Rust error E0107: missing generics for struct `Foo`

What it meansAn incorrect number of generic arguments was provided.

What rustc prints

error[E0107]: missing generics for struct `Foo`
 --> main.rs:3:17
  |
3 | struct Bar { x: Foo }             // error: wrong number of type arguments:
  |                 ^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `T`
 --> main.rs:1:8
  |
1 | struct Foo<T> { x: T }
  |        ^^^ -
help: add missing generic argument
  |
3 | struct Bar { x: Foo<T> }             // error: wrong number of type arguments:
  |                    +++

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.

E0107Types and traits

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

An incorrect number of generic arguments was provided.

Erroneous code example:

struct Foo<T> { x: T }

struct Bar { x: Foo }             // error: wrong number of type arguments:
                                  //        expected 1, found 0
struct Baz<S, T> { x: Foo<S, T> } // error: wrong number of type arguments:
                                  //        expected 1, found 2

fn foo<T, U>(x: T, y: U) {}
fn f() {}

fn main() {
    let x: bool = true;
    foo::<bool>(x);                 // error: wrong number of type arguments:
                                    //        expected 2, found 1
    foo::<bool, i32, i32>(x, 2, 4); // error: wrong number of type arguments:
                                    //        expected 2, found 3
    f::<'static>();                 // error: wrong number of lifetime arguments
                                    //        expected 0, found 1
}

When using/declaring an item with generic arguments, you must provide the exact same number:

struct Foo<T> { x: T }

struct Bar<T> { x: Foo<T> }               // ok!
struct Baz<S, T> { x: Foo<S>, y: Foo<T> } // ok!

fn foo<T, U>(x: T, y: U) {}
fn f() {}

fn main() {
    let x: bool = true;
    foo::<bool, u32>(x, 12);              // ok!
    f();                                  // ok!
}

From rustc --explain E0107 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Rust error E0107: missing generics for struct `Foo`

What rustc prints

The compiler team's explanation

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