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

Erroneous code example:

let x = Vec::new();

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.

In the example above, type Vec has a type parameter T. When calling Vec::new, barring any other later usage of the variable x that allows the compiler to infer what type T is, the compiler needs to be told what it is.

The type can be specified on the variable:

let x: Vec<i32> = Vec::new();

The type can also be specified in the path of the expression:

let x = Vec::<i32>::new();

In cases with more complex types, 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:

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.