From 12ac49afc9e1c33fe94f01dfca9efda4d7fed0b2 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Sun, 17 Nov 2019 18:43:26 -0500
Subject: [PATCH 01/15] Add feature gate for &mut in const fns

---
 src/librustc_feature/active.rs | 3 +++
 src/libsyntax_pos/symbol.rs    | 1 +
 2 files changed, 4 insertions(+)

diff --git a/src/librustc_feature/active.rs b/src/librustc_feature/active.rs
index 16d8ada9f24c0..706e102436517 100644
--- a/src/librustc_feature/active.rs
+++ b/src/librustc_feature/active.rs
@@ -527,6 +527,9 @@ declare_features! (
     /// Allows the use of `#[cfg(sanitize = "option")]`; set when -Zsanitizer is used.
     (active, cfg_sanitize, "1.41.0", Some(39699), None),
 
+    /// Allows using `&mut` in constant functions.
+    (active, const_fn_mut_refs, "1.41.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index 3059b05969106..c98063275dd34 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -203,6 +203,7 @@ symbols! {
         const_constructor,
         const_extern_fn,
         const_fn,
+        const_fn_mut_refs,
         const_fn_union,
         const_generics,
         const_if_match,

From bb2a4238946563ef87eed6fe7e833ca731fa4c79 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Sun, 17 Nov 2019 21:11:46 -0500
Subject: [PATCH 02/15] Allow &mut in const fns when feature gate is enabled

---
 src/librustc_mir/transform/qualify_min_const_fn.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/librustc_mir/transform/qualify_min_const_fn.rs b/src/librustc_mir/transform/qualify_min_const_fn.rs
index e40d6a5952edb..28240f6beca71 100644
--- a/src/librustc_mir/transform/qualify_min_const_fn.rs
+++ b/src/librustc_mir/transform/qualify_min_const_fn.rs
@@ -80,10 +80,14 @@ pub fn is_min_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId, body: &'a Body<'tcx>) -
 fn check_ty(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span, fn_def_id: DefId) -> McfResult {
     for ty in ty.walk() {
         match ty.kind {
-            ty::Ref(_, _, hir::Mutability::Mutable) => return Err((
-                span,
-                "mutable references in const fn are unstable".into(),
-            )),
+            ty::Ref(_, _, hir::Mutability::Mutable) => {
+                if !tcx.features().const_fn_mut_refs {
+                    return Err((
+                        span,
+                        "mutable references in const fn are unstable".into(),
+                    ))
+                }
+            }
             ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
             ty::FnPtr(..) => {
                 if !tcx.const_fn_is_allowed_fn_ptr(fn_def_id) {

From 8b0f5acfcbc178680f3cbec737b13bf0da6c05bd Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 21 Nov 2019 08:45:34 -0500
Subject: [PATCH 03/15] Add tests for mutable borrows in const fns

---
 .../const-eval/feature-gate-const_fn_mut_refs.rs  |  7 +++++++
 .../feature-gate-const_fn_mut_refs.stderr         | 12 ++++++++++++
 src/test/ui/consts/const_fn_mut_refs.rs           | 15 +++++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs
 create mode 100644 src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr
 create mode 100644 src/test/ui/consts/const_fn_mut_refs.rs

diff --git a/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs b/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs
new file mode 100644
index 0000000000000..2207599815ee8
--- /dev/null
+++ b/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs
@@ -0,0 +1,7 @@
+fn main() {
+    foo(&mut 5);
+}
+
+const fn foo(x: &mut i32) -> i32 { //~ ERROR mutable references in const fn are unstable
+    *x + 1
+}
diff --git a/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr b/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr
new file mode 100644
index 0000000000000..26a61d38f8c3c
--- /dev/null
+++ b/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr
@@ -0,0 +1,12 @@
+error[E0723]: mutable references in const fn are unstable
+  --> $DIR/feature-gate-const_fn_mut_refs.rs:5:14
+   |
+LL | const fn foo(x: &mut i32) -> i32 {
+   |              ^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0723`.
diff --git a/src/test/ui/consts/const_fn_mut_refs.rs b/src/test/ui/consts/const_fn_mut_refs.rs
new file mode 100644
index 0000000000000..764c45b386b06
--- /dev/null
+++ b/src/test/ui/consts/const_fn_mut_refs.rs
@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(const_fn_mut_refs)]
+
+struct Foo {
+    x: i32
+}
+
+const fn bar(foo: &mut Foo) -> i32 {
+    foo.x + 1
+}
+
+fn main() {
+    assert_eq!(bar(&mut Foo{x: 0}), 1);
+}

From d24ae2577fad83d581d090816303b0f1842f1514 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 21 Nov 2019 12:38:14 -0500
Subject: [PATCH 04/15] Rename feature gate

---
 src/librustc_feature/active.rs                     | 2 +-
 src/librustc_mir/transform/qualify_min_const_fn.rs | 2 +-
 src/libsyntax_pos/symbol.rs                        | 2 +-
 src/test/ui/consts/const_fn_mut_refs.rs            | 2 +-
 4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/librustc_feature/active.rs b/src/librustc_feature/active.rs
index 706e102436517..8e3ef4f3d9dca 100644
--- a/src/librustc_feature/active.rs
+++ b/src/librustc_feature/active.rs
@@ -528,7 +528,7 @@ declare_features! (
     (active, cfg_sanitize, "1.41.0", Some(39699), None),
 
     /// Allows using `&mut` in constant functions.
-    (active, const_fn_mut_refs, "1.41.0", None, None),
+    (active, const_mut_refs, "1.41.0", Some(57349), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
diff --git a/src/librustc_mir/transform/qualify_min_const_fn.rs b/src/librustc_mir/transform/qualify_min_const_fn.rs
index 28240f6beca71..20bccb59a5cfb 100644
--- a/src/librustc_mir/transform/qualify_min_const_fn.rs
+++ b/src/librustc_mir/transform/qualify_min_const_fn.rs
@@ -81,7 +81,7 @@ fn check_ty(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span, fn_def_id: DefId) -> Mc
     for ty in ty.walk() {
         match ty.kind {
             ty::Ref(_, _, hir::Mutability::Mutable) => {
-                if !tcx.features().const_fn_mut_refs {
+                if !tcx.features().const_mut_refs {
                     return Err((
                         span,
                         "mutable references in const fn are unstable".into(),
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index c98063275dd34..d700f779e1773 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -203,13 +203,13 @@ symbols! {
         const_constructor,
         const_extern_fn,
         const_fn,
-        const_fn_mut_refs,
         const_fn_union,
         const_generics,
         const_if_match,
         const_indexing,
         const_in_array_repeat_expressions,
         const_let,
+        const_mut_refs,
         const_panic,
         const_raw_ptr_deref,
         const_raw_ptr_to_usize_cast,
diff --git a/src/test/ui/consts/const_fn_mut_refs.rs b/src/test/ui/consts/const_fn_mut_refs.rs
index 764c45b386b06..2263a130284a5 100644
--- a/src/test/ui/consts/const_fn_mut_refs.rs
+++ b/src/test/ui/consts/const_fn_mut_refs.rs
@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(const_fn_mut_refs)]
+#![feature(const_mut_refs)]
 
