Error code E0282
The compiler could not infer a type and asked for a type annotation.
Erroneous code example:
#![allow(unused)] fn main() { 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:
#![allow(unused)] fn main() { let x: Vec<i32> = Vec::new(); }
The type can also be specified in the path of the expression:
#![allow(unused)] fn main() { 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:
#![allow(unused)] fn main() { let x: Vec<_> = "hello".chars().rev().collect(); }
Another way to provide the compiler with enough information, is to specify the generic type parameter:
#![allow(unused)] fn main() { let x = "hello".chars().rev().collect::<Vec<char>>(); }
Again, you need not specify the full type if the compiler can infer it:
#![allow(unused)] fn main() { 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:
#![allow(unused)] fn main() { 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.