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Clarified why Sized bound not implicit on trait's implicit Self type. #48210

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kennytm merged 2 commits into from
Feb 15, 2018
Merged

Clarified why Sized bound not implicit on trait's implicit Self type. #48210

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Clarified why `Sized` bound not implicit on trait's implicit `Self` t…
jacob-hughes Feb 14, 2018
38064a9
Review change - Expanded on explanation.
jacob-hughes Feb 14, 2018
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10 changes: 7 additions & 3 deletions src/libcore/marker.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -63,9 +63,13 @@ impl<T: ?Sized> !Send for *mut T { }
/// struct BarUse(Bar<[i32]>); // OK
/// ```
///
/// The one exception is the implicit `Self` type of a trait, which does not
/// get an implicit `Sized` bound. This is because a `Sized` bound prevents
/// the trait from being used to form a [trait object]:
/// The one exception is the implicit `Self` type of a trait. A trait does not
/// have an implicit `Sized` bound as this is incompatible with [trait object]s
/// where, by definition, the trait needs to work with all possible implementors,
/// and thus could be any size.
///
/// Although Rust will let you bind `Sized` to a trait, you won't
/// be able to use it to form a trait object later:
///
/// ```
/// # #![allow(unused_variables)]
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