 struct Foo {
     x: i32

From 1f420b9891c962656eac16831c772cbd56afff24 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 21 Nov 2019 14:33:39 -0500
Subject: [PATCH 05/15] Allow mutable borrows in constant bodies

---
 src/librustc_mir/transform/check_consts/ops.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index a4f12a4e54fa7..117d628af257f 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -183,6 +183,10 @@ impl NonConstOp for Loop {
 #[derive(Debug)]
 pub struct MutBorrow(pub BorrowKind);
 impl NonConstOp for MutBorrow {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_mut_refs)
+    }
+
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
         let kind = self.0;
         if let BorrowKind::Mut { .. } = kind {

From de60f721c4983b1d43a305f0603a7ba6207b888d Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 21 Nov 2019 14:35:19 -0500
Subject: [PATCH 06/15] Move and rewrite tests to use &mut in constants

---
 .../ui/consts/const-mut-refs/const_mut_refs.rs    | 15 +++++++++++++++
 .../feature-gate-const_mut_refs.rs}               |  0
 .../feature-gate-const_mut_refs.stderr}           |  2 +-
 src/test/ui/consts/const_fn_mut_refs.rs           | 15 ---------------
 4 files changed, 16 insertions(+), 16 deletions(-)
 create mode 100644 src/test/ui/consts/const-mut-refs/const_mut_refs.rs
 rename src/test/ui/consts/{const-eval/feature-gate-const_fn_mut_refs.rs => const-mut-refs/feature-gate-const_mut_refs.rs} (100%)
 rename src/test/ui/consts/{const-eval/feature-gate-const_fn_mut_refs.stderr => const-mut-refs/feature-gate-const_mut_refs.stderr} (88%)
 delete mode 100644 src/test/ui/consts/const_fn_mut_refs.rs

diff --git a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
new file mode 100644
index 0000000000000..60a69cb9f3f67
--- /dev/null
+++ b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(const_mut_refs)]
+
+struct Foo {
+    x: usize
+}
+
+const fn bar(foo: &mut Foo) -> usize {
+    foo.x + 1
+}
+
+fn main() {
+    let _: [(); bar(&mut Foo { x: 0 })] = [(); 1];
+}
diff --git a/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs b/src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.rs
similarity index 100%
rename from src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.rs
rename to src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.rs
diff --git a/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr b/src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.stderr
similarity index 88%
rename from src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr
rename to src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.stderr
index 26a61d38f8c3c..4fae119f02604 100644
--- a/src/test/ui/consts/const-eval/feature-gate-const_fn_mut_refs.stderr
+++ b/src/test/ui/consts/const-mut-refs/feature-gate-const_mut_refs.stderr
@@ -1,5 +1,5 @@
 error[E0723]: mutable references in const fn are unstable
-  --> $DIR/feature-gate-const_fn_mut_refs.rs:5:14
+  --> $DIR/feature-gate-const_mut_refs.rs:5:14
    |
 LL | const fn foo(x: &mut i32) -> i32 {
    |              ^
diff --git a/src/test/ui/consts/const_fn_mut_refs.rs b/src/test/ui/consts/const_fn_mut_refs.rs
deleted file mode 100644
index 2263a130284a5..0000000000000
--- a/src/test/ui/consts/const_fn_mut_refs.rs
+++ /dev/null
@@ -1,15 +0,0 @@
-// run-pass
-
-#![feature(const_mut_refs)]
-
-struct Foo {
-    x: i32
-}
-
-const fn bar(foo: &mut Foo) -> i32 {
-    foo.x + 1
-}
-
-fn main() {
-    assert_eq!(bar(&mut Foo{x: 0}), 1);
-}

From d92e9b7374bb2087e0eb4803bfabb030c1397bcd Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Fri, 22 Nov 2019 19:59:34 -0500
Subject: [PATCH 07/15] Allow mutable derefs with feature gate

---
 src/librustc_mir/transform/check_consts/ops.rs      | 6 +++++-
 src/test/ui/consts/const-mut-refs/const_mut_refs.rs | 4 +++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index 117d628af257f..be5845eaa365f 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -216,7 +216,11 @@ impl NonConstOp for MutBorrow {
 
 #[derive(Debug)]
 pub struct MutDeref;
-impl NonConstOp for MutDeref {}
+impl NonConstOp for MutDeref {
+    fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
+        Some(tcx.features().const_mut_refs)
+    }
+}
 
 #[derive(Debug)]
 pub struct Panic;
diff --git a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
index 60a69cb9f3f67..36174b2843d31 100644
--- a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
+++ b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
@@ -7,7 +7,9 @@ struct Foo {
 }
 
 const fn bar(foo: &mut Foo) -> usize {
-    foo.x + 1
+    let x = &mut foo.x;
+    *x = 1;
+    *x
 }
 
 fn main() {

From e31a1368fda9c5b4b4bc39e69754b19d36dff8c2 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Sat, 23 Nov 2019 15:00:14 -0500
Subject: [PATCH 08/15] Extend test for const_mut_refs feature

---
 .../consts/const-mut-refs/const_mut_refs.rs   | 25 ++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
index 36174b2843d31..99006a20b1bcb 100644
--- a/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
+++ b/src/test/ui/consts/const-mut-refs/const_mut_refs.rs
@@ -6,12 +6,31 @@ struct Foo {
     x: usize
 }
 
