drive-by: remove ClosureArgs.tupled_sig

Neither hax nor charon use it and it's potentially confusing.

Author: Nadrieril

engine/lib/import_thir.ml

@@ -1018,10 +1018,8 @@ end) : EXPR = struct
     | Float k ->
         TFloat
           (match k with F16 -> F16 | F32 -> F32 | F64 -> F64 | F128 -> F128)
-    | Arrow signature
-    | Closure { untupled_sig = signature; _ }
-    | FnDef { fn_sig = signature; _ } ->
-        let ({ inputs; output; _ } : Thir.ty_fn_sig) = signature.value in
+    | Arrow fn_sig | Closure { fn_sig; _ } | FnDef { fn_sig; _ } ->
+        let ({ inputs; output; _ } : Thir.ty_fn_sig) = fn_sig.value in
         let inputs =
           if List.is_empty inputs then [ U.unit_typ ]
           else List.map ~f:(c_ty span) inputs

frontend/exporter/src/types/mir.rs

@@ -373,7 +373,7 @@ fn translate_terminator_kind_call<'tcx, S: BaseState<'tcx> + HasMir<'tcx> + HasO
     let sig = match hax_ty.kind() {
         TyKind::Arrow(sig) => sig,
         TyKind::FnDef { fn_sig, .. } => fn_sig,
-        TyKind::Closure(args) => &args.untupled_sig,
+        TyKind::Closure(args) => &args.fn_sig,
         _ => supposely_unreachable_fatal!(
             s,
             "TerminatorKind_Call_expected_fn_type";

frontend/exporter/src/types/ty.rs

@@ -1353,11 +1353,8 @@ pub struct ClosureArgs {
     /// The base kind of this closure. The kinds are ordered by inclusion: any `Fn` works as an
     /// `FnMut`, and any `FnMut` works as an `FnOnce`.
     pub kind: ClosureKind,
-    /// The proper `fn(A, B, C) -> D` signature of the closure.
-    pub untupled_sig: PolyFnSig,
-    /// The signature of the closure as one input and one output, where the input arguments are
-    /// tupled. This is relevant to implementing the `Fn*` traits.
-    pub tupled_sig: PolyFnSig,
+    /// The signature of the function that the closure implements, e.g. `fn(A, B, C) -> D`.
+    pub fn_sig: PolyFnSig,
     /// The set of captured variables. Together they form the state of the closure.
     pub upvar_tys: Vec<Ty>,
 }
@@ -1384,8 +1381,7 @@ impl ClosureArgs {
         ClosureArgs {
             item,
             kind: from.kind().sinto(s),
-            tupled_sig: sig.sinto(s),
-            untupled_sig: tcx
+            fn_sig: tcx
                 .signature_unclosure(sig, rustc_hir::Safety::Safe)
                 .sinto(s),
             upvar_tys: from.upvar_tys().sinto(s),