FP `non-primitive cast`

[matthiaskrgr]

The following code

// run-pass
// Regression test for issue #39292. The object vtable was being
// incorrectly left with a null pointer.

trait Foo<T> {
    fn print<'a>(&'a self) where [(); 0 + 0]: Sized { println!("foo"); }
}

impl<'a> Foo<&'a ()> for () { }

trait Bar: for<'a> Foo<&'a ()> { }

impl Bar for () {}

fn main() {
    (&() as &dyn Bar).print(); // Segfault
}

comiled with rustc (cargo check):

    Blocking waiting for file lock on package cache
    Checking _1773141613043550384 v0.1.0 (/tmp/icemaker_global_tempdir.NKMZ5efnG5rg/rustc_testrunner_tmpdir.LbzPWgo9Z4kp/_1773141613043550384)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.24s

but errors were issued by rust-analyzer diagnostics

at crate _1773141613043550384, file /tmp/icemaker_global_tempdir.NKMZ5efnG5rg/rustc_testrunner_tmpdir.LbzPWgo9Z4kp/_1773141613043550384/src/main.rs: Error RustcHardError("E0605") from LineCol { line: 15, col: 5 } to LineCol { line: 15, col: 20 }: non-primitive cast: `&()` as `&(dyn Bar + 'static)`

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)

[dfireBird]

This happens because coerce while cast checking fails here:

rust-analyzer/crates/hir-ty/src/infer/cast.rs

Lines 132 to 145 in 6574fdb

	// This should always come first so that we apply the coercion, which impacts infer vars.
	if ctx
	.coerce(
	self.source_expr,
	self.expr_ty,
	self.cast_ty,
	AllowTwoPhase::No,
	ExprIsRead::Yes,
	)
	.is_ok()
	{
	ctx.result.coercion_casts.insert(self.source_expr);
	return Ok(());
	}

Not sure why the coerce fails, the types are same in rustc and here.

[ChayimFriedman2]

We should probably re-sync cast checking anyway.

[dfireBird]

Well we can do that but I don't think it's that. I did some digging and the same code in rustc gets coerce_unsize as Ok, while we return Err and it comes from here:

rust-analyzer/crates/hir-ty/src/infer/coerce.rs

Lines 681 to 697 in 9f4667f

	if self
	.delegate
	.infcx()
	.visit_proof_tree(
	Goal::new(self.infcx().interner, self.param_env(), pred),
	&mut CoerceVisitor {
	delegate: &self.delegate,
	errored: false,
	unsize_did,
	coerce_unsized_did,
	span: self.cause.span(),
	},
	)
	.is_break()
	{
	return Err(TypeError::Mismatch);
	}

Which from what I've seen, we have the same goals as rustc, but visit_proof_tree fails for us, specifically this goal.result() results in an Err for us, while in rustc it's an Ok.

I can do some more digging on why it's return an Err but that's state for now.