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Box<dyn T> #8

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mio-19 opened this issue Apr 3, 2020 · 3 comments · May be fixed by #9
Open

Box<dyn T> #8

mio-19 opened this issue Apr 3, 2020 · 3 comments · May be fixed by #9

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@mio-19
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mio-19 commented Apr 3, 2020

I somehow got this error:

error[E0277]: the size for values of type `dyn Values` cannot be known at compilation time
  --> src/lib.rs:54:49
   |
54 |         self.unsafe_write(THREADED_OBJECT_SPACE.create(Box::new(x))).await;
   |                                                 ^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn Values`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required because of the requirements on the impl of `_::_gcmodule::Trace` for `std::boxed::Box<dyn Values>`

https://github.com/The-Lingo/The-Lingo/blob/73f5827b7cc8da1b3ab264cc0d986c917c54abc4/rust/the-lingo/src/lib.rs
@quark-zju
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quark-zju commented Apr 3, 2020 •
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The error is a bit confusing. It does not explain why dyn Values needs to be sized.

It is actually that Box<dyn Values> needs to implement Trace and in trace_impl.rs only Box<T: Trace + Sized> and Box<dyn Trace> implements Trace. Box<dyn Values> does not implement Trace, because dyn Values is not sized.

I think a solution would be adding a new type:

struct BoxValues(Box<dyn Values>);

Implement Trace on it, then use ThreadedCc<BoxValues>.

@mio-19
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mio-19 commented Apr 4, 2020

gcmodule/src/trace_impls.rs

Line 89 in a8681f1

impl<T: Trace> Trace for Box<T> {

But it's impl<T: Trace> Trace for Box<T>, not impl<T: Trace + Sized> Trace for Box<T>

@quark-zju
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By default trait bounds are Sized, unless explicitly specified + ?Sized, which allows unsized types.

@mio-19 mio-19 linked a pull request Apr 4, 2020 that will close this issue
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Box<dyn _> archive-tsao-chi-forks/gcmodule
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@quark-zju @mio-19