-const fn bar(foo: &mut Foo) -> usize {
+const fn foo() -> Foo {
+    Foo { x: 0 }
+}
+
+impl Foo {
+    const fn bar(&mut self) -> usize {
+        self.x = 1;
+        self.x
+    }
+
+}
+
+const fn baz(foo: &mut Foo) -> usize {
     let x = &mut foo.x;
-    *x = 1;
+    *x = 2;
     *x
 }
 
+const fn bazz(foo: &mut Foo) -> usize {
+    foo.x = 3;
+    foo.x
+}
+
 fn main() {
-    let _: [(); bar(&mut Foo { x: 0 })] = [(); 1];
+    let _: [(); foo().bar()] = [(); 1];
+    let _: [(); baz(&mut foo())] = [(); 2];
+    let _: [(); bazz(&mut foo())] = [(); 3];
 }

From 683f5c9c23a07fc6434b3e15207aa12144c90420 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Tue, 26 Nov 2019 10:00:41 -0500
Subject: [PATCH 09/15] Disallow cell borrowing

---
 .../transform/check_consts/ops.rs             | 50 ++++++++++---------
 .../transform/check_consts/validation.rs      | 13 +++--
 2 files changed, 36 insertions(+), 27 deletions(-)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index be5845eaa365f..1d23b6e27f31c 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -1,7 +1,6 @@
 //! Concrete error types for all operations which may be invalid in a certain const context.
 
 use rustc::hir::def_id::DefId;
-use rustc::mir::BorrowKind;
 use rustc::session::config::nightly_options;
 use rustc::ty::TyCtxt;
 use syntax::feature_gate::feature_err;
@@ -181,36 +180,39 @@ impl NonConstOp for Loop {
 }
 
 #[derive(Debug)]
-pub struct MutBorrow(pub BorrowKind);
+pub struct CellBorrow;
+impl NonConstOp for CellBorrow {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        span_err!(item.tcx.sess, span, E0492,
+            "cannot borrow a constant which may contain \
+            interior mutability, create a static instead");
+    }
+}
+
+#[derive(Debug)]
+pub struct MutBorrow;
 impl NonConstOp for MutBorrow {
     fn feature_gate(tcx: TyCtxt<'_>) -> Option<bool> {
         Some(tcx.features().const_mut_refs)
     }
 
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
-        let kind = self.0;
-        if let BorrowKind::Mut { .. } = kind {
-            let mut err = struct_span_err!(item.tcx.sess, span, E0017,
-                                           "references in {}s may only refer \
-                                            to immutable values", item.const_kind());
-            err.span_label(span, format!("{}s require immutable values",
-                                                item.const_kind()));
-            if item.tcx.sess.teach(&err.get_code().unwrap()) {
-                err.note("References in statics and constants may only refer \
-                          to immutable values.\n\n\
-                          Statics are shared everywhere, and if they refer to \
-                          mutable data one might violate memory safety since \
-                          holding multiple mutable references to shared data \
-                          is not allowed.\n\n\
-                          If you really want global mutable state, try using \
-                          static mut or a global UnsafeCell.");
-            }
-            err.emit();
-        } else {
-            span_err!(item.tcx.sess, span, E0492,
-                      "cannot borrow a constant which may contain \
-                       interior mutability, create a static instead");
+        let mut err = struct_span_err!(item.tcx.sess, span, E0017,
+                                       "references in {}s may only refer \
+                                        to immutable values", item.const_kind());
+        err.span_label(span, format!("{}s require immutable values",
+                                            item.const_kind()));
+        if item.tcx.sess.teach(&err.get_code().unwrap()) {
+            err.note("References in statics and constants may only refer \
+                      to immutable values.\n\n\
+                      Statics are shared everywhere, and if they refer to \
+                      mutable data one might violate memory safety since \
+                      holding multiple mutable references to shared data \
+                      is not allowed.\n\n\
+                      If you really want global mutable state, try using \
+                      static mut or a global UnsafeCell.");
         }
+        err.emit();
     }
 }
 
diff --git a/src/librustc_mir/transform/check_consts/validation.rs b/src/librustc_mir/transform/check_consts/validation.rs
index 783c64ece7371..4a55bce767d81 100644
--- a/src/librustc_mir/transform/check_consts/validation.rs
+++ b/src/librustc_mir/transform/check_consts/validation.rs
@@ -360,7 +360,11 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
                 };
 
                 if !is_allowed {
-                    self.check_op(ops::MutBorrow(kind));
+                    if let BorrowKind::Mut{ .. } = kind {
+                        self.check_op(ops::MutBorrow);
+                    } else {
+                        self.check_op(ops::CellBorrow);
+                    }
                 }
             }
 
@@ -385,7 +389,11 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
                 );
 
                 if borrowed_place_has_mut_interior {
-                    self.check_op(ops::MutBorrow(kind));
+                    if let BorrowKind::Mut{ .. } = kind {
+                        self.check_op(ops::MutBorrow);
+                    } else {
+                        self.check_op(ops::CellBorrow);
+                    }
                 }
             }
 
@@ -452,7 +460,6 @@ impl Visitor<'tcx> for Validator<'_, 'mir, 'tcx> {
             }
         }
     }
-
     fn visit_projection_elem(
         &mut self,
         place_base: &PlaceBase<'tcx>,

From 681690db4e41505401a0ebbf3d1e4bec69ead702 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Tue, 26 Nov 2019 11:00:30 -0500
Subject: [PATCH 10/15] Update miri_unleashed tests

---
 .../ui/consts/miri_unleashed/mutable_const.rs |  4 +--
 .../miri_unleashed/mutable_const.stderr       | 22 ++-------------
 .../miri_unleashed/mutable_references.rs      |  5 +---
 .../miri_unleashed/mutable_references.stderr  | 28 ++-----------------
 4 files changed, 6 insertions(+), 53 deletions(-)

diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.rs b/src/test/ui/consts/miri_unleashed/mutable_const.rs
index 44b408494679c..cf5e6d6002581 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_const.rs
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.rs
@@ -12,9 +12,7 @@ const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
 const MUTATING_BEHIND_RAW: () = {
     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
     unsafe {
-        *MUTABLE_BEHIND_RAW = 99 //~ WARN skipping const checks
-        //~^ ERROR any use of this value will cause an error
-        //~^^ tried to modify constant memory
+        *MUTABLE_BEHIND_RAW = 99 //~ ERROR constant contains unimplemented expression type
     }
 };
 
