From bc543d7e6c4c87f99eea4dc6217eee54cd7f18b1 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Mon, 19 Nov 2018 11:19:14 +0100
Subject: [PATCH 01/15] Allow assignments in const contexts

---
 src/librustc_mir/transform/qualify_consts.rs  | 34 ++++++++++++++----
 .../assign-to-static-within-other-static.rs   | 35 +++++++++++++++++++
 ...ssign-to-static-within-other-static.stderr |  8 +++++
 .../const-eval/mod-static-with-const-fn.rs    |  5 +--
 .../mod-static-with-const-fn.stderr           | 15 ++------
 src/test/ui/consts/const_let_assign.rs        | 12 +++++++
 src/test/ui/consts/const_let_assign2.rs       | 25 +++++++++++++
 src/test/ui/consts/const_let_assign3.rs       | 22 ++++++++++++
 src/test/ui/consts/const_let_assign3.stderr   |  9 +++++
 9 files changed, 143 insertions(+), 22 deletions(-)
 create mode 100644 src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
 create mode 100644 src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
 create mode 100644 src/test/ui/consts/const_let_assign.rs
 create mode 100644 src/test/ui/consts/const_let_assign2.rs
 create mode 100644 src/test/ui/consts/const_let_assign3.rs
 create mode 100644 src/test/ui/consts/const_let_assign3.stderr

diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 00309b0a3e905..31a0dc1494ce1 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -243,13 +243,29 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             return;
         }
 
+        if self.tcx.features().const_let {
+            let mut dest = dest;
+            let index = loop {
+                match dest {
+                    Place::Local(index) => break *index,
+                    Place::Projection(proj) => dest = &proj.base,
+                    Place::Promoted(..) | Place::Static(..) => {
+                        // Catch more errors in the destination.
+                        self.visit_place(
+                            dest,
+                            PlaceContext::MutatingUse(MutatingUseContext::Store),
+                            location
+                        );
+                        return;
+                    }
+                }
+            };
+            debug!("store to var {:?}", index);
+            self.local_qualif[index] = Some(self.qualif);
+            return;
+        }
+
         match *dest {
-            Place::Local(index) if (self.mir.local_kind(index) == LocalKind::Var ||
-                                   self.mir.local_kind(index) == LocalKind::Arg) &&
-                                   self.tcx.sess.features_untracked().const_let => {
-                debug!("store to var {:?}", index);
-                self.local_qualif[index] = Some(self.qualif);
-            }
             Place::Local(index) if self.mir.local_kind(index) == LocalKind::Temp ||
                                    self.mir.local_kind(index) == LocalKind::ReturnPointer => {
                 debug!("store to {:?} (temp or return pointer)", index);
@@ -478,6 +494,12 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 
                 // Only allow statics (not consts) to refer to other statics.
                 if self.mode == Mode::Static || self.mode == Mode::StaticMut {
+                    if context.is_mutating_use() {
+                        self.tcx.sess.span_err(
+                            self.span,
+                            "cannot mutate statics in the initializer of another static",
+                        );
+                    }
                     return;
                 }
                 self.add(Qualif::NOT_CONST);
diff --git a/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
new file mode 100644
index 0000000000000..5113d73b38414
--- /dev/null
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
@@ -0,0 +1,35 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// New test for #53818: modifying static memory at compile-time is not allowed.
+// The test should never compile successfully
+
+#![feature(const_raw_ptr_deref)]
+#![feature(const_let)]
+
+use std::cell::UnsafeCell;
+
+struct Foo(UnsafeCell<u32>);
+
+unsafe impl Send for Foo {}
+unsafe impl Sync for Foo {}
+
+static FOO: Foo = Foo(UnsafeCell::new(42));
+
+static BAR: () = unsafe {
+    *FOO.0.get() = 5;
+};
+
+static mut FOO2: u32 = 42;
+static BOO2: () = unsafe {
+    FOO2 = 5;
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
new file mode 100644
index 0000000000000..87f02e8e4cf7e
--- /dev/null
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
@@ -0,0 +1,8 @@
+error: cannot mutate statics in the initializer of another static
+  --> $DIR/assign-to-static-within-other-static.rs:32:5
+   |
+LL |     FOO2 = 5;
+   |     ^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs b/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
index 4136a7b6a724f..600931e49a08e 100644
--- a/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
+++ b/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
@@ -9,7 +9,7 @@
 // except according to those terms.
 
 // New test for #53818: modifying static memory at compile-time is not allowed.
-// The test should never succeed.
+// The test should never compile successfully
 
 #![feature(const_raw_ptr_deref)]
 #![feature(const_let)]
@@ -27,9 +27,6 @@ fn foo() {}
 
 static BAR: () = unsafe {
     *FOO.0.get() = 5;
-    //~^ ERROR statements in statics are unstable (see issue #48821)
-    // This error is caused by a separate bug that the feature gate error is reported
-    // even though the feature gate "const_let" is active.
 
