From f52c72948aa1dd718cc1f168d21c91c584c0a662 Mon Sep 17 00:00:00 2001
From: David Wood <david@davidtw.co>
Date: Mon, 22 Jun 2020 14:00:27 +0100
Subject: [PATCH] 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              | 43 +++++++++++--------
 .../polymorphization/normalized_sig_types.rs  | 25 +++++++++++
 2 files changed, 51 insertions(+), 17 deletions(-)
 create mode 100644 src/test/ui/polymorphization/normalized_sig_types.rs

diff --git a/src/librustc_middle/ty/layout.rs b/src/librustc_middle/ty/layout.rs
index 610a7cd41be2..cb937bf0112a 100644
--- a/src/librustc_middle/ty/layout.rs
+++ b/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(
-                    iter::once(env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
-                    sig.output(),
-                    sig.c_variadic,
-                    sig.unsafety,
-                    sig.abi
-                ))
+                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,
+                    )
+                })
             }
             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),
         }
     }
 }
diff --git a/src/test/ui/polymorphization/normalized_sig_types.rs b/src/test/ui/polymorphization/normalized_sig_types.rs
new file mode 100644
index 000000000000..fa76b7201e8c
--- /dev/null
+++ b/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(|| ());
+}