From c10ad0d888df19e7185e15f811fdb011278f3c20 Mon Sep 17 00:00:00 2001
From: Bastian Kauschke <bastian_kauschke@hotmail.de>
Date: Thu, 6 Aug 2020 22:12:21 +0200
Subject: [PATCH] review

---
 compiler/rustc_middle/src/mir/mod.rs                |  4 ++++
 .../rustc_trait_selection/src/traits/fulfill.rs     | 11 +++++------
 .../rustc_trait_selection/src/traits/select/mod.rs  | 13 ++++++-------
 3 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 197e8dd5341..3c041bbc0ae 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -197,6 +197,10 @@ pub struct Body<'tcx> {
     ///     let _ = [0; std::mem::size_of::<*mut T>()];
     /// }
     /// ```
+    ///
+    /// **WARNING**: Do not change this flags after the MIR was originally created, even if an optimization
+    /// removed the last mention of all generic params. We do not want to rely on optimizations and
+    /// potentially allow things like `[u8; std::mem::size_of::<T>() * 0]` due to this.
     pub is_polymorphic: bool,
 
     predecessor_cache: PredecessorCache,
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 989c6e6dbc2..4818022bf62 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -459,17 +459,16 @@ fn process_obligation(
                 }
 
                 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
-                    const_evaluatable::is_const_evaluatable(
+                    match const_evaluatable::is_const_evaluatable(
                         self.selcx.infcx(),
                         def_id,
                         substs,
                         obligation.param_env,
                         obligation.cause.span,
-                    )
-                    .map_or_else(
-                        |e| ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
-                        |()| ProcessResult::Changed(vec![]),
-                    )
+                    ) {
+                        Ok(()) => ProcessResult::Changed(vec![]),
+                        Err(e) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
+                    }
                 }
 
                 ty::PredicateAtom::ConstEquate(c1, c2) => {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 5a1e1eb89a6..7e8e2baa8a1 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -543,18 +543,17 @@ fn evaluate_predicate_recursively<'o>(
             }
 
             ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
-                const_evaluatable::is_const_evaluatable(
+                match const_evaluatable::is_const_evaluatable(
                     self.infcx,
                     def_id,
                     substs,
                     obligation.param_env,
                     obligation.cause.span,
-                )
-                .map(|()| EvaluatedToOk)
-                .or_else(|e| match e {
-                    ErrorHandled::TooGeneric => Ok(EvaluatedToAmbig),
-                    _ => Ok(EvaluatedToErr),
-                })
+                ) {
+                    Ok(()) => Ok(EvaluatedToOk),
+                    Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig),
+                    Err(_) => Ok(EvaluatedToErr),
+                }
             }
 
             ty::PredicateAtom::ConstEquate(c1, c2) => {