overhaul &mut suggestions in borrowck errors

tests/ui/array-slice-vec/slice-mut-2.stderr

@@ -4,10 +4,10 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
 LL |     let _ = &mut x[2..4];
    |                  ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
    |
-help: consider changing this to be a mutable reference
+help: consider changing this binding's type
    |
-LL |     let x: &[isize] = &mut [1, 2, 3, 4, 5];
-   |                        +++
+LL |     let x: &mut [isize] = &[1, 2, 3, 4, 5];
+   |             +++
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrow-raw-address-of-deref-mutability.stderr

@@ -15,10 +15,10 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `*const` pointer
 LL |     let q = &raw mut *x;
    |             ^^^^^^^^^^^ `x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable
    |
-help: consider changing this to be a mutable pointer
+help: consider specifying this binding's type
    |
-LL |     let x = &mut 0 as *const i32;
-   |              +++
+LL |     let x: *mut i32 = &0 as *const i32;
+   |          ++++++++++
 
 error: aborting due to 2 previous errors
 

tests/ui/borrowck/borrowck-access-permissions.stderr

@@ -43,10 +43,11 @@ error[E0596]: cannot borrow `*ptr_x` as mutable, as it is behind a `*const` poin
 LL |             let _y1 = &mut *ptr_x;
    |                       ^^^^^^^^^^^ `ptr_x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable
    |
-help: consider changing this to be a mutable pointer
+help: consider changing this binding's type
+   |
+LL -         let ptr_x: *const _ = &x;
+LL +         let ptr_x: *mut i32 = &x;
    |
-LL |         let ptr_x: *const _ = &mut x;
-   |                                +++
 
 error[E0596]: cannot borrow `*foo_ref.f` as mutable, as it is behind a `&` reference
   --> $DIR/borrowck-access-permissions.rs:59:18

tests/ui/borrowck/suggestions/overloaded-index-not-mut-but-should-be-mut.fixed

@@ -0,0 +1,21 @@
+//@ run-rustfix
+fn main() {
+    let mut map = std::collections::BTreeMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = map.get_mut(&0).unwrap();
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+
+    let mut map = std::collections::HashMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = map.get_mut(&0).unwrap();
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+
+    let mut vec = vec![String::new(), String::new()];
+    let string = &mut vec[0];
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+}

tests/ui/borrowck/suggestions/overloaded-index-not-mut-but-should-be-mut.rs

@@ -0,0 +1,21 @@
+//@ run-rustfix
+fn main() {
+    let mut map = std::collections::BTreeMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = &map[&0];
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+
+    let mut map = std::collections::HashMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = &map[&0];
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+
+    let mut vec = vec![String::new(), String::new()];
+    let string = &vec[0];
+    string.push_str("test");
+    //~^ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference
+}

tests/ui/borrowck/suggestions/overloaded-index-not-mut-but-should-be-mut.stderr

@@ -0,0 +1,38 @@
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/overloaded-index-not-mut-but-should-be-mut.rs:7:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider using `get_mut`
+   |
+LL -     let string = &map[&0];
+LL +     let string = map.get_mut(&0).unwrap();
+   |
+
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/overloaded-index-not-mut-but-should-be-mut.rs:14:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider using `get_mut`
+   |
+LL -     let string = &map[&0];
+LL +     let string = map.get_mut(&0).unwrap();
+   |
+
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/overloaded-index-not-mut-but-should-be-mut.rs:19:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     let string = &mut vec[0];
+   |                   +++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0596`.

tests/ui/borrowck/suggestions/overloaded-index-without-indexmut.rs

@@ -0,0 +1,16 @@
+use std::ops::Index;
+
+struct MyType;
+impl Index<usize> for MyType {
+    type Output = String;
+    fn index(&self, _idx: usize) -> &String {
+        const { &String::new() }
+    }
+}
+
+fn main() {
+    let x = MyType;
+    let y = &x[0];
+    y.push_str("");
+    //~^ ERROR cannot borrow `*y` as mutable, as it is behind a `&` reference
+}

tests/ui/borrowck/suggestions/overloaded-index-without-indexmut.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow `*y` as mutable, as it is behind a `&` reference
+  --> $DIR/overloaded-index-without-indexmut.rs:14:5
+   |
+LL |     y.push_str("");
+   |     ^ `y` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0596`.