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