Accessing the `FnSig` of a `DefId`

ranjitjhala · December 14, 2021, 6:04pm

Hi all,

(My apologies if this is the wrong place to ask this question!)

We are trying to build a rustc plugin -- and am having a bit of trouble trying to extract the FnSig (type) of a DefId. When you have a LocalDefId (which, as I understand it, means a DefId defined in the current crate, you can simply do something like

let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
let hir_fn_sig = tcx.hir().fn_sig_by_hir_id(hir_id).unwrap();

However, I'd like to do the same thing but for identifiers defined in external crates (e.g. std::process::exit to pick a random example).

Can anyone give some pointers? Maybe there is some API to

Convert a DefId to a Node or HirId and then extract the FnSig ?

Or some other route? [Or do we only have Node and HirId for LocalDefId and should I be looking elsewhere entirely?]

Any pointers will be most welcome, thanks in advance!

Ranjit.

bjorn3 · December 14, 2021, 6:14pm

You can take a look at

github.com

rust-lang/rust/blob/3e21768a0a3fc84befd1cbe825ae6849e9941b73/compiler/rustc_middle/src/ty/layout.rs#L2584-L2677

    
      
          fn fn_sig_for_fn_abi(&self, tcx: TyCtxt<'tcx>) -> ty::PolyFnSig<'tcx> {
              // FIXME(davidtwco,eddyb): A `ParamEnv` should be passed through to this function.
              let ty = self.ty(tcx, ty::ParamEnv::reveal_all());
              match *ty.kind() {
                  ty::FnDef(..) => {
                      // HACK(davidtwco,eddyb): This is a workaround for polymorphization considering
                      // parameters unused if they show up in the signature, but not in the `mir::Body`
                      // (i.e. due to being inside a projection that got normalized, see
                      // `src/test/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
                      // track of a polymorphization `ParamEnv` to allow normalizing later.
                      let mut sig = match *ty.kind() {
                          ty::FnDef(def_id, substs) => tcx
                              .normalize_erasing_regions(tcx.param_env(def_id), tcx.fn_sig(def_id))
                              .subst(tcx, substs),
                          _ => unreachable!(),
                      };
          
          
            if let ty::InstanceDef::VtableShim(..) = self.def {
                          // Modify `fn(self, ...)` to `fn(self: *mut Self, ...)`.
                          sig = sig.map_bound(|mut sig| {

This file has been truncated. show original

ranjitjhala · December 15, 2021, 12:37am

Thanks! That moves things along quite a bit, however I am next stuck on the following.

For the LocalDefId,we get the rustc_hir::hir::FnSig but
For the DefId (following the code above) we get a rustc_middle::ty::FnSig

The two are slightly different, e.g. the constituent types (inputs and outputs) have respectively

rustc_hir::hir::Ty vs
rustc_middle::ty::Ty

Do you know if it is possible to convert the middle versions to the hir versions (or vice versa)?

bjorn3 · December 15, 2021, 11:41am

No, going from hir::Ty -> ty::Ty only happens during hir lowering. The other way around is inpossible. You can convert a LocalDefId to a DefId though and then use the linked code.

camelid · December 15, 2021, 7:28pm

Usually ty::Tys are more useful to perform analyses on, since they are the semantic rather than syntactic representation of a type. The main limitation of ty::Tys I know of is that type aliases are fully expanded and aren't represented.

You can't get HIR from external crates because it's not stored in the crate metadata files.

camelid · December 15, 2021, 7:33pm

TyCtxt::fn_sig() is probably the function you want.

bjorn3 · December 15, 2021, 8:40pm

I believe tcx.fn_sig doesn't work for closures and generators. The code I linked does work for them.

system · March 15, 2022, 8:41pm

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Accessing the `FnSig` of a `DefId`

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