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.stderr b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
index 757f0ffb59ff7..1a18063625be3 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_const.stderr
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
@@ -4,30 +4,12 @@ warning: skipping const checks
 LL | const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
    |                                      ^^^^^^^^^^^^^^^^^^^^
 
-warning: skipping const checks
+error[E0019]: constant contains unimplemented expression type
   --> $DIR/mutable_const.rs:15:9
    |
 LL |         *MUTABLE_BEHIND_RAW = 99
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: any use of this value will cause an error
-  --> $DIR/mutable_const.rs:15:9
-   |
-LL | / const MUTATING_BEHIND_RAW: () = {
-LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
-LL | |     unsafe {
-LL | |         *MUTABLE_BEHIND_RAW = 99
-   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ tried to modify constant memory
-...  |
-LL | |     }
-LL | | };
-   | |__-
-   |
-note: lint level defined here
-  --> $DIR/mutable_const.rs:4:9
-   |
-LL | #![deny(const_err)]
-   |         ^^^^^^^^^
-
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0019`.
diff --git a/src/test/ui/consts/miri_unleashed/mutable_references.rs b/src/test/ui/consts/miri_unleashed/mutable_references.rs
index 59dafcbf4d50c..d95833e3071df 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_references.rs
+++ b/src/test/ui/consts/miri_unleashed/mutable_references.rs
@@ -1,4 +1,5 @@
 // compile-flags: -Zunleash-the-miri-inside-of-you
+#![feature(const_mut_refs)]
 #![allow(const_err)]
 
 use std::cell::UnsafeCell;
@@ -6,15 +7,12 @@ use std::cell::UnsafeCell;
 // a test demonstrating what things we could allow with a smarter const qualification
 
 static FOO: &&mut u32 = &&mut 42;
-//~^ WARN: skipping const checks
 
 static BAR: &mut () = &mut ();
-//~^ WARN: skipping const checks
 
 struct Foo<T>(T);
 
 static BOO: &mut Foo<()> = &mut Foo(());
-//~^ WARN: skipping const checks
 
 struct Meh {
     x: &'static UnsafeCell<i32>,
@@ -28,7 +26,6 @@ static MEH: Meh = Meh {
 };
 
 static OH_YES: &mut i32 = &mut 42;
-//~^ WARN: skipping const checks
 
 fn main() {
     unsafe {
diff --git a/src/test/ui/consts/miri_unleashed/mutable_references.stderr b/src/test/ui/consts/miri_unleashed/mutable_references.stderr
index b9c0af33c39cd..ce1f5f38dd99a 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_references.stderr
+++ b/src/test/ui/consts/miri_unleashed/mutable_references.stderr
@@ -1,35 +1,11 @@
 warning: skipping const checks
-  --> $DIR/mutable_references.rs:8:26
-   |
-LL | static FOO: &&mut u32 = &&mut 42;
-   |                          ^^^^^^^
-
-warning: skipping const checks
-  --> $DIR/mutable_references.rs:11:23
-   |
-LL | static BAR: &mut () = &mut ();
-   |                       ^^^^^^^
-
-warning: skipping const checks
-  --> $DIR/mutable_references.rs:16:28
-   |
-LL | static BOO: &mut Foo<()> = &mut Foo(());
-   |                            ^^^^^^^^^^^^
-
-warning: skipping const checks
-  --> $DIR/mutable_references.rs:26:8
+  --> $DIR/mutable_references.rs:24:8
    |
 LL |     x: &UnsafeCell::new(42),
    |        ^^^^^^^^^^^^^^^^^^^^
 
-warning: skipping const checks
-  --> $DIR/mutable_references.rs:30:27
-   |
-LL | static OH_YES: &mut i32 = &mut 42;
-   |                           ^^^^^^^
-
 error[E0594]: cannot assign to `*OH_YES`, as `OH_YES` is an immutable static item
-  --> $DIR/mutable_references.rs:37:5
+  --> $DIR/mutable_references.rs:34:5
    |
 LL |     *OH_YES = 99;
    |     ^^^^^^^^^^^^ cannot assign

From 5e61e4cadc72141f9e4c80ca8615509f2eaeb51f Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Tue, 26 Nov 2019 22:43:40 -0500
Subject: [PATCH 11/15] Suggest feature for const_mut_refs errors

---
 src/librustc_mir/transform/check_consts/ops.rs     | 10 +++++++---
 src/test/ui/consts/const-eval/issue-65394.stderr   |  9 ++++++---
 src/test/ui/consts/const-multi-ref.stderr          |  9 ++++++---
 src/test/ui/consts/const_let_assign3.stderr        | 14 ++++++++++----
 src/test/ui/consts/projection_qualif.stderr        |  9 ++++++---
 .../consts/static_mut_containing_mut_ref2.stderr   |  9 ++++++---
 6 files changed, 41 insertions(+), 19 deletions(-)

diff --git a/src/librustc_mir/transform/check_consts/ops.rs b/src/librustc_mir/transform/check_consts/ops.rs
index 1d23b6e27f31c..182ba4625667a 100644
--- a/src/librustc_mir/transform/check_consts/ops.rs
+++ b/src/librustc_mir/transform/check_consts/ops.rs
@@ -197,9 +197,13 @@ impl NonConstOp for MutBorrow {
     }
 