     foo();
     //~^ ERROR calls in statics are limited to constant functions, tuple structs and tuple variants
diff --git a/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr b/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
index c2bba27e4d1e2..899fc24f153a5 100644
--- a/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
+++ b/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
@@ -1,18 +1,9 @@
-error[E0658]: statements in statics are unstable (see issue #48821)
-  --> $DIR/mod-static-with-const-fn.rs:29:5
-   |
-LL |     *FOO.0.get() = 5;
-   |     ^^^^^^^^^^^^^^^^
-   |
-   = help: add #![feature(const_let)] to the crate attributes to enable
-
 error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/mod-static-with-const-fn.rs:34:5
+  --> $DIR/mod-static-with-const-fn.rs:31:5
    |
 LL |     foo();
    |     ^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors occurred: E0015, E0658.
-For more information about an error, try `rustc --explain E0015`.
+For more information about this error, try `rustc --explain E0015`.
diff --git a/src/test/ui/consts/const_let_assign.rs b/src/test/ui/consts/const_let_assign.rs
new file mode 100644
index 0000000000000..a3c53a451e106
--- /dev/null
+++ b/src/test/ui/consts/const_let_assign.rs
@@ -0,0 +1,12 @@
+// compile-pass
+
+#![feature(const_let)]
+
+struct S(i32);
+
+const A: () = {
+    let mut s = S(0);
+    s.0 = 1;
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/const_let_assign2.rs b/src/test/ui/consts/const_let_assign2.rs
new file mode 100644
index 0000000000000..0de7396501adc
--- /dev/null
+++ b/src/test/ui/consts/const_let_assign2.rs
@@ -0,0 +1,25 @@
+// compile-pass
+
+#![feature(const_let)]
+#![feature(const_fn)]
+
+pub struct AA {
+    pub data: [u8; 10],
+}
+
+impl AA {
+    pub const fn new() -> Self {
+        let mut res: AA = AA { data: [0; 10] };
+        res.data[0] = 5;
+        res
+    }
+}
+
+static mut BB: AA = AA::new();
+
+fn main() {
+    let ptr = unsafe { &mut BB };
+    for a in ptr.data.iter() {
+        println!("{}", a);
+    }
+}
diff --git a/src/test/ui/consts/const_let_assign3.rs b/src/test/ui/consts/const_let_assign3.rs
new file mode 100644
index 0000000000000..83825456b5c61
--- /dev/null
+++ b/src/test/ui/consts/const_let_assign3.rs
@@ -0,0 +1,22 @@
+#![feature(const_let)]
+#![feature(const_fn)]
+
+struct S {
+    state: u32,
+}
+
+impl S {
+    const fn foo(&mut self, x: u32) {
+        self.state = x;
+    }
+}
+
+const FOO: S = {
+    let mut s = S { state: 42 };
+    s.foo(3); //~ ERROR references in constants may only refer to immutable values
+    s
+};
+
+fn main() {
+    assert_eq!(FOO.state, 3);
+}
diff --git a/src/test/ui/consts/const_let_assign3.stderr b/src/test/ui/consts/const_let_assign3.stderr
new file mode 100644
index 0000000000000..7f9a953c10fe8
--- /dev/null
+++ b/src/test/ui/consts/const_let_assign3.stderr
@@ -0,0 +1,9 @@
+error[E0017]: references in constants may only refer to immutable values
+  --> $DIR/const_let_assign3.rs:16:5
+   |
+LL |     s.foo(3); //~ ERROR references in constants may only refer to immutable values
+   |     ^ constants require immutable values
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0017`.

From 59eff14120a067d04832142c8198f0132db2acb0 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Mon, 19 Nov 2018 13:49:07 +0100
Subject: [PATCH 02/15] Also catch static mutation at evaluation time

---
 src/librustc/ich/impls_ty.rs                  |  1 +
 src/librustc/mir/interpret/error.rs           |  3 ++
 src/librustc_mir/interpret/memory.rs          |  8 ++---
 src/librustc_mir/transform/const_prop.rs      |  1 +
 src/librustc_mir/transform/qualify_consts.rs  |  7 ++++-
 .../assign-to-static-within-other-static-2.rs | 30 +++++++++++++++++++
 ...ign-to-static-within-other-static-2.stderr |  9 ++++++
 .../assign-to-static-within-other-static.rs   | 17 ++---------
 ...ssign-to-static-within-other-static.stderr |  6 ++--
 src/test/ui/error-codes/E0017.rs              |  1 +
 src/test/ui/error-codes/E0017.stderr          | 10 +++++--
 src/test/ui/error-codes/E0388.rs              |  1 +
 src/test/ui/error-codes/E0388.stderr          | 10 +++++--
 src/test/ui/write-to-static-mut-in-static.rs  |  4 +--
 .../ui/write-to-static-mut-in-static.stderr   |  9 ++----
 15 files changed, 82 insertions(+), 35 deletions(-)
 create mode 100644 src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.rs
 create mode 100644 src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.stderr

