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diesel::migrations::run_migrations has a weird type signature #593

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jethrogb opened this Issue Jan 28, 2017 · 2 comments

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@jethrogb @weiznich
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jethrogb commented Jan 28, 2017 •
edited

List: IntoIterator<Item=M>, M: Deref<Target=Migration> means that you can't pass in a slice of &Migration. Arrays don't implement IntoIterator. This means you have to allocate a container of references, but that's not possible because references passed in need to be 'static. So, you need to pass in a container of boxes.
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weiznich commented Jan 29, 2017

It's working without the need of having a box. Iterator itself implements IntoIterator, therefore it is sufficient to call .iter() on the array.
See the embedded_migration code for an example. (In special line 56)
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jethrogb commented Jan 29, 2017 •
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That only works because the references there are 'static. The Box is necessary for the implicit 'static bound on M. But you're right, I can get rid of the collection like so: [Box::new(my_migration) as Box<Migration>].iter().map(|v|&**v)

sgrif added a commit that referenced this issue Feb 2, 2017

@sgrif
Remove the implicit `'static` bound from `run_migrations` 
Rust has the undocumented behavior of implying `+ 'static` any time you
reference a trait object directly. Normally this isn't a problem since
the only time you'd usually do that is as `Box<Trait>`, where you can
just write `Box<Trait + 'a>` instead. We can't do that here, however,
sicne `Deref<Target=Trait + 'a>` is not valid syntax. I'm not sure if
there's a workaround to keep the old code that I'm missing, or if this
is legitimately a problem in the language.

Either way it doesn't matter, we can just implement the trait on box and
ref and everything is fine.

I did not include a changelog entry, as all previous callers of this
function should continue to work, and this is a function that is
documented as "you probably should be using something else"

Fixes #593.
4eb0f62

@sgrif sgrif referenced this issue Feb 2, 2017

Merged

Remove the implicit `'static` bound from `run_migrations` #606

@sgrif sgrif closed this in #606 Feb 2, 2017

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