add new error code

src/librustc_error_codes/error_codes.rs

@@ -444,6 +444,7 @@ E0760: include_str!("./error_codes/E0760.md"),
 E0761: include_str!("./error_codes/E0761.md"),
 E0762: include_str!("./error_codes/E0762.md"),
 E0763: include_str!("./error_codes/E0763.md"),
+E0764: include_str!("./error_codes/E0764.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0764.md

@@ -0,0 +1,39 @@
+Mutable references (`&mut`) can only be used in constant functions, not statics
+or constants. This limitation exists to prevent the creation of constants that
+have a mutable reference in their final value. If you had a constant of `&mut
+i32` type, you could modify the value through that reference, making the
+constant essentially mutable. While there could be a more fine-grained scheme
+in the future that allows mutable references if they are not "leaked" to the
+final value, a more conservative approach was chosen for now. `const fn` do not
+have this problem, as the borrow checker will prevent the `const fn` from
+returning new mutable references.
+
+Erroneous code example:
+
+```compile_fail,E0764
+#![feature(const_fn)]
+#![feature(const_mut_refs)]
+
+fn main() {
+    const OH_NO: &'static mut usize = &mut 1; // error!
+}
+```
+
+Remember: you cannot use a function call inside a constant or static. However,
+you can totally use it in constant functions:
+
+```
+#![feature(const_fn)]
+#![feature(const_mut_refs)]
+
+const fn foo(x: usize) -> usize {
+    let mut y = 1;
+    let z = &mut y;
+    *z += x;
+    y
+}
+
+fn main() {
+    const FOO: usize = foo(10); // ok!
+}
+```

src/librustc_mir/transform/check_consts/ops.rs

@@ -227,7 +227,7 @@ impl NonConstOp for MutBorrow {
             struct_span_err!(
                 ccx.tcx.sess,
                 span,
-                E0019,
+                E0764,
                 "mutable references are not allowed in {}s",
                 ccx.const_kind(),
             )

src/test/ui/check-static-immutable-mut-slices.stderr

@@ -1,9 +1,9 @@
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/check-static-immutable-mut-slices.rs:3:37
    |
 LL | static TEST: &'static mut [isize] = &mut [];
    |                                     ^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/const-eval/issue-65394.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/issue-65394.rs:8:13
    |
 LL |     let r = &mut x;
@@ -12,5 +12,5 @@ LL |     let mut x = Vec::<i32>::new();
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0019, E0493.
-For more information about an error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0493, E0764.
+For more information about an error, try `rustc --explain E0493`.

src/test/ui/consts/const-multi-ref.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const-multi-ref.rs:6:13
    |
 LL |     let p = &mut a;
@@ -12,5 +12,5 @@ LL |     let p = &a;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0019, E0492.
-For more information about an error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0492, E0764.
+For more information about an error, try `rustc --explain E0492`.

src/test/ui/consts/const-mut-refs/const_mut_address_of.stderr

@@ -1,15 +1,15 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_mut_address_of.rs:24:5
    |
 LL |     foo().bar();
    |     ^^^^^ `&mut` is only allowed in `const fn`
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_mut_address_of.rs:26:9
    |
 LL |     baz(&mut foo());
    |         ^^^^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/const-mut-refs/const_mut_refs.stderr

@@ -1,21 +1,21 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_mut_refs.rs:31:17
    |
 LL |     let _: [(); foo().bar()] = [(); 1];
    |                 ^^^^^ `&mut` is only allowed in `const fn`
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_mut_refs.rs:33:21
    |
 LL |     let _: [(); baz(&mut foo())] = [(); 2];
    |                     ^^^^^^^^^^ `&mut` is only allowed in `const fn`
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_mut_refs.rs:35:22
    |
 LL |     let _: [(); bazz(&mut foo())] = [(); 3];
    |                      ^^^^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/const_let_assign3.stderr

@@ -6,13 +6,13 @@ LL |         self.state = x;
    |
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_let_assign3.rs:16:5
    |
 LL |     s.foo(3);
    |     ^ `&mut` is only allowed in `const fn`
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const_let_assign3.rs:22:13
    |
 LL |     let y = &mut x;
@@ -28,4 +28,5 @@ LL |     *y = 42;
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0019, E0764.
+For more information about an error, try `rustc --explain E0019`.

src/test/ui/consts/projection_qualif.mut_refs.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/projection_qualif.rs:10:27
    |
 LL |         let b: *mut u32 = &mut a;
@@ -15,5 +15,5 @@ LL |         unsafe { *b = 5; }
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0019, E0658.
-For more information about an error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0658, E0764.
+For more information about an error, try `rustc --explain E0658`.

