Code
#![feature(type_alias_impl_trait)]
#[diagnostic::on_unimplemented(message = "my custom message")]
trait Marker {}
type Ta = impl Marker;
#[define_opaque(Ta)]
fn construct() -> Ta {
5u32
}
fn main() {}Current output
error[E0271]: type mismatch resolving `Ta == u32`
--> src/main.rs:9:19
|
9 | fn construct() -> Ta {
| ^^ types differDesired output
error[E0277]: my custom message
--> src/main.rs:9:19
|
9 | fn construct() -> Ta {
| ^^ the trait `Marker` is not implemented for `u32`
10 | 5u32
| ---- return type was inferred to be `u32` hereRationale and extra context
With the old solver you would get the good error message "u32 doesn't implement Marker".
With the new solver it's just a type mismatch, no indication why.
A consequence of this is the #[on_unimplemented] message is not shown to the user. We were using it in embassy-executor to give better error messages, see here.
Other cases
Rust Version
rustc 1.100.0-nightly (c656540d6 2026-08-21)
binary: rustc
commit-hash: c656540d6467dee1381f0cbd882412d6bd1cd5ae
commit-date: 2026-08-21
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0
Anything else?
No response
Code
Current output
Desired output
Rationale and extra context
With the old solver you would get the good error message "u32 doesn't implement Marker".
With the new solver it's just a type mismatch, no indication why.
A consequence of this is the
#[on_unimplemented]message is not shown to the user. We were using it inembassy-executorto give better error messages, see here.Other cases
Rust Version
Anything else?
No response