The compiler could not infer a type and asked for a type annotation.

Erroneous code example:

```compile_fail,E0283
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<_>>();
```

We can see a self-contained example below:

```compile_fail,E0283
struct Foo;

impl Into<u32> for Foo {
    fn into(self) -> u32 { 1 }
}

let foo = Foo;
let bar: u32 = foo.into() * 1u32;
```

This error can be solved by adding type annotations that provide the missing
information to the compiler. In this case, the solution is to specify the
trait's type parameter:

```
struct Foo;

impl Into<u32> for Foo {
    fn into(self) -> u32 { 1 }
}

let foo = Foo;
let bar: u32 = Into::<u32>::into(foo) * 1u32;
```