Compiler hangs with uninitialized variable containing a struct that contains itself #153205
Description
The following code causes rustc to hang when built with cargo run --release
trait Apply {
type Output<T>;
}
struct Identity;
impl Apply for Identity {
type Output<T> = T;
}
struct Thing<A: Apply>(A::Output<Self>);
/*
Effectively:
struct Thing_Identity(Thing_Identity);
*/
fn foo<A: Apply>() {
let _x: Thing<A>;
}
fn main() {
foo::<Identity>();
}Compiler output before the hang:
Compiling playground v0.0.1 (/playground)
error[E0391]: cycle detected when computing layout of `Thing<Identity>`
|
= note: ...which requires computing layout of `<Identity as Apply>::Output<Thing<Identity>>`...
= note: ...which again requires computing layout of `Thing<Identity>`, completing the cycle
note: cycle used when optimizing MIR for `main`
--> src/main.rs:20:1
|
20 | fn main() {
| ^^^^^^^^^
= note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
See also #152885
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Reproducible on the playground with version 1.95.0-nightly (2026-02-24 859951e3c7c9d0322c39)