Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0631: type mismatch in closure arguments
What it meansThis error indicates a type mismatch in closure arguments.
What rustc prints
error[E0631]: type mismatch in closure arguments --> main.rs:5:5 | 5 | foo(|x: &str| {}); | ^^^^---------^^^^ | | | | | found signature defined here | expected due to this | = note: expected closure signature `fn(i32) -> _` found closure signature `fn(&str) -> _` note: required by a bound in `foo` --> main.rs:1:11 | 1 | fn foo<F: Fn(i32)>(f: F) { | ^^^^^^^ required by this bound in `foo`
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
This error indicates a type mismatch in closure arguments.
Erroneous code example:
fn foo<F: Fn(i32)>(f: F) {
}
fn main() {
foo(|x: &str| {});
}The error occurs because foo accepts a closure that takes an i32 argument, but in main, it is passed a closure with a &str argument.
This can be resolved by changing the type annotation or removing it entirely if it can be inferred.
fn foo<F: Fn(i32)>(f: F) {
}
fn main() {
foo(|x: i32| {});
}From rustc --explain E0631 (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
- E0617can't pass `f32` to variadic function
- E0620cast to unsized type: `&[usize; 2]` as `[usize]`
- E0634type has conflicting packed representation hints
- E0642patterns aren't allowed in functions without bodies
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?