Rust errors / Ownership and borrowing
Rust compiler error · checked with rustc 1.99.0
Rust error E0228: cannot deduce the lifetime bound for this trait object type from context
What it meansThe lifetime bound for this object type cannot be deduced from context and must be specified.
What rustc prints
error[E0228]: cannot deduce the lifetime bound for this trait object type from context --> main.rs:9:38 | 9 | type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>; | ^^^^^^^^^ | help: please supply an explicit bound | 9 | type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + /* 'a */>; | ++++++++++
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
The lifetime bound for this object type cannot be deduced from context and must be specified.
Erroneous code example:
trait Trait { }
struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
x: &'a i32,
y: &'b i32,
z: T,
}
type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>;When a trait object is used as a type argument of a generic type, Rust will try to infer its lifetime if unspecified. However, this isn't possible when the containing type has more than one lifetime bound.
The above example can be resolved by either reducing the number of lifetime bounds to one or by making the trait object lifetime explicit, like so:
trait Trait { }
struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
x: &'a i32,
y: &'b i32,
z: T,
}
type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + 'b>;For more information, see RFC 599 and its amendment RFC 1156.
From rustc --explain E0228 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
Related errors
- E0382borrow of moved value
- E0499cannot borrow as mutable more than once at a time
- E0502cannot borrow as mutable because it is also borrowed as immutable
- E0106missing lifetime specifier
- E0226only a single explicit lifetime bound is permitted
- E0227ambiguous lifetime bound, explicit lifetime bound required
- E0261use of undeclared lifetime name `'a`
- E0262invalid lifetime parameter name: `'static`
Up next
Often hit nextE0382: borrow of moved valueA related error from the same family.Read it next →Did this answer your question?