Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0121: the placeholder `_` is not allowed within types on item signatures for return types
What it meansThe type placeholder _ was used within a type on an item's signature.
What rustc prints
error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types --> main.rs:1:13 | 1 | fn foo() -> _ { 5 } // error | ^ not allowed in type signatures | help: replace with the correct return type | 1 - fn foo() -> _ { 5 } // error 1 + fn foo() -> i32 { 5 } // error |
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.
E0121Types and traits
Activein rustc 1.99.0
8related errors linked below
The compiler team's explanation
The type placeholder _ was used within a type on an item's signature.
Erroneous code example:
fn foo() -> _ { 5 } // error
static BAR: _ = "test"; // errorIn those cases, you need to provide the type explicitly:
fn foo() -> i32 { 5 } // ok!
static BAR: &str = "test"; // ok!The type placeholder _ can be used outside item's signature as follows:
let x = "a4a".split('4')
.collect::<Vec<_>>(); // No need to precise the Vec's generic type.From rustc --explain E0121 (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
- E0119conflicting implementations of trait for type
- E0120the `Drop` trait may only be implemented for local structs, enums, and unions
- E0128generic parameter defaults cannot reference parameters before they are declared
- E0131The `main` function was defined with generic parameters
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?