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Add details on PhantomData

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This commit is contained in:
Alisdair Owens 2015-07-18 18:42:00 +01:00
parent ea79264ee4
commit f78333e052
1 changed files with 35 additions and 10 deletions
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45
src/librustc_typeck/diagnostics.rs
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@ -2112,10 +2112,8 @@ For more information see the [opt-in builtin traits RFC](https://github.com/rust
"##,
E0392: r##"
This error indicates that a type parameter has been declared but not actually
used.
Here is an example that demonstrates the error:
This error indicates that a type or lifetime parameter has been declared
but not actually used. Here is an example that demonstrates the error:
```
enum Foo<T> {
@ -2123,8 +2121,8 @@ enum Foo<T> {
}
```
The first way to fix this error is by removing the type parameter, as
shown below:
If the type parameter was included by mistake, this error can be fixed
by simply removing the type parameter, as shown below:
```
enum Foo {
@ -2132,7 +2130,8 @@ enum Foo {
}
```
The second method is to actually make use of the type parameter:
Alternatively, if the type parameter was intentionally inserted, it must be
used. A simple fix is shown below:
```
enum Foo<T> {
@ -2140,10 +2139,36 @@ enum Foo<T> {
}
```
See the 'Type Parameters' section of the reference for more details
on this topic:
This error may also commonly be found when working with unsafe code. For
example, when using raw pointers one may wish to specify the lifetime for
which the pointed-at data is valid. An initial attempt (below) causes this
error:
http://doc.rust-lang.org/reference.html#type-parameters-1
```
struct Foo<'a, T> {
x: *const T
}
```
We want to express the constraint that Foo should not outlive `'a`, because
the data pointed to by `T` is only valid for that lifetime. The problem is
that there are no actual uses of `'a`. It's possible to work around this
by adding a PhantomData type to the struct, using it to tell the compiler
to act as if the struct contained a borrowed reference `&'a T`:
```
use std::marker::PhantomData;
struct Foo<'a, T: 'a> {
x: *const T,
phantom: PhantomData<&'a T>
}
```
PhantomData can also be used to express information about unused type parameters.
You can read more about it in the API documentation:
https://doc.rust-lang.org/std/marker/struct.PhantomData.html
"##
}