4 ms·

Types are not generated at runtime, lifetimes are types (although they cannot be constructed). T<'a> and T<'b> are different types altogether (although, being

by kaoD 6y ago

Types are not generated at runtime, lifetimes are types (although they cannot be constructed). T<'a> and T<'b> are different types altogether (although, being generic, they can resolve to the same type if 'a == 'b, but it's not required). All this happens at compile time. When you say T: 'a you say that T is a subtype of 'a hence it lives longer. https://doc.rust-lang.org/nomicon/subtyping.html https://doc.rust-lang.org/nomicon/subtyping.html Maybe you're being confused by misconception 1 here? https://github.com/pretzelhammer/rust-blog/blob/master/posts/common-rust-lifetime-misconceptions.md https://github.com/pretzelhammer/rust-blog/blob/master/posts... Type T, being generic, can be an OwnedStruct, but also an &'a BorrowedStruct or a StructWithReferences<'a>. The owned one is a subtype of 'static since it's owned, but the ones with lifetimes are subtypes of 'a which itself (being a generic lifetime) can be a subtype of 'static or any other lifetime. Again, all this happens and is checked at compile time. This ensures that at runtime, the actual references are alive as per their lifetimes without explicitly checking them anymore.

wokwokwok 6y ago
You misunderstand; what confuses me is how 'static can mean the same thing in both of these contexts. T: 'static does not mean the references in T must exist for the lifetime of the application. Ie. the lifetime constraint on T isn’t the same 'static from &'static where the reference must life for the entire lifetime of the application. Types are static. T: 'static applies to instances at runtime. These instances do not have and are not related to the 'static lifetime which is “the entire length of the application”. I accept: > Type: lifetime means that the type is constrained by the lifetime. I don't accept the explanation: > Because 'static means "the lifetime of the whole process". That means that the type is not constrained. A does not follow from B. If you want something to mean "A type cannot have references in it" then invent a 'noref lifetime. 'static in this context would mean that all members of T must be 'static, which means that all instances of T must be 'static. It does not mean that. I don’t understand why they have the same name; they are not the same thing; it is an example of failure to have orthogonality in the language design in my opinion.
- kaoD 6y ago
  Sorry, I probably edited my comment while you were replying and added a couple links. Check misconception 2 here, I think it addresses your point. https://github.com/pretzelhammer/rust-blog/blob/master/posts/common-rust-lifetime-misconceptions.md#2-if-t-static-then-t-must-be-valid-for-the-entire-program https://github.com/pretzelhammer/rust-blog/blob/master/posts... EDIT to your edit: > 'static in this context would mean that all instances of T must be 'static. You mean in T: 'static? No. It means that any instance passed as type T must be bound by 'static and therefore could be held up to the end of 'static. This does not mean that they're allocated at compile time, it just so happens that static variables (allocated at compile time) are 'static but the causality is reversed. > If you want something to mean "A type cannot have references in it" then invent a 'noref lifetime. It can have references in it! As long as they're bound by 'static. Here's an example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=857389cd5251e037f5ba608566b89b04 https://play.rust-lang.org/?version=stable&mode=debug&editio...
  - deleted 6y ago
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  - wokwokwok 6y ago
    Oh, I've read that. I just maintain that the word 'static is being overloaded here to mean multiple different things. 'static is not the "lifetime of the entire application" when it is used in the context of T: 'static. > It can have references in it! As long as they're bound by 'static. :) ...but it can also have values in it which are not 'static. So is T: 'static, or not? It's arbitrary semantics; ...but my take on it is: - IF you take "x is 'static" as meaning the "X is valid for entire lifetime of the application" then if: - x: &'static 'is static' and must be valid for the entire lifetime of the application. I would expect: - x: T + 'static 'is static' and must be valid for the entire lifetime of the application. I'm happy to agree that's not what it does mean, what I'm saying is that it is inconsistent for it not to mean that.
    - AsyncAwait 6y ago
      > ...but it can also have values in it which are not 'static. I don't think that's right. It can only have references which live at least as long as 'static does (or longer, but 'static is the longest so...)

by kaoD 6y ago

wokwokwok 6y ago

You misunderstand; what confuses me is how 'static can mean the same thing in both of these contexts. T: 'static does not mean the references in T must exist for the lifetime of the application. Ie. the lifetime constraint on T isn’t the same 'static from &'static where the reference must life for the entire lifetime of the application. Types are static. T: 'static applies to instances at runtime. These instances do not have and are not related to the 'static lifetime which is “the entire length of the application”. I accept: > Type: lifetime means that the type is constrained by the lifetime. I don't accept the explanation: > Because 'static means "the lifetime of the whole process". That means that the type is not constrained. A does not follow from B. If you want something to mean "A type cannot have references in it" then invent a 'noref lifetime. 'static in this context would mean that all members of T must be 'static, which means that all instances of T must be 'static. It does not mean that. I don’t understand why they have the same name; they are not the same thing; it is an example of failure to have orthogonality in the language design in my opinion.

kaoD 6y ago
Sorry, I probably edited my comment while you were replying and added a couple links. Check misconception 2 here, I think it addresses your point. https://github.com/pretzelhammer/rust-blog/blob/master/posts/common-rust-lifetime-misconceptions.md#2-if-t-static-then-t-must-be-valid-for-the-entire-program https://github.com/pretzelhammer/rust-blog/blob/master/posts... EDIT to your edit: > 'static in this context would mean that all instances of T must be 'static. You mean in T: 'static? No. It means that any instance passed as type T must be bound by 'static and therefore could be held up to the end of 'static. This does not mean that they're allocated at compile time, it just so happens that static variables (allocated at compile time) are 'static but the causality is reversed. > If you want something to mean "A type cannot have references in it" then invent a 'noref lifetime. It can have references in it! As long as they're bound by 'static. Here's an example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=857389cd5251e037f5ba608566b89b04 https://play.rust-lang.org/?version=stable&mode=debug&editio...
- deleted 6y ago
  [deleted]
- wokwokwok 6y ago
  Oh, I've read that. I just maintain that the word 'static is being overloaded here to mean multiple different things. 'static is not the "lifetime of the entire application" when it is used in the context of T: 'static. > It can have references in it! As long as they're bound by 'static. :) ...but it can also have values in it which are not 'static. So is T: 'static, or not? It's arbitrary semantics; ...but my take on it is: - IF you take "x is 'static" as meaning the "X is valid for entire lifetime of the application" then if: - x: &'static 'is static' and must be valid for the entire lifetime of the application. I would expect: - x: T + 'static 'is static' and must be valid for the entire lifetime of the application. I'm happy to agree that's not what it does mean, what I'm saying is that it is inconsistent for it not to mean that.
  - AsyncAwait 6y ago
    > ...but it can also have values in it which are not 'static. I don't think that's right. It can only have references which live at least as long as 'static does (or longer, but 'static is the longest so...)