diff --git a/src/librustc/ich/impls_ty.rs b/src/librustc/ich/impls_ty.rs
index f3a62975dd9f4..fc970400f6ccf 100644
--- a/src/librustc/ich/impls_ty.rs
+++ b/src/librustc/ich/impls_ty.rs
@@ -423,6 +423,7 @@ impl_stable_hash_for!(
         CalledClosureAsFunction,
         VtableForArgumentlessMethod,
         ModifiedConstantMemory,
+        ModifiedStatic,
         AssumptionNotHeld,
         InlineAsm,
         ReallocateNonBasePtr,
diff --git a/src/librustc/mir/interpret/error.rs b/src/librustc/mir/interpret/error.rs
index f28aa41ed4222..5895b6cab9abd 100644
--- a/src/librustc/mir/interpret/error.rs
+++ b/src/librustc/mir/interpret/error.rs
@@ -276,6 +276,7 @@ pub enum EvalErrorKind<'tcx, O> {
     CalledClosureAsFunction,
     VtableForArgumentlessMethod,
     ModifiedConstantMemory,
+    ModifiedStatic,
     AssumptionNotHeld,
     InlineAsm,
     TypeNotPrimitive(Ty<'tcx>),
@@ -380,6 +381,8 @@ impl<'tcx, O> EvalErrorKind<'tcx, O> {
                 "tried to call a vtable function without arguments",
             ModifiedConstantMemory =>
                 "tried to modify constant memory",
+            ModifiedStatic =>
+                "tried to modify a static's initial value from another static's initializer",
             AssumptionNotHeld =>
                 "`assume` argument was false",
             InlineAsm =>
diff --git a/src/librustc_mir/interpret/memory.rs b/src/librustc_mir/interpret/memory.rs
index e125927e7d273..68a91b0a3bd28 100644
--- a/src/librustc_mir/interpret/memory.rs
+++ b/src/librustc_mir/interpret/memory.rs
@@ -423,10 +423,10 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
             if alloc.mutability == Mutability::Immutable {
                 return err!(ModifiedConstantMemory);
             }
-            let kind = M::STATIC_KIND.expect(
-                "An allocation is being mutated but the machine does not expect that to happen"
-            );
-            Ok((MemoryKind::Machine(kind), alloc.into_owned()))
+            match M::STATIC_KIND {
+                Some(kind) => Ok((MemoryKind::Machine(kind), alloc.into_owned())),
+                None => err!(ModifiedStatic),
+            }
         });
         // Unpack the error type manually because type inference doesn't
         // work otherwise (and we cannot help it because `impl Trait`)
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index 885d70dc4304d..51327464266d5 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -197,6 +197,7 @@ impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
                     | CalledClosureAsFunction
                     | VtableForArgumentlessMethod
                     | ModifiedConstantMemory
+                    | ModifiedStatic
                     | AssumptionNotHeld
                     // FIXME: should probably be removed and turned into a bug! call
                     | TypeNotPrimitive(_)
diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 31a0dc1494ce1..1371b9a997707 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -249,7 +249,8 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                 match dest {
                     Place::Local(index) => break *index,
                     Place::Projection(proj) => dest = &proj.base,
-                    Place::Promoted(..) | Place::Static(..) => {
+                    Place::Promoted(..) => bug!("promoteds don't exist yet during promotion"),
+                    Place::Static(..) => {
                         // Catch more errors in the destination.
                         self.visit_place(
                             dest,
@@ -495,6 +496,10 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                 // Only allow statics (not consts) to refer to other statics.
                 if self.mode == Mode::Static || self.mode == Mode::StaticMut {
                     if context.is_mutating_use() {
+                        // this is not strictly necessary as miri will also bail out
+                        // For interior mutability we can't really catch this statically as that
+                        // goes through raw pointers and intermediate temporaries, so miri has
+                        // to catch this anyway
                         self.tcx.sess.span_err(
                             self.span,
                             "cannot mutate statics in the initializer of another static",
diff --git a/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.rs b/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.rs
new file mode 100644
index 0000000000000..ef0de61d219ad
--- /dev/null
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.rs
@@ -0,0 +1,30 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// New test for #53818: modifying static memory at compile-time is not allowed.
+// The test should never compile successfully
+
+#![feature(const_raw_ptr_deref)]
+#![feature(const_let)]
+
+use std::cell::UnsafeCell;
+
+struct Foo(UnsafeCell<u32>);
+
+unsafe impl Send for Foo {}
+unsafe impl Sync for Foo {}
+
+static FOO: Foo = Foo(UnsafeCell::new(42));
+
+static BAR: () = unsafe {
+    *FOO.0.get() = 5; //~ ERROR could not evaluate static initializer
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.stderr b/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.stderr
new file mode 100644
index 0000000000000..0892b05a69df2
--- /dev/null
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.stderr
@@ -0,0 +1,9 @@
+error[E0080]: could not evaluate static initializer
+  --> $DIR/assign-to-static-within-other-static-2.rs:27:5
+   |
+LL |     *FOO.0.get() = 5; //~ ERROR could not evaluate static initializer
+   |     ^^^^^^^^^^^^^^^^ 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/const-eval/assign-to-static-within-other-static.rs b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
index 5113d73b38414..6f16f644eec68 100644
--- a/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.rs
@@ -16,20 +16,9 @@
 
 use std::cell::UnsafeCell;
 
-struct Foo(UnsafeCell<u32>);
-
-unsafe impl Send for Foo {}
-unsafe impl Sync for Foo {}
-
-static FOO: Foo = Foo(UnsafeCell::new(42));
-
-static BAR: () = unsafe {
-    *FOO.0.get() = 5;
-};
-
-static mut FOO2: u32 = 42;
-static BOO2: () = unsafe {
-    FOO2 = 5;
+static mut FOO: u32 = 42;
+static BOO: () = unsafe {
+    FOO = 5; //~ ERROR cannot mutate statics in the initializer of another static
 };
 
 fn main() {}
diff --git a/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
index 87f02e8e4cf7e..ca652c9df32ad 100644
--- a/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
+++ b/src/test/ui/consts/const-eval/assign-to-static-within-other-static.stderr
@@ -1,8 +1,8 @@
 error: cannot mutate statics in the initializer of another static
-  --> $DIR/assign-to-static-within-other-static.rs:32:5
+  --> $DIR/assign-to-static-within-other-static.rs:21:5
    |
-LL |     FOO2 = 5;
-   |     ^^^^^^^^
+LL |     FOO = 5; //~ ERROR cannot mutate statics in the initializer of another static
+   |     ^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/error-codes/E0017.rs b/src/test/ui/error-codes/E0017.rs
index c98c35a1442ad..bf400fde365be 100644
--- a/src/test/ui/error-codes/E0017.rs
+++ b/src/test/ui/error-codes/E0017.rs
@@ -14,5 +14,6 @@ const C: i32 = 2;
 const CR: &'static mut i32 = &mut C; //~ ERROR E0017
 static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
                                               //~| ERROR cannot borrow
+                                              //~| ERROR cannot mutate statics
 static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
 fn main() {}
diff --git a/src/test/ui/error-codes/E0017.stderr b/src/test/ui/error-codes/E0017.stderr
index 411b9f3139782..94a90d92d3e77 100644
--- a/src/test/ui/error-codes/E0017.stderr
+++ b/src/test/ui/error-codes/E0017.stderr
@@ -4,6 +4,12 @@ error[E0017]: references in constants may only refer to immutable values
 LL | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
    |                              ^^^^^^ constants require immutable values
 
