Update tests for new NLL mutability errors

src/test/compile-fail/E0594.rs

@@ -15,5 +15,5 @@ static NUM: i32 = 18;
 
 fn main() {
     NUM = 20; //[ast]~ ERROR E0594
-              //[mir]~^ ERROR cannot assign to immutable item `NUM`
+              //[mir]~^ ERROR cannot assign to immutable static item `NUM`
 }

src/test/compile-fail/borrowck/borrowck-assign-to-constants.rs

@@ -16,5 +16,5 @@ static foo: isize = 5;
 fn main() {
     // assigning to various global constants
     foo = 6; //[ast]~ ERROR cannot assign to immutable static item
-             //[mir]~^ ERROR cannot assign to immutable item `foo`
+             //[mir]~^ ERROR cannot assign to immutable static item `foo`
 }

src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs

@@ -22,10 +22,10 @@ fn main() {
     let x = 1;
     to_fn_once(move|| { x = 2; });
     //[ast]~^ ERROR: cannot assign to immutable captured outer variable
-    //[mir]~^^ ERROR: cannot assign to immutable item `x`
+    //[mir]~^^ ERROR: cannot assign to `x`, as it is not declared as mutable
 
     let s = std::io::stdin();
     to_fn_once(move|| { s.read_to_end(&mut Vec::new()); });
     //[ast]~^ ERROR: cannot borrow immutable captured outer variable
-    //[mir]~^^ ERROR: cannot borrow immutable item `s` as mutable
+    //[mir]~^^ ERROR: cannot borrow `s` as mutable, as it is not declared as mutable
 }

src/test/compile-fail/nll/constant-thread-locals-issue-47053.rs

@@ -17,5 +17,5 @@
 static FOO: isize = 5;
 
 fn main() {
-    FOO = 6; //~ ERROR cannot assign to immutable item `FOO` [E0594]
+    FOO = 6; //~ ERROR cannot assign to immutable static item `FOO` [E0594]
 }

src/test/ui/augmented-assignments.nll.stderr

@@ -14,7 +14,7 @@ LL | |     x;  //~ value moved here
    | |_____move out of `x` occurs here
    |       borrow later used here
 
-error[E0596]: cannot borrow immutable item `y` as mutable
+error[E0596]: cannot borrow `y` as mutable, as it is not declared as mutable
   --> $DIR/augmented-assignments.rs:30:5
    |
 LL |     let y = Int(2);

src/test/ui/borrowck/issue-45983.nll.stderr

@@ -14,21 +14,14 @@ LL |     give_any(|y| x = Some(y));
    |               |
    |               lifetime `'1` appears in this argument
 
-error[E0594]: cannot assign to immutable item `x`
+error[E0594]: cannot assign to `x`, as it is not declared as mutable
   --> $DIR/issue-45983.rs:17:18
    |
-LL |     give_any(|y| x = Some(y));
-   |                  ^^^^^^^^^^^ cannot assign
-
-error[E0596]: cannot borrow immutable item `x` as mutable
-  --> $DIR/issue-45983.rs:17:14
-   |
 LL |     let x = None;
    |         - help: consider changing this to be mutable: `mut x`
 LL |     give_any(|y| x = Some(y));
-   |              ^^^^^^^^^^^^^^^ cannot borrow as mutable
+   |                  ^^^^^^^^^^^ cannot assign
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-Some errors occurred: E0594, E0596.
-For more information about an error, try `rustc --explain E0594`.
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/borrowck/mut-borrow-of-mut-ref.nll.stderr

@@ -1,4 +1,4 @@
-error[E0596]: cannot borrow immutable item `b` as mutable
+error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
   --> $DIR/mut-borrow-of-mut-ref.rs:18:7
    |
 LL | fn f(b: &mut i32) {