Cycle detected while trying to transmute future

[Amulet9]

I tried this code:

use std::mem::MaybeUninit;

fn main() {
    let fut = async {};
    let cast: [MaybeUninit<u8>; 1] = unsafe { std::mem::transmute(fut) };
}

I was playing around and expected this to succeed, instead, it resulted in this:

error[E0391]: cycle detected when type-checking `main`
 --> src/main.rs:3:1
  |
3 | fn main() {
  | ^^^^^^^^^
  |
  = note: ...which requires computing layout of `{async block@src/main.rs:4:15: 4:23}`...
note: ...which requires optimizing MIR for `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires elaborating drops for `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires coroutine witness types for `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires promoting constants in MIR for `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires checking if `main::{closure#0}` contains FFI-unwind calls...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires building MIR for `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires match-checking `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
note: ...which requires type-checking `main::{closure#0}`...
 --> src/main.rs:4:15
  |
4 |     let fut = async {};
  |               ^^^^^^^^
  = note: ...which again requires type-checking `main`, completing the cycle
  = note: cycle used when running analysis passes on this crate
  = 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

Meta

rustc --version --verbose:

rustc 1.80.0-nightly (87293c958 2024-05-08)
binary: rustc
commit-hash: 87293c9585a7fb2cc83ca9949ae79661d5d3c31a
commit-date: 2024-05-08
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4