+error: cannot mutate statics in the initializer of another static
+  --> $DIR/E0017.rs:15:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
+   |                                       ^^^^^^
+
 error[E0017]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:15:39
    |
@@ -17,12 +23,12 @@ LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
    |                                            ^
 
 error[E0017]: references in statics may only refer to immutable values
-  --> $DIR/E0017.rs:17:38
+  --> $DIR/E0017.rs:18:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
    |                                      ^^^^^^ statics require immutable values
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors occurred: E0017, E0596.
 For more information about an error, try `rustc --explain E0017`.
diff --git a/src/test/ui/error-codes/E0388.rs b/src/test/ui/error-codes/E0388.rs
index c002badfef64c..3203798c70958 100644
--- a/src/test/ui/error-codes/E0388.rs
+++ b/src/test/ui/error-codes/E0388.rs
@@ -14,6 +14,7 @@ const C: i32 = 2;
 const CR: &'static mut i32 = &mut C; //~ ERROR E0017
 static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
                                               //~| ERROR cannot borrow
+                                              //~| ERROR cannot mutate statics
 static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
 
 fn main() {}
diff --git a/src/test/ui/error-codes/E0388.stderr b/src/test/ui/error-codes/E0388.stderr
index d2263cd4034b8..46efda9147b6d 100644
--- a/src/test/ui/error-codes/E0388.stderr
+++ b/src/test/ui/error-codes/E0388.stderr
@@ -4,6 +4,12 @@ error[E0017]: references in constants may only refer to immutable values
 LL | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
    |                              ^^^^^^ constants require immutable values
 
+error: cannot mutate statics in the initializer of another static
+  --> $DIR/E0388.rs:15:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
+   |                                       ^^^^^^
+
 error[E0017]: references in statics may only refer to immutable values
   --> $DIR/E0388.rs:15:39
    |
@@ -17,12 +23,12 @@ LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
    |                                            ^
 
 error[E0017]: references in statics may only refer to immutable values
-  --> $DIR/E0388.rs:17:38
+  --> $DIR/E0388.rs:18:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
    |                                      ^^^^^^ statics require immutable values
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors occurred: E0017, E0596.
 For more information about an error, try `rustc --explain E0017`.
diff --git a/src/test/ui/write-to-static-mut-in-static.rs b/src/test/ui/write-to-static-mut-in-static.rs
index 1ea74f73723b7..191f09b54ee73 100644
--- a/src/test/ui/write-to-static-mut-in-static.rs
+++ b/src/test/ui/write-to-static-mut-in-static.rs
@@ -12,10 +12,10 @@
 
 pub static mut A: u32 = 0;
 pub static mut B: () = unsafe { A = 1; };
-//~^ ERROR statements in statics are unstable
+//~^ ERROR cannot mutate statics in the initializer of another static
 
 pub static mut C: u32 = unsafe { C = 1; 0 };
-//~^ ERROR statements in statics are unstable
+//~^ ERROR cannot mutate statics in the initializer of another static
 
 pub static D: u32 = D;
 
diff --git a/src/test/ui/write-to-static-mut-in-static.stderr b/src/test/ui/write-to-static-mut-in-static.stderr
index f07d240746fdf..673a71b4642f3 100644
--- a/src/test/ui/write-to-static-mut-in-static.stderr
+++ b/src/test/ui/write-to-static-mut-in-static.stderr
@@ -1,19 +1,14 @@
-error[E0658]: statements in statics are unstable (see issue #48821)
+error: cannot mutate statics in the initializer of another static
   --> $DIR/write-to-static-mut-in-static.rs:14:33
    |
 LL | pub static mut B: () = unsafe { A = 1; };
    |                                 ^^^^^
-   |
-   = help: add #![feature(const_let)] to the crate attributes to enable
 
-error[E0658]: statements in statics are unstable (see issue #48821)
+error: cannot mutate statics in the initializer of another static
   --> $DIR/write-to-static-mut-in-static.rs:17:34
    |
 LL | pub static mut C: u32 = unsafe { C = 1; 0 };
    |                                  ^^^^^
-   |
-   = help: add #![feature(const_let)] to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0658`.