src/test/ui/consts/projection_qualif.stock.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/projection_qualif.rs:10:27
    |
 LL |         let b: *mut u32 = &mut a;
@@ -23,5 +23,5 @@ LL |         unsafe { *b = 5; }
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0019, E0658.
+Some errors have detailed explanations: E0019, E0658, E0764.
 For more information about an error, try `rustc --explain E0019`.

src/test/ui/consts/read_from_static_mut_ref.stderr

@@ -1,9 +1,9 @@
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/read_from_static_mut_ref.rs:5:26
    |
 LL | static OH_NO: &mut i32 = &mut 42;
    |                          ^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/static_mut_containing_mut_ref2.mut_refs.stderr

@@ -1,9 +1,9 @@
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/static_mut_containing_mut_ref2.rs:7:46
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0019`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/static_mut_containing_mut_ref2.stock.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/static_mut_containing_mut_ref2.rs:7:46
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
@@ -14,4 +14,5 @@ LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 4
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0019, E0764.
+For more information about an error, try `rustc --explain E0019`.

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 E0019
-static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0019
+const CR: &'static mut i32 = &mut C; //~ ERROR E0764
+static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0764
                                               //~| ERROR E0019
                                               //~| ERROR cannot borrow
-static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0019
-static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M }; //~ ERROR E0019
+static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0764
+static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M }; //~ ERROR E0764
 fn main() {}

src/test/ui/error-codes/E0017.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/E0017.rs:5:30
    |
 LL | const CR: &'static mut i32 = &mut C;
@@ -12,7 +12,7 @@ LL | static STATIC_REF: &'static mut i32 = &mut X;
    |
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/E0017.rs:6:39
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
@@ -24,19 +24,19 @@ error[E0596]: cannot borrow immutable static item `X` as mutable
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ cannot borrow as mutable
 
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/E0017.rs:9:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C;
    |                                      ^^^^^^ `&mut` is only allowed in `const fn`
 
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/E0017.rs:10:52
    |
 LL | static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M };
    |                                                    ^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0019, E0596.
+Some errors have detailed explanations: E0019, E0596, E0764.
 For more information about an error, try `rustc --explain E0019`.

src/test/ui/error-codes/E0388.rs

@@ -1,10 +1,10 @@
 static X: i32 = 1;
 const C: i32 = 2;
 
-const CR: &'static mut i32 = &mut C; //~ ERROR E0019
+const CR: &'static mut i32 = &mut C; //~ ERROR E0764
 static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0019
                                               //~| ERROR cannot borrow
-                                              //~| ERROR E0019
-static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0019
+                                              //~| ERROR E0764
+static CONST_REF: &'static mut i32 = &mut C; //~ ERROR E0764
 
 fn main() {}

src/test/ui/error-codes/E0388.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/E0388.rs:4:30
    |
 LL | const CR: &'static mut i32 = &mut C;
@@ -12,7 +12,7 @@ LL | static STATIC_REF: &'static mut i32 = &mut X;
    |
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/E0388.rs:5:39
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
@@ -24,13 +24,13 @@ error[E0596]: cannot borrow immutable static item `X` as mutable
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ cannot borrow as mutable
 
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/E0388.rs:8:38
    |
 LL | static CONST_REF: &'static mut i32 = &mut C;
    |                                      ^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0019, E0596.
+Some errors have detailed explanations: E0019, E0596, E0764.
 For more information about an error, try `rustc --explain E0019`.

src/test/ui/issues/issue-17718-const-bad-values.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/issue-17718-const-bad-values.rs:1:34
    |
 LL | const C1: &'static mut [usize] = &mut [];
@@ -20,13 +20,13 @@ LL | const C2: &'static mut usize = unsafe { &mut S };
    |
    = help: consider extracting the value of the `static` to a `const`, and referring to that
 
-error[E0019]: mutable references are not allowed in constants
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/issue-17718-const-bad-values.rs:5:41
    |
 LL | const C2: &'static mut usize = unsafe { &mut S };
    |                                         ^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0013, E0019.
+Some errors have detailed explanations: E0013, E0764.
 For more information about an error, try `rustc --explain E0013`.

src/test/ui/issues/issue-46604.rs

@@ -1,4 +1,4 @@
-static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR E0019
+static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];   //~ ERROR E0764
 fn write<T: AsRef<[u8]>>(buffer: T) { }
 
 fn main() {

src/test/ui/issues/issue-46604.stderr

@@ -1,4 +1,4 @@
-error[E0019]: mutable references are not allowed in statics
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/issue-46604.rs:1:25
    |
 LL | static buf: &mut [u8] = &mut [1u8,2,3,4,5,7];
@@ -12,5 +12,5 @@ LL |     buf[0]=2;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0019, E0594.
-For more information about an error, try `rustc --explain E0019`.
+Some errors have detailed explanations: E0594, E0764.
+For more information about an error, try `rustc --explain E0594`.