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That actually makes perfect sense to me. I ran into that issue trying to port something else to rust, just didn't realize it was this same issue. In go you ju

That actually makes perfect sense to me. I ran into that issue trying to port something else to rust, just didn't realize it was this same issue. In go you just define an interface and then make a slice of that interface and put things in it. In rust I ended up having to do Vec<Box<dyn Checker>> I think initially I tried just doing a Vec<Checker> and when that failed I ended up putting something like "How do I make a vec of an impl in rust" and found a code sample. That's where the compiler just saying "The types don't match" is not very helpful.

estebank 6y ago
If I'm following correctly, you had a situation like the following, right? https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=09e5dd1233c7bee0539f3346b201ceb0 https://play.rust-lang.org/?version=stable&mode=debug&editio... error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time --> src/main.rs:5:12 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn std::fmt::Display` error[E0277]: a value of type `Vec<dyn std::fmt::Display>` cannot be built from an iterator over elements of type `{integer}` --> src/main.rs:5:45 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^ value of type `Vec<dyn std::fmt::Display>` cannot be built from `std::iter::Iterator<Item={integer}>` | = help: the trait `FromIterator<{integer}>` is not implemented for `Vec<dyn std::fmt::Display>` error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time --> src/main.rs:5:31 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn std::fmt::Display` I can see a bunch of places where we could improve the output that we haven't gotten to yet: - The third error shouldn't have been emitted in the first place, the first two are more than enough - The first error has a note, but that note with a little bit of work could be turned into a structured suggestion for boxing or borrowing - For the second suggestion we could detect this case in particular where the result would be !Sized and also suggest Boxing. Edit: filed https://github.com/rust-lang/rust/issues/84346 https://github.com/rust-lang/rust/issues/84346 It is also somehow unfortunate that `impl Trait` in locals isn't yet in stable, but once it is it would let you write `let z: Vec<impl Display> = x.into_iter().collect();`, but as you can see here, that doesn't currently work even on nightly: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=d7b11f2a92a74445a4bf1973df7424dc https://play.rust-lang.org/?version=nightly&mode=debug&editi...
brundolf 6y ago
I think the fundamental issue is that most languages these days use dynamic-dispatch invisibly and by default. Rust seeks to empower users to be more efficient by default (which is good), but sometimes, especially on this iterators case, it creates deeply confusing/frustrating barriers that have to be explicitly stepped around.

estebank 6y ago

If I'm following correctly, you had a situation like the following, right? https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=09e5dd1233c7bee0539f3346b201ceb0 https://play.rust-lang.org/?version=stable&mode=debug&editio... error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time --> src/main.rs:5:12 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn std::fmt::Display` error[E0277]: a value of type `Vec<dyn std::fmt::Display>` cannot be built from an iterator over elements of type `{integer}` --> src/main.rs:5:45 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^ value of type `Vec<dyn std::fmt::Display>` cannot be built from `std::iter::Iterator<Item={integer}>` | = help: the trait `FromIterator<{integer}>` is not implemented for `Vec<dyn std::fmt::Display>` error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time --> src/main.rs:5:31 | 5 | let y: Vec<dyn Display> = x.into_iter().collect(); | ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn std::fmt::Display` I can see a bunch of places where we could improve the output that we haven't gotten to yet: - The third error shouldn't have been emitted in the first place, the first two are more than enough - The first error has a note, but that note with a little bit of work could be turned into a structured suggestion for boxing or borrowing - For the second suggestion we could detect this case in particular where the result would be !Sized and also suggest Boxing. Edit: filed https://github.com/rust-lang/rust/issues/84346 https://github.com/rust-lang/rust/issues/84346 It is also somehow unfortunate that `impl Trait` in locals isn't yet in stable, but once it is it would let you write `let z: Vec<impl Display> = x.into_iter().collect();`, but as you can see here, that doesn't currently work even on nightly: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=d7b11f2a92a74445a4bf1973df7424dc https://play.rust-lang.org/?version=nightly&mode=debug&editi...

brundolf 6y ago

I think the fundamental issue is that most languages these days use dynamic-dispatch invisibly and by default. Rust seeks to empower users to be more efficient by default (which is good), but sometimes, especially on this iterators case, it creates deeply confusing/frustrating barriers that have to be explicitly stepped around.