pub struct Generalization<T> {
    pub value: T,
    pub needs_wf: bool,
}
Expand description

Result from a generalization operation. This includes not only the generalized type, but also a bool flag indicating whether further WF checks are needed.

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§value: T§needs_wf: bool

If true, then the generalized type may not be well-formed, even if the source type is well-formed, so we should add an additional check to enforce that it is. This arises in particular around ‘bivariant’ type parameters that are only constrained by a where-clause. As an example, imagine a type:

struct Foo<A, B> where A: Iterator<Item = B> {
    data: A
}

here, A will be covariant, but B is unconstrained. However, whatever it is, for Foo to be WF, it must be equal to A::Item. If we have an input Foo<?A, ?B>, then after generalization we will wind up with a type like Foo<?C, ?D>. When we enforce that Foo<?A, ?B> <: Foo<?C, ?D> (or >:), we will wind up with the requirement that ?A <: ?C, but no particular relationship between ?B and ?D (after all, we do not know the variance of the normalized form of A::Item with respect to A). If we do nothing else, this may mean that ?D goes unconstrained (as in #41677). So, in this scenario where we create a new type variable in a bivariant context, we set the needs_wf flag to true. This will force the calling code to check that WF(Foo<?C, ?D>) holds, which in turn implies that ?C::Item == ?D. So once ?C is constrained, that should suffice to restrict ?D.

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impl<T: Debug> Debug for Generalization<T>

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fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more

Auto Trait Implementations§

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impl<T> RefUnwindSafe for Generalization<T>where T: RefUnwindSafe,

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impl<T> Send for Generalization<T>where T: Send,

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impl<T> Sync for Generalization<T>where T: Sync,

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impl<T> Unpin for Generalization<T>where T: Unpin,

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impl<T> UnwindSafe for Generalization<T>where T: UnwindSafe,

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impl<T> Any for Twhere T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for Twhere U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.

Layout§

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.