[ICE]: `find_attr!` panics for `SyntheticCoroutineBody` (in Miri, related to rustc_no_writable)

[quiode]

Some time ago, I introduced some changes in Miri (rust-lang/miri#4988) that made it use the find_attr! macro. I then ran Miri with those changes on some crates and found a compiler panic. As the panic originates inside the find_attr! logic and not from Miri, I assume this is the right place to open the issue.

It seems that when the find_attr! is called on an external crate on a specific type of function, the logic does not handle it correctly. To be more precise, in decoder.rs in get_item_attrs:

 self.root
            .tables
            .attributes
            .get(self, id)
            .unwrap_or_else(|| {
                // Structure and variant constructors don't have any attributes encoded for them,
                // but we assume that someone passing a constructor ID actually wants to look at
                // the attributes on the corresponding struct or variant.
                let def_key = self.def_key(id);
                assert_eq!(def_key.disambiguated_data.data, DefPathData::Ctor);
                let parent_id = def_key.parent.expect("no parent for a constructor");
                self.root
                    .tables
                    .attributes
                    .get(self, parent_id)
                    .expect("no encoded attributes for a structure or variant")
            })
            .decode((self, tcx))

the assert_eq fails. It seems like this function only expects this unwrap to happen for constructors, but apparently it also happens for SyntheticCoroutineBody.

Initially, I found this panic in the agent-client-protocol crate, version 0.12.1 using the test agent-client-protocol::jsonrpc_advanced test_bidirectional_communication when running Miri with tree borrows and implicit writes enabled on it. But since then also many more crates can show this error. I could also replicate it here inside the Miri test suite: quiode@dd72b0b.

Error output

thread 'rustc' ($TID) panicked at /rustc-dev//compiler/rustc_metadata/src/rmeta/decoder.rs:LL:CC:
assertion `left == right` failed
  left: SyntheticCoroutineBody
 right: Ctor
stack backtrace:

error: the compiler unexpectedly panicked. This is a bug

query stack during panic:
#0 [attrs_for_def] collecting attributes of `closure_ice_lib::line_sink::{closure#0}::{synthetic#0}`
end of query stack

Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
  --> closure-ice-lib/src/lib.rs:LL:CC
   |

(RUST_BACKTRACE=1 produces the same output)

[quiode]

I already mentioned this to @JoJoDeveloping but it would be nice of you could have a look at it @RalfJung

[RalfJung]

Uh... I don't know much about this part of the compiler, sorry. Looks like it's going to be a bit more work to preserve this attribute cross-crate. OTOH we do already successfully use it cross-crate, after all we set the attribute in std and that seems to work fine? So maybe it's not about this being cross crate but find_attr! just not supporting all possible DefId that represent functions?

Cc @oli-obk @jdonszelmann