The compiler could not infer a type and asked for a type annotation.
Erroneous code example:
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:
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.