FP `expected bool, found ()`

The following code

// https://github.com/rust-lang/rust/issues/5741
//@ run-pass
#![allow(unreachable_code)]
#![allow(unreachable_code)]

pub fn main() {
    return//@ run-pass
    while {
    return;
    while true {};
} {};
}

comiled with rustc (cargo check):

    Blocking waiting for file lock on package cache
    Blocking waiting for file lock on package cache
    Blocking waiting for file lock on package cache
    Checking _1775848665622375556 v0.1.0 (/tmp/icemaker_global_tempdir.NKMZ5efnG5rg/rustc_testrunner_tmpdir.9JCMOMjDo2r6/_1775848665622375556)
warning: denote infinite loops with `loop { ... }`
  --> src/main.rs:10:5
   |
10 |     while true {};
   |     ^^^^^^^^^^ help: use `loop`
   |
   = note: `#[warn(while_true)]` on by default

warning: `_1775848665622375556` (bin "_1775848665622375556") generated 1 warning (run `cargo fix --bin "_1775848665622375556" -p _1775848665622375556` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s

but errors were issued by rust-analyzer diagnostics

at crate _1775848665622375556, file /tmp/icemaker_global_tempdir.NKMZ5efnG5rg/rustc_testrunner_tmpdir.9JCMOMjDo2r6/_1775848665622375556/src/main.rs: Error RustcHardError("E0308") from LineCol { line: 10, col: 0 } to LineCol { line: 10, col: 1 }: expected bool, found ()
at crate _1775848665622375556, file /tmp/icemaker_global_tempdir.NKMZ5efnG5rg/rustc_testrunner_tmpdir.9JCMOMjDo2r6/_1775848665622375556/src/main.rs: WeakWarning Clippy("needless_return") from LineCol { line: 6, col: 4 } to LineCol { line: 10, col: 5 }: replace return <expr>; with <expr>

diagnostic scan complete


Error: diagnostic error detected

Stack backtrace:
   0: <anyhow::Error>::msg::<&str>
             at /home/matthias/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/anyhow-1.0.102/src/backtrace.rs:10:14
   1: anyhow::__private::format_err
             at /home/matthias/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/anyhow-1.0.102/src/lib.rs:687:13
   2: <rust_analyzer::cli::flags::Diagnostics>::run_
             at /home/matthias/vcs/github/rust-analyzer/crates/rust-analyzer/src/cli/diagnostics.rs:123:13
   3: <rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}
             at /home/matthias/vcs/github/rust-analyzer/crates/rust-analyzer/src/cli/diagnostics.rs:20:29
   4: <stdx::thread::Builder>::spawn::<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}
             at /home/matthias/vcs/github/rust-analyzer/crates/stdx/src/thread.rs:74:13
   5: std::sys::backtrace::__rust_begin_short_backtrace::<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/backtrace.rs:166:18
   6: std::thread::lifecycle::spawn_unchecked::<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}::{closure#0}
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/lifecycle.rs:70:13
   7: <core::panic::unwind_safe::AssertUnwindSafe<std::thread::lifecycle::spawn_unchecked<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panic/unwind_safe.rs:275:9
   8: std::panicking::catch_unwind::do_call::<core::panic::unwind_safe::AssertUnwindSafe<std::thread::lifecycle::spawn_unchecked<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}::{closure#0}>, core::result::Result<(), anyhow::Error>>
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:576:43
   9: std::panicking::catch_unwind::<core::result::Result<(), anyhow::Error>, core::panic::unwind_safe::AssertUnwindSafe<std::thread::lifecycle::spawn_unchecked<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}::{closure#0}>>
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:544:19
  10: std::panic::catch_unwind::<core::panic::unwind_safe::AssertUnwindSafe<std::thread::lifecycle::spawn_unchecked<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}::{closure#0}>, core::result::Result<(), anyhow::Error>>
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:359:14
  11: std::thread::lifecycle::spawn_unchecked::<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1}
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/lifecycle.rs:68:26
  12: <std::thread::lifecycle::spawn_unchecked<<stdx::thread::Builder>::spawn<<rust_analyzer::cli::flags::Diagnostics>::run::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#0}, core::result::Result<(), anyhow::Error>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
             at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
  13: <alloc::boxed::Box<dyn core::ops::function::FnOnce<(), Output = ()> + core::marker::Send> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/34baba5394fcbda4cba7b7c1964a6db421c77c91/library/alloc/src/boxed.rs:2320:9
  14: <std::sys::thread::unix::Thread>::new::thread_start
             at /rustc/34baba5394fcbda4cba7b7c1964a6db421c77c91/library/std/src/sys/thread/unix.rs:123:17
  15: <unknown>
  16: <unknown>

version information

rust-analyzer 0.0.0 (4bd323d51e 2026-08-17)

Start with crates/rust-analyzer/src/cli/diagnostics.rs at the reported failure near line 123, then reproduce the issue with the supplied Rust snippet and the rust-analyzer diagnostics command. Done means the valid cargo-check result does not produce the spurious E0308 diagnostic and the diagnostics scan completes without reporting an error.