Auto merge of #47261 - estebank:immutable-arg, r=petrochenkov

Assignment to immutable argument: diagnostic tweak

Re #46659.

src/test/compile-fail/issue-45199.rs

@@ -31,11 +31,11 @@ fn test_call() {
 }
 
 fn test_args(b: Box<i32>) {  //[ast]~ NOTE first assignment
-                                //[mir]~^ NOTE first assignment
+                                //[mir]~^ NOTE argument not declared as `mut`
     b = Box::new(2);            //[ast]~ ERROR cannot assign twice to immutable variable
-                                //[mir]~^ ERROR cannot assign twice to immutable variable `b`
+                                //[mir]~^ ERROR cannot assign to immutable argument `b`
                                 //[ast]~| NOTE cannot assign twice to immutable
-                                //[mir]~| NOTE cannot assign twice to immutable
+                                //[mir]~| NOTE cannot assign to immutable argument
 }
 
 fn main() {}

src/test/ui/borrowck/immutable-arg.rs

@@ -0,0 +1,20 @@
+// 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.
+
+//compile-flags: -Z emit-end-regions -Z borrowck=compare
+
+fn foo(_x: u32) {
+    _x = 4;
+    //~^ ERROR cannot assign to immutable argument `_x` (Mir)
+    //~^^ ERROR cannot assign twice to immutable variable `_x` (Ast)
+}
+
+fn main() {}
+

src/test/ui/borrowck/immutable-arg.stderr

@@ -0,0 +1,18 @@
+error[E0384]: cannot assign twice to immutable variable `_x` (Ast)
+  --> $DIR/immutable-arg.rs:14:5
+   |
+13 | fn foo(_x: u32) {
+   |        -- first assignment to `_x`
+14 |     _x = 4;
+   |     ^^^^^^ cannot assign twice to immutable variable
+
+error[E0384]: cannot assign to immutable argument `_x` (Mir)
+  --> $DIR/immutable-arg.rs:14:5
+   |
+13 | fn foo(_x: u32) {
+   |        -- argument not declared as `mut`
+14 |     _x = 4;
+   |     ^^^^^^ cannot assign to immutable argument
+
+error: aborting due to 2 previous errors
+