The first piece of code throws an error, all the others work. Why? Error! #[derive(Debug, Default)] struct StructName<T=u32> { // default generic parameter type field: T, } fn main() { let a = StructName { ..Default::default() }; // ^^^^^^^^^^ Error: …

[image] steffahn: <Clone>::clone(&0) Nevermind, judging by the compiler error messages, this will actually try to use a trait object, so it won’t work for Clone. So only Clone::clone(&0) is an option.

Some example where both the trait and the type has a parameter: let x: Vec<u8> = …; //////////////////////////////////////////////////// // method call let _r: &[u8] = x.as_ref(); // fully qualified let _r: &[u8] = <Vec<u8> as AsRef<[u8]>>::as_ref(&x); //////////////////////////////////////////…

Thanks! That's a lot of possible variations. I didn't realize it was so flexible. I had only ever seen the explicit form <type as trait> I also just learned that ::<>:: is legal and equivalent to ::<_>:: I wouldn't have thought to try that.

[image] m51: I also just learned that ::<>:: is legal and equivalent to ::<_>:: In general, it’s more like it’s equivalent to not giving any parameters at all (so in these cases ::<>:: is like ::). It’s a bit of a weird corner case, I’m not actually sure why it is allowed like this but it is …

I kind of figured it was like that. The example only had a single type parameter, but I get it. Cool stuff.

[image] m51: I didn't realize it was so flexible. Here’s some more variation: let _r: &[u8] = <Vec::<u8> as AsRef::<[u8]>>::as_ref(&x); let _r: &[u8] = <Vec<u8> as AsRef::<[u8]>>::as_ref(&x); let _r: &[u8] = <Vec::<u8> as AsRef<[u8]>>::as_ref(&x); let _r: &[u8] = <Vec::<u8>>::as_ref(&x); let…

[image] steffahn: Even in places where the :: part of the turbofish is not required it is still allowed . I actually just read that in the reference. It is necessary to disambiguate in expressions, but not in types, although it is still allowed. That's a dizzying amount of variety. …

Thank you steffahn! here is another interesting example of an error: #[derive(Debug, Default)] struct StructName<A = u32, B = String> { field_a: A, field_b: B, } fn main() { let a = StructName { field_a: String::from("Spring"), ..<StructName>::default() }; pri…

It seems to me that I have begun to better understand what is happening here. These two examples work, but having to put the right amount of underscores ... I can't found an elegant way to write this. Example A: #[derive(Debug, Default)] struct StructName<A=u32, B=bool, C=u8> { field_a: A, …

#[derive(Debug, Default)] struct StructName<A = u32, B = String> { field_a: A, field_b: B, } fn main() { let a = StructName::<_> { field_a: String::from("Spring"), ..<_>::default() // ^^^^^^^ emoticon of how I feel writing this ;-) }; println!("{:?}", a); }…

Strange behavior default generic parameter type

help

mgeisler May 15, 2021, 11:15am 20

I think it's related to this thread where @quinedot helped me:

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Strange behavior default generic parameter type

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