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The compiler could not infer a type and asked for a type annotation.
Erroneous code example:
```compile_fail,E0282
let x = "hello".chars().rev().collect();
```
This error indicates that type inference did not result in one unique possible
type, and extra information is required. In most cases this can be provided
by adding a type annotation. Sometimes you need to specify a generic type
parameter manually.
A common example is the `collect` method on `Iterator`. It has a generic type
parameter with a `FromIterator` bound, which for a `char` iterator is
implemented by `Vec` and `String` among others. Consider the following snippet
that reverses the characters of a string:
In the first code example, the compiler cannot infer what the type of `x` should
be: `Vec<char>` and `String` are both suitable candidates. To specify which type
to use, you can use a type annotation on `x`:
```
let x: Vec<char> = "hello".chars().rev().collect();
```
It is not necessary to annotate the full type. Once the ambiguity is resolved,
the compiler can infer the rest:
```
let x: Vec<_> = "hello".chars().rev().collect();
```
Another way to provide the compiler with enough information, is to specify the
generic type parameter:
```
let x = "hello".chars().rev().collect::<Vec<char>>();
```
Again, you need not specify the full type if the compiler can infer it:
```
let x = "hello".chars().rev().collect::<Vec<_>>();
```
Apart from a method or function with a generic type parameter, this error can
occur when a type parameter of a struct or trait cannot be inferred. In that
case it is not always possible to use a type annotation, because all candidates
have the same return type. For instance:
```compile_fail,E0282
struct Foo<T> {
num: T,
}
impl<T> Foo<T> {
fn bar() -> i32 {
0
}
fn baz() {
let number = Foo::bar();
}
}
```
This will fail because the compiler does not know which instance of `Foo` to
call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.
|