From 7d5b5ff24df9aa98e2be212b8bc7887fcc5ac16c Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Mon, 19 Nov 2018 19:03:03 +0100
Subject: [PATCH 03/15] Update nll stderr files

---
 src/test/ui/error-codes/E0017.nll.stderr | 10 ++++++++--
 src/test/ui/error-codes/E0388.nll.stderr | 10 ++++++++--
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/test/ui/error-codes/E0017.nll.stderr b/src/test/ui/error-codes/E0017.nll.stderr
index 08708d213d3f6..8f8d87739b8cd 100644
--- a/src/test/ui/error-codes/E0017.nll.stderr
+++ b/src/test/ui/error-codes/E0017.nll.stderr
@@ -4,6 +4,12 @@ error[E0017]: references in constants may only refer to immutable values
 LL | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
    |                              ^^^^^^ constants require immutable values
 
+error: cannot mutate statics in the initializer of another static
+  --> $DIR/E0017.rs:15:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
+   |                                       ^^^^^^
+
 error[E0017]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:15:39
    |
@@ -17,12 +23,12 @@ LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
    |                                       ^^^^^^ cannot borrow as mutable
 
 error[E0017]: references in statics may only refer to immutable values
-  --> $DIR/E0017.rs:17:38
+  --> $DIR/E0017.rs:18:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
    |                                      ^^^^^^ statics require immutable values
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors occurred: E0017, E0596.
 For more information about an error, try `rustc --explain E0017`.
diff --git a/src/test/ui/error-codes/E0388.nll.stderr b/src/test/ui/error-codes/E0388.nll.stderr
index a048374a49725..7d25d41f18b62 100644
--- a/src/test/ui/error-codes/E0388.nll.stderr
+++ b/src/test/ui/error-codes/E0388.nll.stderr
@@ -4,6 +4,12 @@ error[E0017]: references in constants may only refer to immutable values
 LL | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
    |                              ^^^^^^ constants require immutable values
 
+error: cannot mutate statics in the initializer of another static
+  --> $DIR/E0388.rs:15:39
+   |
+LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
+   |                                       ^^^^^^
+
 error[E0017]: references in statics may only refer to immutable values
   --> $DIR/E0388.rs:15:39
    |
@@ -17,12 +23,12 @@ LL | static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0017
    |                                       ^^^^^^ cannot borrow as mutable
 
 error[E0017]: references in statics may only refer to immutable values
-  --> $DIR/E0388.rs:17:38
+  --> $DIR/E0388.rs:18:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0017
    |                                      ^^^^^^ statics require immutable values
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors occurred: E0017, E0596.
 For more information about an error, try `rustc --explain E0017`.

From f98235ed9d8be7a6966764384a99e18ead1195e2 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 11:11:09 +0100
Subject: [PATCH 04/15] Document runtime static mutation checks

---
 src/librustc_mir/interpret/machine.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/librustc_mir/interpret/machine.rs b/src/librustc_mir/interpret/machine.rs
index 57640dc48f13f..4d8f40c81301e 100644
--- a/src/librustc_mir/interpret/machine.rs
+++ b/src/librustc_mir/interpret/machine.rs
@@ -89,7 +89,8 @@ pub trait Machine<'a, 'mir, 'tcx>: Sized {
         Default +
         Clone;
 
-    /// The memory kind to use for copied statics -- or None if those are not supported.
+    /// The memory kind to use for copied statics -- or None if statics should not be mutated
+    /// and thus any such attempt will cause a `ModifiedStatic` error is raised.
     /// Statics are copied under two circumstances: When they are mutated, and when
     /// `static_with_default_tag` or `find_foreign_static` (see below) returns an owned allocation
     /// that is added to the memory so that the work is not done twice.

From 6bcb0d6152e0b817da7af23bdb2abf13a8b2db43 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 11:13:19 +0100
Subject: [PATCH 05/15] Explain missing error in test

---
 src/test/ui/consts/const-eval/mod-static-with-const-fn.rs     | 3 +++
 src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs b/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
index 600931e49a08e..01fcc8307c0c1 100644
--- a/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
+++ b/src/test/ui/consts/const-eval/mod-static-with-const-fn.rs
@@ -27,6 +27,9 @@ fn foo() {}
 
 static BAR: () = unsafe {
     *FOO.0.get() = 5;
+    // we do not error on the above access, because that is not detectable statically. Instead,
+    // const evaluation will error when trying to evaluate it. Due to the error below, we never even
+    // attempt to const evaluate `BAR`, so we don't see the error
 
     foo();
     //~^ ERROR calls in statics are limited to constant functions, tuple structs and tuple variants
diff --git a/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr b/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
index 899fc24f153a5..01ad1fc9a21b9 100644
--- a/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
+++ b/src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr
@@ -1,5 +1,5 @@
 error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/mod-static-with-const-fn.rs:31:5
+  --> $DIR/mod-static-with-const-fn.rs:34:5
    |
 LL |     foo();
    |     ^^^^^

From f70abe8d07023bc58cac03a989ddbd37332fa10f Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 11:33:46 +0100
Subject: [PATCH 06/15] Add sanity test for promotion and `const_let`

---
 src/test/ui/consts/promote_const_let.nll.stderr | 14 ++++++++++++++
 src/test/ui/consts/promote_const_let.rs         |  8 ++++++++
 src/test/ui/consts/promote_const_let.stderr     | 13 +++++++++++++
 3 files changed, 35 insertions(+)
 create mode 100644 src/test/ui/consts/promote_const_let.nll.stderr
 create mode 100644 src/test/ui/consts/promote_const_let.rs
 create mode 100644 src/test/ui/consts/promote_const_let.stderr

