From 4f405119e8305b26d0ee9e857887d2720b5c5c2a Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Sat, 21 Mar 2020 22:37:51 -0400
Subject: [PATCH 1/2] Resolve worsened debug build codegen

---
 src/librustc_mir/transform/const_prop.rs      |  9 +--
 .../const_prop/control-flow-simplification.rs | 64 +++++++++++++++++++
 2 files changed, 65 insertions(+), 8 deletions(-)
 create mode 100644 src/test/mir-opt/const_prop/control-flow-simplification.rs

diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index 8d7cafc34b356..1c6ae0918c7a1 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -324,14 +324,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     ) -> ConstPropagator<'mir, 'tcx> {
         let def_id = source.def_id();
         let substs = &InternalSubsts::identity_for_item(tcx, def_id);
-        let mut param_env = tcx.param_env(def_id);
-
-        // If we're evaluating inside a monomorphic function, then use `Reveal::All` because
-        // we want to see the same instances that codegen will see. This allows us to `resolve()`
-        // specializations.
-        if !substs.needs_subst() {
-            param_env = param_env.with_reveal_all();
-        }
+        let param_env = tcx.param_env(def_id).with_reveal_all();
 
         let span = tcx.def_span(def_id);
         let mut ecx = InterpCx::new(tcx.at(span), param_env, ConstPropMachine, ());
diff --git a/src/test/mir-opt/const_prop/control-flow-simplification.rs b/src/test/mir-opt/const_prop/control-flow-simplification.rs
new file mode 100644
index 0000000000000..0e9f33b15e2cc
--- /dev/null
+++ b/src/test/mir-opt/const_prop/control-flow-simplification.rs
@@ -0,0 +1,64 @@
+// compile-flags: -Zmir-opt-level=1
+
+trait NeedsDrop:Sized{
+    const NEEDS:bool=std::mem::needs_drop::<Self>();
+}
+
+impl<This> NeedsDrop for This{}
+
+fn hello<T>(){
+    if <bool>::NEEDS {
+        panic!()
+    }
+}
+
+pub fn main() {
+    hello::<()>();
+    hello::<Vec<()>>();
+}
+
+// END RUST SOURCE
+// START rustc.hello.ConstProp.before.mir
+// let mut _0: ();
+// let mut _1: bool;
+// let mut _2: !;
+// bb0: {
+//   StorageLive(_1);
+//   _1 = const <bool as NeedsDrop>::NEEDS;
+//   switchInt(_1) -> [false: bb1, otherwise: bb2];
+// }
+// bb1: {
+//   _0 = ();
+//   StorageDead(_1);
+//   return;
+// }
+// bb2: {
+//   StorageLive(_2);
+//   const std::rt::begin_panic::<&str>(const "explicit panic");
+// }
+// END rustc.hello.ConstProp.before.mir
+// START rustc.hello.ConstProp.after.mir
+// let mut _0: ();
+// let mut _1: bool;
+// let mut _2: !;
+// bb0: {
+//   StorageLive(_1);
+//   _1 = const false;
+//   switchInt(const false) -> [false: bb1, otherwise: bb2];
+// }
+// bb1: {
+//   _0 = ();
+//   StorageDead(_1);
+//   return;
+// }
+// bb2: {
+//   StorageLive(_2);
+//   const std::rt::begin_panic::<&str>(const "explicit panic");
+// }
+// END rustc.hello.ConstProp.after.mir
+// START rustc.hello.PreCodegen.before.mir
+// let mut _0: ();
+// bb0: {
+//   return;
+// }
+// END rustc.hello.PreCodegen.before.mir

From 61d3be8de784e7ec2642c06daa5ff95f226d65d3 Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Sat, 21 Mar 2020 23:08:32 -0400
Subject: [PATCH 2/2] Use Reveal::All in MIR inliner

---
 src/librustc_mir/transform/inline.rs | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/src/librustc_mir/transform/inline.rs b/src/librustc_mir/transform/inline.rs
index 52c7225a850fb..16c32e138fdf0 100644
--- a/src/librustc_mir/transform/inline.rs
+++ b/src/librustc_mir/transform/inline.rs
@@ -3,8 +3,8 @@
 use rustc::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc::mir::visit::*;
 use rustc::mir::*;
-use rustc::ty::subst::{InternalSubsts, Subst, SubstsRef};
-use rustc::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt, TypeFoldable};
+use rustc::ty::subst::{Subst, SubstsRef};
+use rustc::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
 use rustc_attr as attr;
 use rustc_hir::def_id::DefId;
 use rustc_index::bit_set::BitSet;
@@ -66,14 +66,7 @@ impl Inliner<'tcx> {
 
         let mut callsites = VecDeque::new();
 
-        let mut param_env = self.tcx.param_env(self.source.def_id());
-
-        let substs = &InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
-
-        // For monomorphic functions, we can use `Reveal::All` to resolve specialized instances.
-        if !substs.needs_subst() {
-            param_env = param_env.with_reveal_all();
-        }
+        let param_env = self.tcx.param_env(self.source.def_id()).with_reveal_all();
 
         // Only do inlining into fn bodies.
         let id = self.tcx.hir().as_local_hir_id(self.source.def_id()).unwrap();