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
-        let mut err = struct_span_err!(item.tcx.sess, span, E0017,
-                                       "references in {}s may only refer \
-                                        to immutable values", item.const_kind());
+        let mut err = feature_err(
+            &item.tcx.sess.parse_sess,
+            sym::const_mut_refs,
+            span,
+            &format!("references in {}s may only refer \
+                      to immutable values", item.const_kind())
+        );
         err.span_label(span, format!("{}s require immutable values",
                                             item.const_kind()));
         if item.tcx.sess.teach(&err.get_code().unwrap()) {
diff --git a/src/test/ui/consts/const-eval/issue-65394.stderr b/src/test/ui/consts/const-eval/issue-65394.stderr
index acf5cbaede665..54b35073340fd 100644
--- a/src/test/ui/consts/const-eval/issue-65394.stderr
+++ b/src/test/ui/consts/const-eval/issue-65394.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/issue-65394.rs:8:13
    |
 LL |     let r = &mut x;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0493]: destructors cannot be evaluated at compile-time
   --> $DIR/issue-65394.rs:7:9
@@ -12,5 +15,5 @@ LL |     let mut x = Vec::<i32>::new();
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0493.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0493, E0658.
+For more information about an error, try `rustc --explain E0493`.
diff --git a/src/test/ui/consts/const-multi-ref.stderr b/src/test/ui/consts/const-multi-ref.stderr
index ed3837e9c9ee5..0809c77c1b604 100644
--- a/src/test/ui/consts/const-multi-ref.stderr
+++ b/src/test/ui/consts/const-multi-ref.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const-multi-ref.rs:6:13
    |
 LL |     let p = &mut a;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
   --> $DIR/const-multi-ref.rs:16:13
@@ -12,5 +15,5 @@ LL |     let p = &a;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0492.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0492, E0658.
+For more information about an error, try `rustc --explain E0492`.
diff --git a/src/test/ui/consts/const_let_assign3.stderr b/src/test/ui/consts/const_let_assign3.stderr
index 53b960b4ec0e0..7852874944b22 100644
--- a/src/test/ui/consts/const_let_assign3.stderr
+++ b/src/test/ui/consts/const_let_assign3.stderr
@@ -4,17 +4,23 @@ error[E0019]: constant function contains unimplemented expression type
 LL |         self.state = x;
    |         ^^^^^^^^^^^^^^
 
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const_let_assign3.rs:16:5
    |
 LL |     s.foo(3);
    |     ^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const_let_assign3.rs:22:13
    |
 LL |     let y = &mut x;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: constant contains unimplemented expression type
   --> $DIR/const_let_assign3.rs:24:5
@@ -24,5 +30,5 @@ LL |     *y = 42;
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0017, E0019.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/consts/projection_qualif.stderr b/src/test/ui/consts/projection_qualif.stderr
index 0efb6bfd10a17..41c7b62812c08 100644
--- a/src/test/ui/consts/projection_qualif.stderr
+++ b/src/test/ui/consts/projection_qualif.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/projection_qualif.rs:6:27
    |
 LL |         let b: *mut u32 = &mut a;
    |                           ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: dereferencing raw pointers in constants is unstable
   --> $DIR/projection_qualif.rs:7:18
@@ -21,5 +24,5 @@ LL |         unsafe { *b = 5; }
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0017, E0019, E0658.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/consts/static_mut_containing_mut_ref2.stderr b/src/test/ui/consts/static_mut_containing_mut_ref2.stderr
index ca691b07be063..e0074687874fe 100644
--- a/src/test/ui/consts/static_mut_containing_mut_ref2.stderr
+++ b/src/test/ui/consts/static_mut_containing_mut_ref2.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/static_mut_containing_mut_ref2.rs:3:46
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: static contains unimplemented expression type
   --> $DIR/static_mut_containing_mut_ref2.rs:3:45
@@ -12,5 +15,5 @@ LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 4
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0019.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.

From 19ddfb545bb6e70ba3281bdce4f40ae8e564a53a Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 28 Nov 2019 11:38:07 -0500
Subject: [PATCH 12/15] Update miri unleashed tests

---
 .../ui/consts/miri_unleashed/mutable_const.rs |  3 ++-
 .../miri_unleashed/mutable_const.stderr       | 23 ++++++++++++++-----
 .../miri_unleashed/mutable_references.rs      |  5 ++++
 .../miri_unleashed/mutable_references.stderr  |  4 ++--
 .../consts/miri_unleashed/read_from_static.rs | 11 +++++++++
 5 files changed, 37 insertions(+), 9 deletions(-)
 create mode 100644 src/test/ui/consts/miri_unleashed/read_from_static.rs

diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.rs b/src/test/ui/consts/miri_unleashed/mutable_const.rs
index cf5e6d6002581..972f59549ea74 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_const.rs
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.rs
@@ -1,6 +1,7 @@
 // compile-flags: -Zunleash-the-miri-inside-of-you
 
 #![feature(const_raw_ptr_deref)]
+#![feature(const_mut_refs)]
 #![deny(const_err)]
 
 use std::cell::UnsafeCell;
@@ -12,7 +13,7 @@ const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
 const MUTATING_BEHIND_RAW: () = {
     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
     unsafe {
-        *MUTABLE_BEHIND_RAW = 99 //~ ERROR constant contains unimplemented expression type
+        *MUTABLE_BEHIND_RAW = 99 //~ ERROR any use of this value will cause an error
     }
 };
 
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.stderr b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
index 1a18063625be3..9daca765c7cd0 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_const.stderr
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
@@ -1,15 +1,26 @@
 warning: skipping const checks
-  --> $DIR/mutable_const.rs:9:38
+  --> $DIR/mutable_const.rs:10:38
    |
 LL | const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
    |                                      ^^^^^^^^^^^^^^^^^^^^
 
-error[E0019]: constant contains unimplemented expression type
-  --> $DIR/mutable_const.rs:15:9
+error: any use of this value will cause an error
+  --> $DIR/mutable_const.rs:16:9
    |
-LL |         *MUTABLE_BEHIND_RAW = 99
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+LL | / const MUTATING_BEHIND_RAW: () = {
+LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
+LL | |     unsafe {
+LL | |         *MUTABLE_BEHIND_RAW = 99
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ tried to modify constant memory
+LL | |     }
+LL | | };
+   | |__-
+   |
+note: lint level defined here
+  --> $DIR/mutable_const.rs:5:9
+   |
+LL | #![deny(const_err)]
+   |         ^^^^^^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0019`.
diff --git a/src/test/ui/consts/miri_unleashed/mutable_references.rs b/src/test/ui/consts/miri_unleashed/mutable_references.rs
index d95833e3071df..fe3c4ee70f2a2 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_references.rs
+++ b/src/test/ui/consts/miri_unleashed/mutable_references.rs
@@ -6,12 +6,16 @@ use std::cell::UnsafeCell;
 
 // a test demonstrating what things we could allow with a smarter const qualification
 
+// this is fine because is not possible to mutate through an immutable reference.
 static FOO: &&mut u32 = &&mut 42;
 
+// this is fine because accessing an immutable static `BAR` is equivalent to accessing `*&BAR`
+// which puts the mutable reference behind an immutable one.
 static BAR: &mut () = &mut ();
 
 struct Foo<T>(T);
 
