ty: normalize fn sigs before subst

This commit normalizes function signatures for instances before
substituting, a workaround for polymorphization considering
parameters unused when they show up in the signature, but not the
body (due to being normalized).

Unfortunately, this causes test output to change with the parallel
compiler only.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_middle/ty/layout.rs

@@ -2302,10 +2302,19 @@ impl<'tcx> ty::Instance<'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(..) |
-            // Shims currently have type FnPtr. Not sure this should remain.
-            ty::FnPtr(_) => {
-                let mut sig = ty.fn_sig(tcx);
+            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| {
@@ -2321,13 +2330,15 @@ impl<'tcx> ty::Instance<'tcx> {
                 let sig = substs.as_closure().sig();
 
                 let env_ty = tcx.closure_env_ty(def_id, substs).unwrap();
-                sig.map_bound(|sig| tcx.mk_fn_sig(
+                sig.map_bound(|sig| {
+                    tcx.mk_fn_sig(
                         iter::once(env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
                         sig.output(),
                         sig.c_variadic,
                         sig.unsafety,
-                    sig.abi
-                ))
+                        sig.abi,
+                    )
+                })
             }
             ty::Generator(_, substs, _) => {
                 let sig = substs.as_generator().poly_sig();
@@ -2343,10 +2354,8 @@ impl<'tcx> ty::Instance<'tcx> {
                 sig.map_bound(|sig| {
                     let state_did = tcx.require_lang_item(GeneratorStateLangItem, None);
                     let state_adt_ref = tcx.adt_def(state_did);
-                    let state_substs = tcx.intern_substs(&[
-                        sig.yield_ty.into(),
-                        sig.return_ty.into(),
-                    ]);
+                    let state_substs =
+                        tcx.intern_substs(&[sig.yield_ty.into(), sig.return_ty.into()]);
                     let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
 
                     tcx.mk_fn_sig(
@@ -2354,11 +2363,11 @@ impl<'tcx> ty::Instance<'tcx> {
                         &ret_ty,
                         false,
                         hir::Unsafety::Normal,
-                        rustc_target::spec::abi::Abi::Rust
+                        rustc_target::spec::abi::Abi::Rust,
                     )
                 })
             }
-            _ => bug!("unexpected type {:?} in Instance::fn_sig", ty)
+            _ => bug!("unexpected type {:?} in Instance::fn_sig", ty),
         }
     }
 }

src/test/ui/polymorphization/normalized_sig_types.rs

@@ -0,0 +1,25 @@
+// build-pass
+
+pub trait ParallelIterator: Sized {
+    fn drive<C: Consumer<()>>(_: C) {
+        C::into_folder();
+    }
+}
+
+pub trait Consumer<T>: Sized {
+    type Result;
+    fn into_folder() -> Self::Result;
+}
+
+impl ParallelIterator for () {}
+
+impl<F: Fn(), T> Consumer<T> for F {
+    type Result = ();
+    fn into_folder() -> Self::Result {
+        unimplemented!()
+    }
+}
+
+fn main() {
+    <()>::drive(|| ());
+}