diff --git a/src/test/ui/consts/promote_const_let.nll.stderr b/src/test/ui/consts/promote_const_let.nll.stderr
new file mode 100644
index 0000000000000..d8749bb5fd90b
--- /dev/null
+++ b/src/test/ui/consts/promote_const_let.nll.stderr
@@ -0,0 +1,14 @@
+error[E0597]: `y` does not live long enough
+  --> $DIR/promote_const_let.rs:6:9
+   |
+LL |     let x: &'static u32 = {
+   |            ------------ type annotation requires that `y` is borrowed for `'static`
+LL |         let y = 42;
+LL |         &y //~ ERROR does not live long enough
+   |         ^^ borrowed value does not live long enough
+LL |     };
+   |     - `y` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/src/test/ui/consts/promote_const_let.rs b/src/test/ui/consts/promote_const_let.rs
new file mode 100644
index 0000000000000..1ba406a957d2d
--- /dev/null
+++ b/src/test/ui/consts/promote_const_let.rs
@@ -0,0 +1,8 @@
+#![feature(const_let)]
+
+fn main() {
+    let x: &'static u32 = {
+        let y = 42;
+        &y //~ ERROR does not live long enough
+    };
+}
\ No newline at end of file
diff --git a/src/test/ui/consts/promote_const_let.stderr b/src/test/ui/consts/promote_const_let.stderr
new file mode 100644
index 0000000000000..6bbb7495fb0dc
--- /dev/null
+++ b/src/test/ui/consts/promote_const_let.stderr
@@ -0,0 +1,13 @@
+error[E0597]: `y` does not live long enough
+  --> $DIR/promote_const_let.rs:6:10
+   |
+LL |         &y //~ ERROR does not live long enough
+   |          ^ borrowed value does not live long enough
+LL |     };
+   |     - borrowed value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

From 7d96f2c4814cdf07860d31e8e8b9695eb9628972 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 12:25:44 +0100
Subject: [PATCH 07/15] Document qualify_consts more

---
 src/librustc_mir/transform/qualify_consts.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 1371b9a997707..f8ecd9634579a 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -247,11 +247,16 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             let mut dest = dest;
             let index = loop {
                 match dest {
+                    // with `const_let` active, we treat all locals equal
                     Place::Local(index) => break *index,
+                    // projections are transparent for assignments
+                    // we qualify the entire destination at once, even if just a field would have
+                    // stricter qualification
                     Place::Projection(proj) => dest = &proj.base,
                     Place::Promoted(..) => bug!("promoteds don't exist yet during promotion"),
                     Place::Static(..) => {
-                        // Catch more errors in the destination.
+                        // Catch more errors in the destination. `visit_place` also checks that we
+                        // do not try to access statics from constants or try to mutate statics
                         self.visit_place(
                             dest,
                             PlaceContext::MutatingUse(MutatingUseContext::Store),

From 3e081c7e824eab1e5d87462e5e6a46be14c529ea Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 12:28:24 +0100
Subject: [PATCH 08/15] Trailing newline

---
 src/test/ui/consts/promote_const_let.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/test/ui/consts/promote_const_let.rs b/src/test/ui/consts/promote_const_let.rs
index 1ba406a957d2d..8de9b00eb111d 100644
--- a/src/test/ui/consts/promote_const_let.rs
+++ b/src/test/ui/consts/promote_const_let.rs
@@ -5,4 +5,4 @@ fn main() {
         let y = 42;
         &y //~ ERROR does not live long enough
     };
-}
\ No newline at end of file
+}

From 37fcd5cad5c314632aede576ae5fbbb1c3ede228 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 20 Nov 2018 12:29:05 +0100
Subject: [PATCH 09/15] Grammar nit

---
 src/librustc_mir/interpret/machine.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/librustc_mir/interpret/machine.rs b/src/librustc_mir/interpret/machine.rs
index 4d8f40c81301e..dd247a96fa48a 100644
--- a/src/librustc_mir/interpret/machine.rs
+++ b/src/librustc_mir/interpret/machine.rs
@@ -90,7 +90,7 @@ pub trait Machine<'a, 'mir, 'tcx>: Sized {
         Clone;
 
     /// The memory kind to use for copied statics -- or None if statics should not be mutated
-    /// and thus any such attempt will cause a `ModifiedStatic` error is raised.
+    /// and thus any such attempt will cause a `ModifiedStatic` error to be raised.
     /// Statics are copied under two circumstances: When they are mutated, and when
     /// `static_with_default_tag` or `find_foreign_static` (see below) returns an owned allocation
     /// that is added to the memory so that the work is not done twice.

From 301ce8b2aa6156ae8dc535ca234e95b04e7c4e4b Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 10:37:18 +0100
Subject: [PATCH 10/15] Properly assign to aggregate fields

---
 src/librustc_mir/transform/qualify_consts.rs |  7 ++++++-
 src/test/ui/consts/partial_qualif.rs         | 11 +++++++++++
 src/test/ui/consts/partial_qualif.stderr     |  9 +++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/consts/partial_qualif.rs
 create mode 100644 src/test/ui/consts/partial_qualif.stderr

diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index f8ecd9634579a..51e590fe29291 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -267,7 +267,12 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                 }
             };
             debug!("store to var {:?}", index);
-            self.local_qualif[index] = Some(self.qualif);
+            match &mut self.local_qualif[index] {
+                // update
+                Some(ref mut qualif) => *qualif = *qualif | self.qualif,
+                // or insert
+                qualif @ None => *qualif = Some(self.qualif),
+            }
             return;
         }
 
diff --git a/src/test/ui/consts/partial_qualif.rs b/src/test/ui/consts/partial_qualif.rs
new file mode 100644
index 0000000000000..bb3fc92d4fb4e
--- /dev/null
+++ b/src/test/ui/consts/partial_qualif.rs
@@ -0,0 +1,11 @@
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+const FOO: &(Cell<usize>, bool) = {
+    let mut a = (Cell::new(0), false);
+    a.1 = true; // resets `qualif(a)` to `qualif(true)`
+    &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
+};
+
+fn main() {}
\ No newline at end of file
diff --git a/src/test/ui/consts/partial_qualif.stderr b/src/test/ui/consts/partial_qualif.stderr
new file mode 100644
index 0000000000000..d695f64e2c3b5
--- /dev/null
+++ b/src/test/ui/consts/partial_qualif.stderr
@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/partial_qualif.rs:8:5
+   |
+LL |     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
+   |     ^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0492`.

