RA sometimes panics and raises an error incorrectly for closure -> fn pointer coercion

[ElliottjPierce]

I'm encountering an issue where RA falsely claims that closure to fn pointer coercion is not allowed.
Ex: "expected unsafe fn(ErasedUninitRef<'_>), found {closure#591559}".
Further, RA appears to be panicking when this happens.

Other issues may be related, but I couldn't find one.
If I missed one, hopefully another data point helps.

Here's the log:

thread 'Worker6' (20744104) panicked at /Users/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rowan-0.15.18/src/cursor.rs:755:9:
Bad offset: range 17053..17071 offset 17036
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: rowan::cursor::SyntaxNode::token_at_offset
   3: ide_assists::handlers::extract_variable::extract_token_range_of
   4: ide_assists::handlers::extract_variable::extract_variable
   5: ide_assists::assists
   6: salsa::cancelled::Cancelled::catch
   7: std::thread::local::LocalKey<T>::with
   8: ide::Analysis::assists_with_fixes
   9: rust_analyzer::handlers::request::handle_code_action
  10: core::ops::function::FnOnce::call_once{{vtable.shim}}
  11: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
additional context:
   0:
version: 0.3.2913-standalone (de5824b6d0 2026-05-24)
request: textDocument/codeAction CodeActionParams {
    text_document: TextDocumentIdentifier {
        uri: Url {
            scheme: "file",
            cannot_be_a_base: false,
            username: "",
            password: None,
            host: None,
            port: None,
            path: "path/to/lib/macros/src/lib.rs",
            query: None,
            fragment: None,
        },
    },
    range: Range {
        start: Position {
            line: 385,
            character: 64,
        },
        end: Position {
            line: 385,
            character: 79,
        },
    },
    context: CodeActionContext {
        diagnostics: [],
        only: None,
        trigger_kind: None,
    },
    work_done_progress_params: WorkDoneProgressParams {
        work_done_token: None,
    },
    partial_result_params: PartialResultParams {
        partial_result_token: None,
    },
}

For additional context, I've had this issue with rust 1.95 and 1.96. The issue may have existed before then; it only breaks when I open the offending file, and this is just when I noticed it. I am using the Zed editor primarily, but this also happens in VS Code. Both cargo check and cargo build across various profiles, etc work. It's only RA that fails here.

The offending code looks something like this:

pub enum Foo {
    A,
    B,
}

pub trait Initializer {
    const INITIALIZER: unsafe fn(*mut u8);
}

impl Initializer for Foo {
    const INITIALIZER: unsafe fn(*mut u8) = const {
        let func: fn(*mut u8) = |dst| unsafe {
            dst.cast::<Self>().write(Self::A);
        };
        func
    };
}

Interestingly, RA doesn't break with just this code, but does in my case, the only major difference being that the implementation is coming from a derive macro. I was not able to reproduce the whole error in a separate example (and not for lack of trying), but I was able to verify that the panic was coming from the derive macro file. Why RA reports it as an error is still a mystery to me. I suspect it has something to do with how the closure captures the Self type. This is allowed, (It's captured variables that aren't.) but maybe RA doesn't see that when it's coming from complex derive macros?

Here's a zip of the min reproduction I made for the panic: reproduce_ra_false_positive.zip. If there's anything you'd like me to try to help reproduce it or investigate, I'm happy to help. Thanks for looking into this!

[ChayimFriedman2]

The attached code does not reproduce the error for me.

Also, this panic is very unlikely to be related to the error.

[ChayimFriedman2]

Also, what is your rust-analyzer version and what is your rust-analyzer settings?

[ElliottjPierce]

RA settings are default except for:

"initialization_options": {
        "assist": { "expressionFillDefault": "default" },
        "check": {
          "command": "clippy"
        },
        "imports": {
          "preferNoStd": true
        }
      }

The version is 0.3.2913-standalone (de5824b6d0 2026-05-24) according to the log. At least, I think that version is referring to RA. (I just did rustup upgrade before opening this, so I'm hopping that's the latest.)

Also, I'm getting more false positives now related to other things, like imports and crate features, which is strange. Cargo still works fine, etc, but RA doesn't. After a cargo clean, those other things seem to be fixed, but not this closure fn pointer coercion.

The false error only shows up when the file is open and goes away when the file is closed, which makes sense. I'm sorry I haven't been able to build a re-production of it, but I'll give it another shot when I get the chance unless anything obvious stands out to you before then. Thanks for helping me with this!