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

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

Reproduce the example in src/lib.rs with the listed rustc versions and confirm the unreachable_code warning. Trace the compiler's unreachable-code handling for the todo!() as core::iter::Empty<()> expression, then add a regression test showing that this case produces no warning and run the relevant compiler tests.