From 3c290a5326755d5f978caf66cfd61b05652169d5 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 10:42:40 +0100
Subject: [PATCH 11/15] Ensure assignments don't allow skipping projection
 checks

---
 src/librustc_mir/transform/qualify_consts.rs | 11 ++++++++++-
 src/test/ui/consts/projection_qualif.rs      | 14 ++++++++++++++
 src/test/ui/consts/projection_qualif.stderr  | 18 ++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/consts/projection_qualif.rs
 create mode 100644 src/test/ui/consts/projection_qualif.stderr

diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 51e590fe29291..571fe99897058 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -252,7 +252,16 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                     // projections are transparent for assignments
                     // we qualify the entire destination at once, even if just a field would have
                     // stricter qualification
-                    Place::Projection(proj) => dest = &proj.base,
+                    Place::Projection(proj) => {
+                        // Catch more errors in the destination. `visit_place` also checks various
+                        // projection rules like union field access and raw pointer deref
+                        self.visit_place(
+                            dest,
+                            PlaceContext::MutatingUse(MutatingUseContext::Store),
+                            location
+                        );
+                        dest = &proj.base;
+                    },
                     Place::Promoted(..) => bug!("promoteds don't exist yet during promotion"),
                     Place::Static(..) => {
                         // Catch more errors in the destination. `visit_place` also checks that we
diff --git a/src/test/ui/consts/projection_qualif.rs b/src/test/ui/consts/projection_qualif.rs
new file mode 100644
index 0000000000000..4806fecee4319
--- /dev/null
+++ b/src/test/ui/consts/projection_qualif.rs
@@ -0,0 +1,14 @@
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+const FOO: &u32 = {
+    let mut a = 42;
+    {
+        let b: *mut u32 = &mut a; //~ ERROR may only refer to immutable values
+        unsafe { *b = 5; } //~ ERROR dereferencing raw pointers in constants
+    }
+    &{a}
+};
+
+fn main() {}
\ No newline at end of file
diff --git a/src/test/ui/consts/projection_qualif.stderr b/src/test/ui/consts/projection_qualif.stderr
new file mode 100644
index 0000000000000..d5252f199beac
--- /dev/null
+++ b/src/test/ui/consts/projection_qualif.stderr
@@ -0,0 +1,18 @@
+error[E0017]: references in constants may only refer to immutable values
+  --> $DIR/projection_qualif.rs:8:27
+   |
+LL |         let b: *mut u32 = &mut a; //~ ERROR may only refer to immutable values
+   |                           ^^^^^^ constants require immutable values
+
+error[E0658]: dereferencing raw pointers in constants is unstable (see issue #51911)
+  --> $DIR/projection_qualif.rs:9:18
+   |
+LL |         unsafe { *b = 5; } //~ ERROR dereferencing raw pointers in constants
+   |                  ^^^^^^
+   |
+   = help: add #![feature(const_raw_ptr_deref)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0017, E0658.
+For more information about an error, try `rustc --explain E0017`.

From 42a3f730c75c4e4b62eb58d4e09a8ec833e13804 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 11:13:49 +0100
Subject: [PATCH 12/15] Tidy

---
 src/test/ui/consts/partial_qualif.rs    | 2 +-
 src/test/ui/consts/projection_qualif.rs | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/test/ui/consts/partial_qualif.rs b/src/test/ui/consts/partial_qualif.rs
index bb3fc92d4fb4e..7cf9ee1d94b0e 100644
--- a/src/test/ui/consts/partial_qualif.rs
+++ b/src/test/ui/consts/partial_qualif.rs
@@ -8,4 +8,4 @@ const FOO: &(Cell<usize>, bool) = {
     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
 };
 
-fn main() {}
\ No newline at end of file
+fn main() {}
diff --git a/src/test/ui/consts/projection_qualif.rs b/src/test/ui/consts/projection_qualif.rs
index 4806fecee4319..34e8eaba9f28f 100644
--- a/src/test/ui/consts/projection_qualif.rs
+++ b/src/test/ui/consts/projection_qualif.rs
@@ -11,4 +11,4 @@ const FOO: &u32 = {
     &{a}
 };
 
-fn main() {}
\ No newline at end of file
+fn main() {}

From e05b61ccd8fcb4bb4fd12085fc13522b940e7618 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 11:47:16 +0100
Subject: [PATCH 13/15] Fix a comment

---
 src/test/ui/consts/partial_qualif.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/test/ui/consts/partial_qualif.rs b/src/test/ui/consts/partial_qualif.rs
index 7cf9ee1d94b0e..4ce41f80f82c8 100644
--- a/src/test/ui/consts/partial_qualif.rs
+++ b/src/test/ui/consts/partial_qualif.rs
@@ -4,7 +4,7 @@ use std::cell::Cell;
 
 const FOO: &(Cell<usize>, bool) = {
     let mut a = (Cell::new(0), false);
-    a.1 = true; // resets `qualif(a)` to `qualif(true)`
+    a.1 = true; // sets `qualif(a)` to `qualif(a) | qualif(true)`
     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
 };
 

From 22aebd57c8ed5344260059abebbb938683ab4d91 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 11:47:44 +0100
Subject: [PATCH 14/15] Add regression test for overwriting qualifs by
 assignment

