unconditional_recursion lint false positive on abort-on-drop type.

[chorman0773]

Code

struct DropBomb;

impl Drop for DropBomb {
    fn drop(&mut self) {
        let _this = DropBomb;
        panic!()
    }
}

Current output

warning: function cannot return without recursing
 --> src/lib.rs:5:5
  |
5 |     fn drop(&mut self) {
  |     ^^^^^^^^^^^^^^^^^^ cannot return without recursing
...
8 |     }
  |     - recursive call site
  |
  = help: a `loop` may express intention better if this is on purpose
  = note: `#[warn(unconditional_recursion)]` on by default

Desired output

No output; compiles as intended.

Rationale and extra context

The lint is semantically incorrect, in that the recursion occurs at most once (after that, the entire program is aborted due to double-panic). The help message is also not useful, because replacing the panic!() with a loop {} changes the program from aborting when dropping DropBomb, to hanging indefinitely.

This is a common construction in no_std for an AbortOnDrop type, usually to stop a program from unwinding out of a piece of unsafe code and leaving broken invariants.

Other cases

Rust Version

rustc 1.87.0-nightly (aa8f0fd71 2025-03-23)
binary: rustc
commit-hash: aa8f0fd7163a2f23aa958faed30c9c2b77b934a5
commit-date: 2025-03-23
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.1

Anything else?

Playground Link: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=8dbe0e6fb92d8f66787d7c3f14c386a5

[chorman0773]

@rustbot label +L-unconditional_recursion

[tmiasko]

The warning is correct about recursion with -Cpanic=unwind. Nested panics are allowed since 1.71.

[Jules-Bertholet]

Yes, testing on playground shows that this does in fact recurse forever.

[chorman0773]

Wasn't that defined to abort since forever?
Is there no way to abort on unwind from no_std then? I'm pretty sure that breaks just about any crate that relies on no-unwind for soundness.

[tmiasko]

It is possible to abort by panicking in a function that cannot unwind:

pub extern "C" fn abort() {
  panic!()
}