Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0741: `A` must implement `ConstParamTy` to be used as the type of a const generic parameter
What it meansA non-structural-match type was used as the type of a const generic parameter.
What rustc prints
error[E0741]: `A` must implement `ConstParamTy` to be used as the type of a const generic parameter --> main.rs:5:19 | 5 | struct B<const X: A>; // error! | ^ | help: add `#[derive(ConstParamTy, PartialEq, Eq)]` to the struct | 3 + #[derive(ConstParamTy, PartialEq, Eq)] 4 | struct 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
A non-structural-match type was used as the type of a const generic parameter.
Erroneous code example:
#![feature(adt_const_params)]
struct A;
struct B<const X: A>; // error!Only structural-match types, which are types that derive PartialEq and Eq and implement ConstParamTy, may be used as the types of const generic parameters.
To fix the previous code example, we derive PartialEq, Eq, and ConstParamTy:
#![feature(adt_const_params)]
use std::marker::ConstParamTy;
#[derive(PartialEq, Eq, ConstParamTy)] // We derive both traits here.
struct A;
struct B<const X: A>; // ok!From rustc --explain E0741 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
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- E0743C-variadic type `...` may not be nested inside another type
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