Rollup merge of #40841 - arielb1:immutable-blame, r=pnkfelix

borrowck: consolidate `mut` suggestions

This converts all of borrowck's `mut` suggestions to a new
`mc::ImmutabilityBlame` API instead of the current mix of various hacks.

Fixes #35937.
Fixes #40823.
Fixes #40859.

cc @estebank
r? @pnkfelix

src/test/compile-fail/augmented-assignments.rs

@@ -27,7 +27,7 @@ fn main() {
     x;  //~ value moved here
 
     let y = Int(2);
-    //~^use `mut y` here to make mutable
+    //~^ consider changing this to `mut y`
     y   //~ error: cannot borrow immutable local variable `y` as mutable
         //~| cannot borrow
     +=

src/test/compile-fail/borrowck/borrowck-issue-14498.rs

@@ -23,7 +23,7 @@ fn indirect_write_to_imm_box() {
     let mut x: isize = 1;
     let y: Box<_> = box &mut x;
     let p = &y;
-    ***p = 2; //~ ERROR cannot assign to data in an immutable container
+    ***p = 2; //~ ERROR cannot assign to data in a `&` reference
     drop(p);
 }
 
@@ -43,7 +43,6 @@ fn borrow_in_var_from_var_via_imm_box() {
     let p = &y;
     let q = &***p;
     **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
-    //~^         ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }
@@ -64,7 +63,6 @@ fn borrow_in_var_from_field_via_imm_box() {
     let p = &y;
     let q = &***p;
     **y = 2; //~ ERROR cannot assign to `**y` because it is borrowed
-    //~^         ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }
@@ -85,7 +83,6 @@ fn borrow_in_field_from_var_via_imm_box() {
     let p = &y.a;
     let q = &***p;
     **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
-    //~^           ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }
@@ -106,7 +103,6 @@ fn borrow_in_field_from_field_via_imm_box() {
     let p = &y.a;
     let q = &***p;
     **y.a = 2; //~ ERROR cannot assign to `**y.a` because it is borrowed
-    //~^           ERROR cannot assign to data in an immutable container
     drop(p);
     drop(q);
 }

src/test/compile-fail/issue-33819.rs

@@ -12,7 +12,7 @@ fn main() {
     match op {
         Some(ref v) => { let a = &mut v; },
         //~^ ERROR:cannot borrow immutable
-        //~| use `ref mut v` here to make mutable
+        //~| cannot borrow mutably
         None => {},
     }
 }

src/test/compile-fail/mut-suggestion.rs

@@ -17,7 +17,7 @@ impl S {
 }
 
 fn func(arg: S) {
-    //~^ here to make mutable
+    //~^ consider changing this to `mut arg`
     arg.mutate();
     //~^ ERROR cannot borrow immutable argument
     //~| cannot borrow mutably
@@ -25,7 +25,7 @@ fn func(arg: S) {
 
 fn main() {
     let local = S;
-    //~^ here to make mutable
+    //~^ consider changing this to `mut local`
     local.mutate();
     //~^ ERROR cannot borrow immutable local variable
     //~| cannot borrow mutably