unreachable_code lint falsely warns entire function body is unreachable

Code

extern crate futures;
use futures::executor::block_on;
use futures::future::join;

fn test() {
    let a = async {};

    let b = async { loop {} };

    block_on(join(a, b));
}

Current output

warning: unreachable expression
  --> src/lib.rs:5:11
   |
 5 |   fn test() {
   |  ___________^
 6 | |     let a = async {};
 7 | |
 8 | |     let b = async { loop {} };
 9 | |
10 | |     block_on(join(a, b));
   | |     -------------------- any code following this expression is unreachable
11 | | }
   | |_^ unreachable expression
   |
note: this expression has type `((), !)`, which is uninhabited
  --> src/lib.rs:10:5
   |
10 |     block_on(join(a, b));
   |     ^^^^^^^^^^^^^^^^^^^^
   = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

Desired output

Rationale and extra context

I don't think this lint should trigger at all in this case, since it's the last expression in the function. It doesn't trigger in the same way if there is anything after it, and the block_on and join seem to be load bearing.

Other cases

Rust Version

Nightly:
> rustc --version --verbose
rustc 1.93.0-nightly (518b42830 2025-11-16)
binary: rustc
commit-hash: 518b428304e0008859cb1fd81d1ac20efb2a064a
commit-date: 2025-11-16
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.5

Beta:
> rustc --version --verbose
rustc 1.92.0-beta.3 (f3f12444a 2025-11-09)
binary: rustc
commit-hash: f3f12444a017add0468f683f3a10656d29438a05
commit-date: 2025-11-09
host: x86_64-unknown-linux-gnu
release: 1.92.0-beta.3
LLVM version: 21.1.3

Anything else?

No response

The lint also fires for match block_on(join(a, b)) {} despite the exhaustive match on the uninhabited type.

searched nightlies: from nightly-2025-08-01 to nightly-2025-10-16
regressed nightly: nightly-2025-10-13
searched commit range: be0ade2...2300c2a
regressed commit: ff6dc92

bisected with cargo-bisect-rustc v0.6.11

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2025-08-01 --end=2025-10-16

Minimized:

//@ edition: 2024
enum Never {}

fn make_never() -> Never {
    loop {}
}

fn check() -> () {
    make_never();
}

fn main() {
    check();
}

@cjgillot

#142390 cc @cjgillot - I guess the warning is correct but the 'unreachable expression' span is wrong. Seems like it's using the span of the implicit _0 = const () return value assignment, which is that of the whole function

Auto merge of #149044 - clubby789:implicit-return-span, r=cjgillot

Auto merge of rust-lang#149044 - clubby789:implicit-return-span, r=cj…

Auto merge of #149044 - clubby789:implicit-return-span, r=cjgillot

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`unreachable_code` lint falsely warns entire function body is unreachable #149042

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Current output

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Rationale and extra context

Other cases

Rust Version

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unreachable_code lint falsely warns entire function body is unreachable #149042

Description

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

Activity

ryanavella commented on Nov 17, 2025

clubby789 commented on Nov 17, 2025

clubby789 commented on Nov 17, 2025

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`unreachable_code` lint falsely warns entire function body is unreachable #149042