1.92.0 warns about unreachable code, but that code can't be removed without type errors

[ericseppanen]

I tried this code:

use std::process::ExitCode;

enum Never {}

fn never_returns() -> Never {
    todo!()
}

pub fn run() -> ExitCode {
    never_returns();
    ExitCode::FAILURE
}

And rustc raises an "unreachable expression" warning. This seems reasonable. So I remove that line:

use std::process::ExitCode;

enum Never {}

fn never_returns() -> Never {
    todo!()
}

pub fn run() -> ExitCode {
    never_returns();
}

I expected to see this happen:

• either rustc can tell that that path is unreachable, and it should not have type errors,
• or rustc can't tell it's unreachable, and the "unreachable expression" warning should not appear.

Instead, this happened: rustc seems to simultaneously indicate that path is unreachable, AND some expression is needed there to satisfy the type-checker.

Is there really no way to write the run function without invoking #[allow(unreachable_code)]? That seems odd. (Sorry, I shouldn't have asked. Didn't mean to nerd-snipe.)

Meta

rustc --version --verbose:

rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.93.1
LLVM version: 21.1.8

(Happens on any stable version >= 1.92.0; and also on nightly 2026-02-11)

[theemathas]

This produces a warning on 1.92.0 but not 1.91.0

[workingjubilee]

Is there really no way to write the run function without invoking #[allow(unreachable_code)]? That seems odd.

@ericseppanen I believe the type error is quite intentional, but the lint firing is less clearly intended as it seems to be one of the many instances of "our regression test suite for the unused-thingy lints sure is growing a lot ever since #142390 landed."

[eggyal]

@ericseppanen I'm sure you already realise this but perhaps for the benefit of anyone else who lands here: if never_returns is declared with the ! return type, then all versions (certainly from 1.91 through to 1.93.1) exhibit the same behaviour (warning that code following the call to never_returns() is unreachable and otherwise type checking run without problem):

use std::process::ExitCode;

fn never_returns() -> ! {
    todo!()
}

pub fn run() -> ExitCode {
    never_returns();
}

The issue with the code in the OP is of course that, whilst enum Never {} is an unoccupied type and therefore can never be constructed/returned, rustc does not examine type information for a function call beyond what is in that function's signature.

[ericseppanen]

I'm sure you already realise this but perhaps for the benefit of anyone else who lands here: if never_returns is declared with the ! return type, then all versions (certainly from 1.91 through to 1.93.1) exhibit the same behaviour (warning that code following the call to never_returns() is unreachable and otherwise type checking run without problem) (...)

@eggyal The example I gave was reduced from some real code that returns Result<Never, anyhow::Error>, and on stable it's not possible to use Result<!, _>.