tait: "cycle detected when computing type" when using transmute.

[Dirbaio]

playground

#![feature(type_alias_impl_trait)]

type Tait = impl Sized;

fn main() {
    let foo1: Tait = 0u32;
    let foo2: Tait = unsafe { core::mem::transmute(foo1) };
}

gives

error[[E0391]](https://doc.rust-lang.org/nightly/error-index.html#E0391): cycle detected when computing type of `Tait::{opaque#0}`
 --> src/main.rs:3:13
  |
3 | type Tait = impl Sized;
  |             ^^^^^^^^^^
  |
note: ...which requires type-checking `main`...
 --> src/main.rs:5:1
  |
5 | fn main() {
  | ^^^^^^^^^
  = note: ...which requires computing layout of `Tait`...
  = note: ...which requires normalizing `Tait`...
  = note: ...which again requires computing type of `Tait::{opaque#0}`, completing the cycle
note: cycle used when checking item types in top-level module
 --> src/main.rs:1:1
  |
1 | / #![feature(type_alias_impl_trait)]
2 | |
3 | | type Tait = impl Sized;
4 | |
... |
7 | |     let foo2: Tait = unsafe { core::mem::transmute(foo1) };
8 | | }
  | |_^

error[[E0512]](https://doc.rust-lang.org/nightly/error-index.html#E0512): cannot transmute between types of different sizes, or dependently-sized types
 --> src/main.rs:7:31
  |
7 |     let foo2: Tait = unsafe { core::mem::transmute(foo1) };
  |                               ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `Tait` does not have a fixed size

I find it quite confusing that the transmute is not allowed. They're the same type after all.

@rustbot label A-impl-trait F-type_alias_impl_trait

[RodBurman]

Using nightly toolchain:

% cargo -v -V
cargo 1.87.0-nightly (2622e844b 2025-02-28)
release: 1.87.0-nightly
commit-hash: 2622e844bc1e2e6123e54e94e4706f7b6195ce3d
commit-date: 2025-02-28
host: aarch64-apple-darwin
libgit2: 1.9.0 (sys:0.20.0 vendored)
libcurl: 8.7.1 (sys:0.4.79+curl-8.12.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 3.4.1 11 Feb 2025
os: Mac OS 15.3.1 [64-bit]

Gives a simpler error:

% cargo build
   Compiling taiter v0.1.0 (/Users/rod/code/rust/triage/taiter)
error[E0308]: mismatched types
 --> src/main.rs:6:22
  |
3 | type Tait = impl Sized;
  |             ---------- the expected opaque type
...
6 |     let foo1: Tait = 0u32;
  |               ----   ^^^^ expected opaque type, found `u32`
  |               |
  |               expected due to this
  |
  = note: expected opaque type `Tait`
                    found type `u32`
note: this item must have the opaque type in its signature in order to be able to register hidden types
 --> src/main.rs:5:4
  |
5 | fn main() {
  |    ^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `taiter` (bin "taiter") due to 1 previous error

Note the current stable build 1.85 gives the same error along with the addition of:

error[E0658]: `impl Trait` in type aliases is unstable

which error is to be expected.

[oli-obk]

Considering that

#![feature(type_alias_impl_trait)]

type Tait = impl Sized;

fn bar(_: Tait) {
    let foo1: Tait = 0u32;
    let foo2: Tait = unsafe { core::mem::transmute(foo1) };
}

Compiles fine now, I consider this fixed.

I don't think it needs a dedicated test either. The new system is much more robust