---
 src/test/ui/consts/qualif_overwrite.rs     | 13 +++++++++++++
 src/test/ui/consts/qualif_overwrite.stderr |  9 +++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 src/test/ui/consts/qualif_overwrite.rs
 create mode 100644 src/test/ui/consts/qualif_overwrite.stderr

diff --git a/src/test/ui/consts/qualif_overwrite.rs b/src/test/ui/consts/qualif_overwrite.rs
new file mode 100644
index 0000000000000..65e182945bf79
--- /dev/null
+++ b/src/test/ui/consts/qualif_overwrite.rs
@@ -0,0 +1,13 @@
+// compile-pass
+
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+const FOO: &Option<Cell<usize>> = {
+    let mut a = Some(Cell::new(0));
+    a = None; // resets `qualif(a)` to `qualif(None)`
+    &{a}
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/qualif_overwrite.stderr b/src/test/ui/consts/qualif_overwrite.stderr
new file mode 100644
index 0000000000000..b24c0fa70f063
--- /dev/null
+++ b/src/test/ui/consts/qualif_overwrite.stderr
@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/qualif_overwrite.rs:8:5
+   |
+LL |     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
+   |     ^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0492`.

From 6db8c6c6d9fd1f8226b3afd9e18c6fb3cd2f1176 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Wed, 21 Nov 2018 11:58:28 +0100
Subject: [PATCH 15/15] Explain why we do not overwrite qualification of locals

---
 src/librustc_mir/transform/qualify_consts.rs |  7 +++++--
 src/test/ui/consts/qualif_overwrite.rs       | 10 ++++++----
 src/test/ui/consts/qualif_overwrite.stderr   |  2 +-
 src/test/ui/consts/qualif_overwrite_2.rs     | 13 +++++++++++++
 src/test/ui/consts/qualif_overwrite_2.stderr |  9 +++++++++
 5 files changed, 34 insertions(+), 7 deletions(-)
 create mode 100644 src/test/ui/consts/qualif_overwrite_2.rs
 create mode 100644 src/test/ui/consts/qualif_overwrite_2.stderr

diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 571fe99897058..399c6dbcb7ca6 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -277,9 +277,12 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             };
             debug!("store to var {:?}", index);
             match &mut self.local_qualif[index] {
-                // update
+                // this is overly restrictive, because even full assignments do not clear the qualif
+                // While we could special case full assignments, this would be inconsistent with
+                // aggregates where we overwrite all fields via assignments, which would not get
+                // that feature.
                 Some(ref mut qualif) => *qualif = *qualif | self.qualif,
-                // or insert
+                // insert new qualification
                 qualif @ None => *qualif = Some(self.qualif),
             }
             return;
diff --git a/src/test/ui/consts/qualif_overwrite.rs b/src/test/ui/consts/qualif_overwrite.rs
index 65e182945bf79..806a74ee4530b 100644
--- a/src/test/ui/consts/qualif_overwrite.rs
+++ b/src/test/ui/consts/qualif_overwrite.rs
@@ -1,13 +1,15 @@
-// compile-pass
-
 #![feature(const_let)]
 
 use std::cell::Cell;
 
+// this is overly conservative. The reset to `None` should clear `a` of all qualifications
+// while we could fix this, it would be inconsistent with `qualif_overwrite_2.rs`.
+// We can fix this properly in the future by allowing constants that do not depend on generics
+// to be checked by an analysis on the final value instead of looking at the body.
 const FOO: &Option<Cell<usize>> = {
     let mut a = Some(Cell::new(0));
-    a = None; // resets `qualif(a)` to `qualif(None)`
-    &{a}
+    a = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
+    &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
 };
 
 fn main() {}
diff --git a/src/test/ui/consts/qualif_overwrite.stderr b/src/test/ui/consts/qualif_overwrite.stderr
index b24c0fa70f063..4fac64bf8063f 100644
--- a/src/test/ui/consts/qualif_overwrite.stderr
+++ b/src/test/ui/consts/qualif_overwrite.stderr
@@ -1,5 +1,5 @@
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
-  --> $DIR/qualif_overwrite.rs:8:5
+  --> $DIR/qualif_overwrite.rs:12:5
    |
 LL |     &{a} //~ ERROR cannot borrow a constant which may contain interior mutability
    |     ^^^^
diff --git a/src/test/ui/consts/qualif_overwrite_2.rs b/src/test/ui/consts/qualif_overwrite_2.rs
new file mode 100644
index 0000000000000..29557a3da4781
--- /dev/null
+++ b/src/test/ui/consts/qualif_overwrite_2.rs
@@ -0,0 +1,13 @@
+#![feature(const_let)]
+
+use std::cell::Cell;
+
+// const qualification is not smart enough to know about fields and always assumes that there might
+// be other fields that caused the qualification
+const FOO: &Option<Cell<usize>> = {
+    let mut a = (Some(Cell::new(0)),);
+    a.0 = None; // sets `qualif(a)` to `qualif(a) | qualif(None)`
+    &{a.0} //~ ERROR cannot borrow a constant which may contain interior mutability
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/qualif_overwrite_2.stderr b/src/test/ui/consts/qualif_overwrite_2.stderr
new file mode 100644
index 0000000000000..181b728c7b76f
--- /dev/null
+++ b/src/test/ui/consts/qualif_overwrite_2.stderr
@@ -0,0 +1,9 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/qualif_overwrite_2.rs:10:5
+   |
+LL |     &{a.0} //~ ERROR cannot borrow a constant which may contain interior mutability
+   |     ^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0492`.