Using `as` to get `todo!` to work with `impl` return gives false unreachable code warning

[ChaiTRex]

Code

pub fn f() -> impl Iterator<Item = ()> {
    todo!() as core::iter::Empty<()>
}

Current output

warning: unreachable expression
 --> src/lib.rs:2:5
  |
2 |     todo!() as core::iter::Empty<()>
  |     -------^^^^^^^^^^^^^^^^^^^^^^^^^
  |     |
  |     unreachable expression
  |     any code following this expression is unreachable
  |
  = note: `#[warn(unreachable_code)]` on by default

warning: `playground` (lib) generated 1 warning

Desired output

No warning

Rationale and extra context

Using todo!() with a return type that is impl currently causes a type error. Using as to give it a specific type fixes that but brings this incorrect warning message.

as T by itself shouldn't really be considered an expression.

Other cases

No response

Rust Version

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: aarch64-apple-darwin
release: 1.77.2
LLVM version: 17.0.6

rustc 1.78.0-beta.6 (27011d5dc 2024-04-09)
binary: rustc
commit-hash: 27011d5dc09a8d1ac2289d024f54ecac9fc70d47
commit-date: 2024-04-09
host: aarch64-apple-darwin
release: 1.78.0-beta.6
LLVM version: 18.1.2

rustc 1.79.0-nightly (aa067fb98 2024-04-10)
binary: rustc
commit-hash: aa067fb984d36462548bb785da221bfaf38253f0
commit-date: 2024-04-10
host: aarch64-apple-darwin
release: 1.79.0-nightly
LLVM version: 18.1.3

Anything else?

No response

[cuviper]

as T by itself shouldn't really be considered an expression.

It's literally a type cast expression.

If we had type ascription, then I would agree that shouldn't look unreachable.

[ChaiTRex]

OK, then perhaps the problem is that todo!() as T should not be considered unreachable as if it comes after a statement that panics or returns or whatever, as it doesn't come after such an expression. It is that expression.

[cuviper]

I would say that the panicking todo!() is a subexpression of the todo!() as T expression, and it does render the rest of the expression unreachable, just like if you had todo!() + 1 or foo(todo!()).

Don't get me wrong though, I am sympathetic to the need for a type in todo!() with -> impl Trait. We just don't have a good way to write that yet.

[cuviper]

For iterators, if you're willing to have a lazy panic until the iterator is actually used, then this isn't too bad:

pub fn f() -> impl Iterator<Item = ()> {
    core::iter::repeat_with(|| todo!())
}

[eggyal]

Relevant recent discussion on IRLO.