

It would had been useful to mention that life time could be coerced. That is a reason to use just a' when actually both parameters have different life times.

[@andriesvanwyk3226]

Very clear explanation! Thank you!

[@chlbri]

You explain very well some difficult notions.

[@marinhobrandao]

I loved to know you like The Wheel of Time as well :D

[@sakuranooka]

Ok, I understand half of this. What I don't understand is e.g. why I need to add /'a/ so many times. Why for instance do we have to add it in angle brackets after the function name? The simple answer to this would be "because Rust requires it", but I'm interested in the longer answer to "why does Rust require it? Is there anything that wouldn't work if in a language Rust2.0 only the input and output arguments had lifetime annotations, but not the function itself?". What I don't understand either is why x and y have the same lifetime annotation, even though we just said x and y can have different lifetimes. And could you show an example where we need more than one lifetime specification, e.g. /'a/ and /'b/? Thanks for these otherwise extremely helpful videos :)

[@healthytips4818]

hopefully someday rust just use AI to generate this lifetime annotation :D

[@swapode]

x and y have the same lifetime annotation because the return value's lifetime depends on both of them. It doesn't matter that x and y can have different lifetimes, it's important that the values of both x and y outlive the return value, since the return value can be a pointer to either one. I made a simple modification to the longest() function, to demonstrate how two different lifetimes might be useful: fn weird_longest(x:&'a str, y:&'b str) -> Result { if x.len() > y.len() { Ok(x) } else { Err(y) } } While this also would compile with a single lifetime, it'd mean different things on the call side. Two different lifetimes would allow us to discard one of the actual values, while with the same lifetime we'd have to keep both of them around until the return value goes out of scope. I'm actually not quite sure about "why does Rust require it?". I can't come up with a counter example on the spot, something to think about. One reason might be something like monomorphization, explicitly declaring the lifetimes makes it clear that this is a generic function that might be compiled several times for different calls (what if both parameters have static lifetime?) but I can't quite wrap my head around it.

[@pynchia4119]

can you please explain the syntax on line 5 @7.30 ? Don't types follow the colon? Are you assigning a value to the arguments of the call to longest? If so, why isn't an equal symbol used? Are you casting their type instead? And what is the need of using .as_str() in the first place?

[@aidenlilley1319]

I don't understand the first part of the question but the point of calling .as_str() is to get the reference, as this video is about reference lifetimes.

[@Gramini]

Do you mean the "x: string1.as_str()"? The pattern "var: type = value" only applies to creating variables. The things inside longest(…) are not variables, but passing parameters. When calling functions you can use named parameters, meaning the parameters are not passed in the order they are defined in but by the names they are defined as. To do so you specify the parameter name, a colon and then the value. "x: string1" means "For the parameter named x use the value string1". These grey, smaller text is generated by the IDE/code editor as hints and are not actually there.

[@stanislavstudzinskyi2347]

Outstanding quality. Thank you so much for your labor!

[@dulanjala]

this is a nice little tutorial with so much value. Thanks!

[@gaelanm3920]

Let's start out by talking about ding-a-ling references. In all seriousness, great video.

[@LimitedWard]

This was really helpful. So far this seems to be the "ugliest" part of Rust's syntax, though, I guess it's just a direct consequence of having a borrow checker. What's the general consensus on the use of lifetime annotations? Should we be writing code to explicitly avoid needing them whenever possible?

[@Daxlo]

I'd love it if someone who knows could answer this question! I think they become important when you have references in multiple data structures. For example, two Vectors contain a reference to the same value.

[@kevinmcfarlane2752]

What tends to happen with languages is that they simplify the syntax over time. Though in the meantime they add more concepts with more syntax. Lol.

[@thebsv]

Have a small question about the last example, what is the lifetime of the variable ann which has a generic type T? Why wasn't it explicitly mentioned, like the other input parameters? Would 'ann' also have the same lifetime as x and y, meaning 'a?

[@AndresFH7233]

Dude great explanation, I finally got the concept!

[@lee45283]

Great video, thanks for the content!

[@walter0331]

I spent an afternoon trying to figure out how lifetimes works, finally understood after watching this vid 🤦🏻‍♂️

[@murugarajuperumalla5508]

Super, this video is enough to understand lifetime, thanks a lot ❤️❤️

[@kiddkai]

Great video, help me understand the lifetime annotation!

[@Timjstewart]

So thorough and interesting! Thanks!

[@jacques-dev]

Wowie. Thanks Bogdan!

[@vijaysinghbisht3016]

Thank you very much.. Explained very nicely.. Can you do a day event?

[@gauravpoudel7288]

AWESOME EXPLANATION.!! thANKS SO MUCH!

[@tourdesource]

Clear as day. Thank you!

