Merge pull request #276 from JohnTitor/phantomdata-link

Add a link to show why unused lifetimes on structs are forbidden
pull/280/head
Eric Huss 3 years ago committed by GitHub
commit fa1326c429
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23

@ -13,10 +13,12 @@ struct Iter<'a, T: 'a> {
```
However because `'a` is unused within the struct's body, it's *unbounded*.
Because of the troubles this has historically caused, unbounded lifetimes and
types are *forbidden* in struct definitions. Therefore we must somehow refer
to these types in the body. Correctly doing this is necessary to have
correct variance and drop checking.
[Because of the troubles this has historically caused][unused-param],
unbounded lifetimes and types are *forbidden* in struct definitions.
Therefore we must somehow refer to these types in the body.
Correctly doing this is necessary to have correct variance and drop checking.
[unused-param]: https://rust-lang.github.io/rfcs/0738-variance.html#the-corner-case-unused-parameters-and-parameters-that-are-only-used-unsafely
We do this using `PhantomData`, which is a special marker type. `PhantomData`
consumes no space, but simulates a field of the given type for the purpose of
@ -25,7 +27,7 @@ the type-system the kind of variance that you want, while also providing other
useful things such as the information needed by drop check.
Iter logically contains a bunch of `&'a T`s, so this is exactly what we tell
the PhantomData to simulate:
the `PhantomData` to simulate:
```rust
use std::marker;
@ -63,7 +65,7 @@ soundness. This will in turn allow people to create unsoundness using
Vec's destructor.
In order to tell dropck that we *do* own values of type T, and therefore may
drop some T's when *we* drop, we must add an extra PhantomData saying exactly
drop some T's when *we* drop, we must add an extra `PhantomData` saying exactly
that:
```rust

Loading…
Cancel
Save