+// this is fine for the same reason as `BAR`.
 static BOO: &mut Foo<()> = &mut Foo(());
 
 struct Meh {
@@ -25,6 +29,7 @@ static MEH: Meh = Meh {
     //~^ WARN: skipping const checks
 };
 
+// this is fine for the same reason as `BAR`.
 static OH_YES: &mut i32 = &mut 42;
 
 fn main() {
diff --git a/src/test/ui/consts/miri_unleashed/mutable_references.stderr b/src/test/ui/consts/miri_unleashed/mutable_references.stderr
index ce1f5f38dd99a..3e1300c63c17d 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_references.stderr
+++ b/src/test/ui/consts/miri_unleashed/mutable_references.stderr
@@ -1,11 +1,11 @@
 warning: skipping const checks
-  --> $DIR/mutable_references.rs:24:8
+  --> $DIR/mutable_references.rs:28:8
    |
 LL |     x: &UnsafeCell::new(42),
    |        ^^^^^^^^^^^^^^^^^^^^
 
 error[E0594]: cannot assign to `*OH_YES`, as `OH_YES` is an immutable static item
-  --> $DIR/mutable_references.rs:34:5
+  --> $DIR/mutable_references.rs:39:5
    |
 LL |     *OH_YES = 99;
    |     ^^^^^^^^^^^^ cannot assign
diff --git a/src/test/ui/consts/miri_unleashed/read_from_static.rs b/src/test/ui/consts/miri_unleashed/read_from_static.rs
new file mode 100644
index 0000000000000..821c501c9fcc5
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/read_from_static.rs
@@ -0,0 +1,11 @@
+// run-pass
+// compile-flags: -Zunleash-the-miri-inside-of-you
+#![feature(const_mut_refs)]
+#![allow(const_err)]
+
+static OH_YES: &mut i32 = &mut 42;
+
+fn main() {
+    // Make sure `OH_YES` can be read.
+    assert_eq!(*OH_YES, 42);
+}

From dc0117a42c000428914ed3ca42ba1a287f622694 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 28 Nov 2019 11:57:34 -0500
Subject: [PATCH 13/15] Add dual tests for const_mut_refs

---
 src/test/ui/consts/projection_qualif.mut_refs.stderr | 12 ++++++++++++
 src/test/ui/consts/projection_qualif.rs              |  8 ++++++--
 ..._qualif.stderr => projection_qualif.stock.stderr} |  6 +++---
 .../static_mut_containing_mut_ref2.mut_refs.stderr   |  9 +++++++++
 src/test/ui/consts/static_mut_containing_mut_ref2.rs |  9 +++++++--
 ...r => static_mut_containing_mut_ref2.stock.stderr} |  4 ++--
 6 files changed, 39 insertions(+), 9 deletions(-)
 create mode 100644 src/test/ui/consts/projection_qualif.mut_refs.stderr
 rename src/test/ui/consts/{projection_qualif.stderr => projection_qualif.stock.stderr} (89%)
 create mode 100644 src/test/ui/consts/static_mut_containing_mut_ref2.mut_refs.stderr
 rename src/test/ui/consts/{static_mut_containing_mut_ref2.stderr => static_mut_containing_mut_ref2.stock.stderr} (89%)

diff --git a/src/test/ui/consts/projection_qualif.mut_refs.stderr b/src/test/ui/consts/projection_qualif.mut_refs.stderr
new file mode 100644
index 0000000000000..23538777c9df3
--- /dev/null
+++ b/src/test/ui/consts/projection_qualif.mut_refs.stderr
@@ -0,0 +1,12 @@
+error[E0658]: dereferencing raw pointers in constants is unstable
+  --> $DIR/projection_qualif.rs:11:18
+   |
+LL |         unsafe { *b = 5; }
+   |                  ^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/51911
+   = help: add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/consts/projection_qualif.rs b/src/test/ui/consts/projection_qualif.rs
index dedb7db592089..cfe8e7f03d5e4 100644
--- a/src/test/ui/consts/projection_qualif.rs
+++ b/src/test/ui/consts/projection_qualif.rs
@@ -1,11 +1,15 @@
+// revisions: stock mut_refs
+
+#![cfg_attr(mut_refs, feature(const_mut_refs))]
+
 use std::cell::Cell;
 
 const FOO: &u32 = {
     let mut a = 42;
     {
-        let b: *mut u32 = &mut a; //~ ERROR may only refer to immutable values
+        let b: *mut u32 = &mut a; //[stock]~ ERROR may only refer to immutable values
         unsafe { *b = 5; } //~ ERROR dereferencing raw pointers in constants
-        //~^ contains unimplemented expression
+        //[stock]~^ contains unimplemented expression
     }
     &{a}
 };
diff --git a/src/test/ui/consts/projection_qualif.stderr b/src/test/ui/consts/projection_qualif.stock.stderr
similarity index 89%
rename from src/test/ui/consts/projection_qualif.stderr
rename to src/test/ui/consts/projection_qualif.stock.stderr
index 41c7b62812c08..472d2607453d0 100644
--- a/src/test/ui/consts/projection_qualif.stderr
+++ b/src/test/ui/consts/projection_qualif.stock.stderr
@@ -1,5 +1,5 @@
 error[E0658]: references in constants may only refer to immutable values
-  --> $DIR/projection_qualif.rs:6:27
+  --> $DIR/projection_qualif.rs:10:27
    |
 LL |         let b: *mut u32 = &mut a;
    |                           ^^^^^^ constants require immutable values
@@ -8,7 +8,7 @@ LL |         let b: *mut u32 = &mut a;
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: dereferencing raw pointers in constants is unstable
-  --> $DIR/projection_qualif.rs:7:18
+  --> $DIR/projection_qualif.rs:11:18
    |
 LL |         unsafe { *b = 5; }
    |                  ^^^^^^
@@ -17,7 +17,7 @@ LL |         unsafe { *b = 5; }
    = help: add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
 
