Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0666: nested `impl Trait` is not allowed
What it meansimpl Trait types cannot appear nested in the generic arguments of other impl Trait types.
What rustc prints
error[E0666]: nested `impl Trait` is not allowed
--> main.rs:5:30
|
5 | bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
| --------------------^^^^^^^^^^^^^^^^^-
| | |
| | nested `impl Trait` here
| outer `impl Trait`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.
E0666Types and traits
Activein rustc 1.99.0
8related errors linked below
The compiler team's explanation
impl Trait types cannot appear nested in the generic arguments of other impl Trait types.
Erroneous code example:
trait MyGenericTrait<T> {}
trait MyInnerTrait {}
fn foo(
bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
) {}Type parameters for impl Trait types must be explicitly defined as named generic parameters:
trait MyGenericTrait<T> {}
trait MyInnerTrait {}
fn foo<T: MyInnerTrait>(
bar: impl MyGenericTrait<T>, // ok!
) {}From rustc --explain E0666 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
Related errors
- E0308mismatched types
- E0277the trait bound is not satisfied
- E0599no method named X found in the current scope
- E0433failed to resolve
- E0644closure/coroutine type that references itself
- E0665`#[derive(Default)]` on enum with no `#[default]`
- E0689can't call method `neg` on ambiguous numeric type `{float}`
- E0690transparent struct needs at most one non-trivial field, but has 2
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?