`cargo test` fails when `GlobalAlloc::Function` is not `RigidTy::FnPtr`

[dkcumming]

Trying to produce artefacts for stable mir json on test code of a cargo project via cargo test panics.

Reproducer

cd stable-mir-json
cargo build --release && cargo run --bin cargo_stable_mir_json -- $PWD
cd ..
cargo new --lib smir-test-fail
cd smir-test-fail
RUSTC=~/.stable-mir-json/release.sh cargo test

The test fails with:

thread 'rustc' panicked at src/printer.rs:688:13:
assertion failed: kind.unwrap().is_fn_ptr()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Which is from line 688 of stable_mir_json::printer introduced in this PR:

//-snip-
        GlobalAlloc::Function(inst) => {
            assert!(kind.unwrap().is_fn_ptr()); // <-- line 688
            entry.or_insert(AllocInfo::Function(inst));
        }
//-snip-

Printing kind and inst gives

Some(RigidTy(Ref(Region { kind: ReErased }, Ty { id: 116, kind: RigidTy(Adt(AdtDef(DefId { id: 30, name: "tests::test::TestDescAndFn" }), GenericArgs([]))) }, Not)))

Instance { kind: Shim, def: "_ZN4core3ops8function6FnOnce9call_once17h4aa4ff19edd7fd65E", args: GenericArgs([Type(Ty { id: 103, kind: RigidTy(Closure(ClosureDef(DefId { id: 17, name: "tests::it_works::{closure#0}" }), GenericArgs([Type(Ty { id: 106, kind: RigidTy(Int(I8)) }), Type(Ty { id: 117, kind: RigidTy(FnPtr(Binder { value: FnSig { inputs_and_output: [Ty { id: 2, kind: RigidTy(Tuple([])) }, Ty { id: 64, kind: RigidTy(Adt(AdtDef(DefId { id: 23, name: "std::result::Result" }), GenericArgs([Type(Ty { id: 2, kind: RigidTy(Tuple([])) }), Type(Ty { id: 15, kind: RigidTy(Adt(AdtDef(DefId { id: 4, name: "std::string::String" }), GenericArgs([]))) })]))) }], c_variadic: false, safety: Safe, abi: RustCall }, bound_vars: [] })) }), Type(Ty { id: 2, kind: RigidTy(Tuple([])) })]))) }), Type(Ty { id: 2, kind: RigidTy(Tuple([])) })]) }

which is from the #[test] attribute, which is a compiler built in.