 error[E0019]: constant contains unimplemented expression type
-  --> $DIR/projection_qualif.rs:7:18
+  --> $DIR/projection_qualif.rs:11:18
    |
 LL |         unsafe { *b = 5; }
    |                  ^^^^^^
diff --git a/src/test/ui/consts/static_mut_containing_mut_ref2.mut_refs.stderr b/src/test/ui/consts/static_mut_containing_mut_ref2.mut_refs.stderr
new file mode 100644
index 0000000000000..b43fbc86f99f2
--- /dev/null
+++ b/src/test/ui/consts/static_mut_containing_mut_ref2.mut_refs.stderr
@@ -0,0 +1,9 @@
+error[E0080]: could not evaluate static initializer
+  --> $DIR/static_mut_containing_mut_ref2.rs:7:45
+   |
+LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
+   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ tried to modify a static's initial value from another static's initializer
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/src/test/ui/consts/static_mut_containing_mut_ref2.rs b/src/test/ui/consts/static_mut_containing_mut_ref2.rs
index ef378fa84518e..74162fbd54b04 100644
--- a/src/test/ui/consts/static_mut_containing_mut_ref2.rs
+++ b/src/test/ui/consts/static_mut_containing_mut_ref2.rs
@@ -1,7 +1,12 @@
+// revisions: stock mut_refs
+
+#![cfg_attr(mut_refs, feature(const_mut_refs))]
+
 static mut STDERR_BUFFER_SPACE: u8 = 0;
 
 pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
-//~^ ERROR references in statics may only refer to immutable values
-//~| ERROR static contains unimplemented expression type
+//[mut_refs]~^ ERROR could not evaluate static initializer
+//[stock]~^^ ERROR references in statics may only refer to immutable values
+//[stock]~| ERROR static contains unimplemented expression type
 
 fn main() {}
diff --git a/src/test/ui/consts/static_mut_containing_mut_ref2.stderr b/src/test/ui/consts/static_mut_containing_mut_ref2.stock.stderr
similarity index 89%
rename from src/test/ui/consts/static_mut_containing_mut_ref2.stderr
rename to src/test/ui/consts/static_mut_containing_mut_ref2.stock.stderr
index e0074687874fe..430cef94dc3cb 100644
--- a/src/test/ui/consts/static_mut_containing_mut_ref2.stderr
+++ b/src/test/ui/consts/static_mut_containing_mut_ref2.stock.stderr
@@ -1,5 +1,5 @@
 error[E0658]: references in statics may only refer to immutable values
-  --> $DIR/static_mut_containing_mut_ref2.rs:3:46
+  --> $DIR/static_mut_containing_mut_ref2.rs:7:46
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^ statics require immutable values
@@ -8,7 +8,7 @@ LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 4
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: static contains unimplemented expression type
-  --> $DIR/static_mut_containing_mut_ref2.rs:3:45
+  --> $DIR/static_mut_containing_mut_ref2.rs:7:45
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

From 416b439ffbdf6840bb3c663fea0246d3ead9a5dc Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Thu, 28 Nov 2019 12:20:28 -0500
Subject: [PATCH 14/15] Correct other tests related to const_mut_refs

---
 .../compile-fail/consts/const-fn-error.rs     |  2 +-
 .../check-static-immutable-mut-slices.stderr  |  7 ++-
 src/test/ui/error-codes/E0017.rs              |  8 ++--
 src/test/ui/error-codes/E0017.stderr          | 24 ++++++++---
 src/test/ui/error-codes/E0388.rs              | 10 +++++
 src/test/ui/error-codes/E0388.stderr          | 43 +++++++++++++++++++
 .../issue-17718-const-bad-values.stderr       | 12 ++++--
 src/test/ui/issues/issue-46604.rs             |  2 +-
 src/test/ui/issues/issue-46604.stderr         |  9 ++--
 9 files changed, 97 insertions(+), 20 deletions(-)
 create mode 100644 src/test/ui/error-codes/E0388.rs
 create mode 100644 src/test/ui/error-codes/E0388.stderr

diff --git a/src/test/compile-fail/consts/const-fn-error.rs b/src/test/compile-fail/consts/const-fn-error.rs
index 1a4fc72e81786..5d26059644d17 100644
--- a/src/test/compile-fail/consts/const-fn-error.rs
+++ b/src/test/compile-fail/consts/const-fn-error.rs
@@ -6,7 +6,7 @@ const fn f(x: usize) -> usize {
     let mut sum = 0;
     for i in 0..x {
         //~^ ERROR E0015
-        //~| ERROR E0017
+        //~| ERROR E0658
         //~| ERROR E0080
         //~| ERROR E0744
         //~| ERROR E0019
diff --git a/src/test/ui/check-static-immutable-mut-slices.stderr b/src/test/ui/check-static-immutable-mut-slices.stderr
index 4f4bf16a0ff69..39da824ede57c 100644
--- a/src/test/ui/check-static-immutable-mut-slices.stderr
+++ b/src/test/ui/check-static-immutable-mut-slices.stderr
@@ -1,9 +1,12 @@
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/check-static-immutable-mut-slices.rs:3:37
    |
 LL | static TEST: &'static mut [isize] = &mut [];
    |                                     ^^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0017`.
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/error-codes/E0017.rs b/src/test/ui/error-codes/E0017.rs
index 3bc518c2c2b71..64be41170d0c8 100644
--- a/src/test/ui/error-codes/E0017.rs
+++ b/src/test/ui/error-codes/E0017.rs
@@ -2,10 +2,10 @@ static X: i32 = 1;
 const C: i32 = 2;
 static mut M: i32 = 3;
 
-const CR: &'static mut i32 = &mut C; //~ ERROR E0017
-static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
+const CR: &'static mut i32 = &mut C; //~ ERROR E0658
+static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0658
                                               //~| ERROR E0019
                                               //~| ERROR cannot borrow
-static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
-static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M }; //~ ERROR E0017
+static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0658
+static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M }; //~ ERROR E0658
 fn main() {}
diff --git a/src/test/ui/error-codes/E0017.stderr b/src/test/ui/error-codes/E0017.stderr
index 8c8660adceb7a..9a87195a9d05e 100644
--- a/src/test/ui/error-codes/E0017.stderr
+++ b/src/test/ui/error-codes/E0017.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/E0017.rs:5:30
    |
 LL | const CR: &'static mut i32 = &mut C;
    |                              ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: static contains unimplemented expression type
   --> $DIR/E0017.rs:6:39
@@ -10,11 +13,14 @@ error[E0019]: static contains unimplemented expression type
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^
 
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:6:39
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0596]: cannot borrow immutable static item `X` as mutable
   --> $DIR/E0017.rs:6:39
@@ -22,19 +28,25 @@ error[E0596]: cannot borrow immutable static item `X` as mutable
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ cannot borrow as mutable
 