[@rahatsshowcase8614]

You explained really well! and after your explanation I was like is there any single case even where i will need 2 lifetime annotations like 'a and 'b to express my logic! because i think i could do my lifetime annotations for any function or struct just by using 'a only! because if u look deeply for functions it's just annotating all your parameters with 'a which have the possible to be returned from your function and return 'a on the other hand for struct as the struct will become invalid if a single property containing a ref becomes invalid which means we can always pass a single lifetime through the whole struct? because i have tried 5-10 examples and all are working with a single annotation perfectly as expected! so let me know your opinion regarding this!

[@madhank93]

Very nicely done. Thank you 🙏

[@user-ko5yr9mo6d]

намного понятнее чем в книжках. Спасибо)

[@Scramblejams]

I believe this could use an update now that the non-lexical lifetime analyzer has been updated, correct? For example, I do not get an error with rust 1.72.1 on the code sample at 13:00.

[@hungnguyenthanh2800]

Very nice explanination!!!!

[@abraham7966]

This is sooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo helpful. I am reading the Rust book at the moment and I am about to learn lifetimes but this gives me such an amazing foundation!!!! Thanks for explaining this concept! It is great to have people that cares about explaining confusing and hard topics rather than the same easy subjects.

[@berliangigihprakoso6948]

very good man, your videos is easy for beginer like me.❤👍

[@aaronzolotor430]

This was amazingly helpful!

[@kvasco-iu8nq]

Superb! really good explanation, got some small pieces that help me to get my head around this lifetime thingy. in the fn longest example, m 11:02 it works if we use string slices instead of Strings, my guess is because these implement the copy trait ? hence the values are moved so the reference to result is valid at the end ?

[@shhhhhhKoiHai]

This is great explanation. Love it. Thanks.

[@NoNameEntered]

Great video and examples! Thank you!

[@mr.bulldops7692]

Lifetimes being relational is a revelation.

[@jamesbarrow]

Smashed the like button. Didn't smash subscribe, already done that before ;)

[@rjlacanlaled9419]

Does this still apply, or are compilers more advanced now that we don't need to use generic lifetime annotations anymore?

[@funkdefied1]

Thank you!

[@Jordan4Ibanez]

I wish I could like this video more than once

[@pygeekrfoo820]

Chap 10 is a kick to the nuts.

[@dadestor]

Great video and explanation, thanks

[@zombieshoe]

Hi! This might sound silly but I was just wondering why the Rust compiler can't automatically assign the lifetime based on which values are returned. For example, in the "longest" function, why can't it return a reference to x with the same lifetime as the input reference to x OR a reference to y with the same input lifetime as the reference to y?

[@alagaika8515]

Because this question can not in general be answered at compile time.

[@zombieshoe]

@@alagaika8515 But my question was why. When I think about it in my head, I could imagine a way of determining how to configure a compiler to do what I suggested. Maybe you could give an example of where there'd be ambiguity?

[@alagaika8515]

@@zombieshoe I think the problem is that a lifetime is not some information that is stored together with the value of the variable. It is a compiler restriction on what functions can be defined. If you defined the largest function without explicit lifetime hints and used the last example where one string is defined in the outer scope together with the result reference and another in the inner scope, but the values of those strings are only determined at runtime by user input - how should the compiler determine if the code results in some access to a freed memory address or is valid?

[@zombieshoe]

@@alagaika8515 I think you and I would both agree that the value of the string shouldn't matter and that instead what is important is if it's None or Some. It's been quite some time since I watched the video so I may have to refer back to the last example, but it seems to me that a user defined string is no different than a precompiled one in the sense that that only difference is the location of the pointer and NOT the lifetime of the variable. And so yeah I'd assume returning the string that has the outer definition would outlive that of the inner definition. Again, I could totally be missing something, so I'm sorry if I sound silly lol

[@alagaika8515]

@@zombieshoeThe value of the string determines which one is longer (has more characters) and therefore if the function returns a reference to x or to y. One of these will be freed before the other, so in order to ensure that there never remains a reference that is no longer valid, the compiler can only ensure that the result will be valid at least as long as the shortest lived input. This could probably also be a fallback rule to use if no explicit lifetime annotations are given, not sure what reasons the rust designers had for this decision.

[@unduloid]

So when would you use mixed lifetimes?

[@bob55320]

The video is great and I really appreciate the explanation. However, the background music is quite annoying, even if it is very low volume. I kept wondering if someone was playing music in the other room. I would personally prefer that there was no background music at all. Hopefully you find this feedback constructive and please understand that I am writing it with the best of intentions! Thanks!

[@kg3217]

Thank you : ) Thanks again Thanks a lot Thank you for explaining it so well 🙏