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:9:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C;
    |                                      ^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:10:52
    |
 LL | static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M };
    |                                                    ^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0017, E0019, E0596.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0596, E0658.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/error-codes/E0388.rs b/src/test/ui/error-codes/E0388.rs
new file mode 100644
index 0000000000000..5954e3490b06c
--- /dev/null
+++ b/src/test/ui/error-codes/E0388.rs
@@ -0,0 +1,10 @@
+static X: i32 = 1;
+const C: i32 = 2;
+
+const CR: &'static mut i32 = &mut C; //~ ERROR E0658
+static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0658
+                                              //~| ERROR cannot borrow
+                                              //~| ERROR E0019
+static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0658
+
+fn main() {}
diff --git a/src/test/ui/error-codes/E0388.stderr b/src/test/ui/error-codes/E0388.stderr
new file mode 100644
index 0000000000000..986307d3f123f
--- /dev/null
+++ b/src/test/ui/error-codes/E0388.stderr
@@ -0,0 +1,43 @@
+error[E0658]: references in constants may only refer to immutable values
+  --> $DIR/E0388.rs:4:30
+   |
+LL | const CR: &'static mut i32 = &mut C;
+   |                              ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error[E0019]: static contains unimplemented expression type
+  --> $DIR/E0388.rs:5:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X;
+   |                                       ^^^^^^
+
+error[E0658]: references in statics may only refer to immutable values
+  --> $DIR/E0388.rs:5:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X;
+   |                                       ^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error[E0596]: cannot borrow immutable static item `X` as mutable
+  --> $DIR/E0388.rs:5:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X;
+   |                                       ^^^^^^ cannot borrow as mutable
+
+error[E0658]: references in statics may only refer to immutable values
+  --> $DIR/E0388.rs:8:38
+   |
+LL | static CONST_REF: &'static mut i32 = &mut C;
+   |                                      ^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0019, E0596, E0658.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/issues/issue-17718-const-bad-values.stderr b/src/test/ui/issues/issue-17718-const-bad-values.stderr
index 8d875d37d85f9..7e4a62ac96957 100644
--- a/src/test/ui/issues/issue-17718-const-bad-values.stderr
+++ b/src/test/ui/issues/issue-17718-const-bad-values.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/issue-17718-const-bad-values.rs:1:34
    |
 LL | const C1: &'static mut [usize] = &mut [];
    |                                  ^^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0013]: constants cannot refer to statics, use a constant instead
   --> $DIR/issue-17718-const-bad-values.rs:5:46
@@ -10,13 +13,16 @@ error[E0013]: constants cannot refer to statics, use a constant instead
 LL | const C2: &'static mut usize = unsafe { &mut S };
    |                                              ^
 
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/issue-17718-const-bad-values.rs:5:41
    |
 LL | const C2: &'static mut usize = unsafe { &mut S };
    |                                         ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0013, E0017.
+Some errors have detailed explanations: E0013, E0658.
 For more information about an error, try `rustc --explain E0013`.
diff --git a/src/test/ui/issues/issue-46604.rs b/src/test/ui/issues/issue-46604.rs
index 4f1ad38dbdd94..e1967eb765542 100644
--- a/src/test/ui/issues/issue-46604.rs
+++ b/src/test/ui/issues/issue-46604.rs
@@ -1,4 +1,4 @@
-static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR E0017
+static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR E0658
 fn write<T: AsRef<[u8]>>(buffer: T) { }
 
 fn main() {
diff --git a/src/test/ui/issues/issue-46604.stderr b/src/test/ui/issues/issue-46604.stderr
index c72f580f24391..32c7ecbf72e95 100644
--- a/src/test/ui/issues/issue-46604.stderr
+++ b/src/test/ui/issues/issue-46604.stderr
@@ -1,8 +1,11 @@
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/issue-46604.rs:1:25
    |
 LL | static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];
    |                         ^^^^^^^^^^^^^^^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0594]: cannot assign to `buf[_]`, as `buf` is an immutable static item
   --> $DIR/issue-46604.rs:6:5
@@ -12,5 +15,5 @@ LL |     buf[0]=2;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0594.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0594, E0658.
+For more information about an error, try `rustc --explain E0594`.

From e01ad6a01abce35f59543bf38a280a05eb7f6929 Mon Sep 17 00:00:00 2001
From: Christian Poveda <christianpoveda@protonmail.com>
Date: Fri, 29 Nov 2019 21:54:27 -0500
Subject: [PATCH 15/15] Remove E0017 from error codes index

---
 src/librustc_error_codes/error_codes.rs       |  1 -
 src/librustc_error_codes/error_codes/E0017.md | 20 -------------------
 2 files changed, 21 deletions(-)
 delete mode 100644 src/librustc_error_codes/error_codes/E0017.md

diff --git a/src/librustc_error_codes/error_codes.rs b/src/librustc_error_codes/error_codes.rs
index 7f111b42403b5..259fd51c4362e 100644
--- a/src/librustc_error_codes/error_codes.rs
+++ b/src/librustc_error_codes/error_codes.rs
@@ -18,7 +18,6 @@ E0010: include_str!("./error_codes/E0010.md"),
 E0013: include_str!("./error_codes/E0013.md"),
 E0014: include_str!("./error_codes/E0014.md"),
 E0015: include_str!("./error_codes/E0015.md"),
-E0017: include_str!("./error_codes/E0017.md"),
 E0019: include_str!("./error_codes/E0019.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),
diff --git a/src/librustc_error_codes/error_codes/E0017.md b/src/librustc_error_codes/error_codes/E0017.md
deleted file mode 100644
index d5e6857b4d6f8..0000000000000
--- a/src/librustc_error_codes/error_codes/E0017.md
+++ /dev/null
@@ -1,20 +0,0 @@
-References in statics and constants may only refer to immutable values.
-
-Erroneous code example:
-
-```compile_fail,E0017
-static X: i32 = 1;
-const C: i32 = 2;
-
-// these three are not allowed:
-const CR: &mut i32 = &mut C;
-static STATIC_REF: &'static mut i32 = &mut X;
-static CONST_REF: &'static mut i32 = &mut C;
-```
-
-Statics are shared everywhere, and if they refer to mutable data one might
-violate memory safety since holding multiple mutable references to shared data
-is not allowed.
-
-If you really want global mutable state, try using `static mut` or a global
-`UnsafeCell`.