Improve diagnostic labels and add note.

This commit improves diagnostic labels to mention which field a borrow
overlaps with and adds a note explaining that the fields overlap.

src/test/ui/borrowck/borrowck-borrow-from-owned-ptr.stderr

@@ -138,7 +138,7 @@ error[E0502]: cannot borrow `foo` (via `foo.bar2`) as immutable because `foo` is
 LL |     let bar1 = &mut foo.bar1;
    |                     -------- mutable borrow occurs here (via `foo.bar1`)
 LL |     let _foo1 = &foo.bar2; //~ ERROR cannot borrow
-   |                  ^^^^^^^^ immutable borrow occurs here (via `foo.bar2`)
+   |                  ^^^^^^^^ immutable borrow of `foo.bar2` -- which overlaps with `foo.bar1` -- occurs here
 LL |     *bar1;
 LL | }
    | - mutable borrow ends here

src/test/ui/borrowck/borrowck-box-insensitivity.ast.stderr

@@ -61,7 +61,7 @@ error[E0502]: cannot borrow `a` (via `a.y`) as immutable because `a` is also bor
 LL |     let _x = &mut a.x;
    |                   --- mutable borrow occurs here (via `a.x`)
 LL |     let _y = &a.y; //[ast]~ ERROR cannot borrow
-   |               ^^^ immutable borrow occurs here (via `a.y`)
+   |               ^^^ immutable borrow of `a.y` -- which overlaps with `a.x` -- occurs here
 ...
 LL | }
    | - mutable borrow ends here
@@ -72,7 +72,7 @@ error[E0502]: cannot borrow `a` (via `a.y`) as mutable because `a` is also borro
 LL |     let _x = &a.x;
    |               --- immutable borrow occurs here (via `a.x`)
 LL |     let _y = &mut a.y; //[ast]~ ERROR cannot borrow
-   |                   ^^^ mutable borrow occurs here (via `a.y`)
+   |                   ^^^ mutable borrow of `a.y` -- which overlaps with `a.x` -- occurs here
 ...
 LL | }
    | - immutable borrow ends here

src/test/ui/borrowck/borrowck-box-insensitivity.rs

@@ -83,14 +83,14 @@ fn borrow_after_mut_borrow() {
     let mut a: Box<_> = box A { x: box 0, y: 1 };
     let _x = &mut a.x;
     let _y = &a.y; //[ast]~ ERROR cannot borrow
-    //[ast]~^ immutable borrow occurs here (via `a.y`)
+    //[ast]~^ immutable borrow of `a.y` -- which overlaps with `a.x` -- occurs here
     use_mut(_x);
 }
 fn mut_borrow_after_borrow() {
     let mut a: Box<_> = box A { x: box 0, y: 1 };
     let _x = &a.x;
     let _y = &mut a.y; //[ast]~ ERROR cannot borrow
-    //[ast]~^ mutable borrow occurs here (via `a.y`)
+    //[ast]~^ mutable borrow of `a.y` -- which overlaps with `a.x` -- occurs here
     use_imm(_x);
 }
 fn copy_after_move_nested() {

src/test/ui/borrowck/borrowck-union-borrow.nll.stderr

@@ -24,9 +24,11 @@ error[E0502]: cannot borrow `u` (via `u.b`) as mutable because it is also borrow
 LL |             let ra = &u.a;
    |                      ---- immutable borrow occurs here (via `u.a`)
 LL |             let rmb = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
-   |                       ^^^^^^^^ mutable borrow occurs here (via `u.b`)
+   |                       ^^^^^^^^ mutable borrow of `u.b` -- which overlaps with `u.a` -- occurs here
 LL |             drop(ra);
    |                  -- immutable borrow later used here
+   |
+   = note: `u.b` is a field of the union `U`, so it overlaps the field `u.a`
 
 error[E0506]: cannot assign to `u.b` because it is borrowed
   --> $DIR/borrowck-union-borrow.rs:51:13
@@ -84,9 +86,11 @@ error[E0502]: cannot borrow `u` (via `u.b`) as immutable because it is also borr
 LL |             let rma = &mut u.a;
    |                       -------- mutable borrow occurs here (via `u.a`)
 LL |             let rb = &u.b; //~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
-   |                      ^^^^ immutable borrow occurs here (via `u.b`)
+   |                      ^^^^ immutable borrow of `u.b` -- which overlaps with `u.a` -- occurs here
 LL |             drop(rma);
    |                  --- mutable borrow later used here
+   |
+   = note: `u.b` is a field of the union `U`, so it overlaps the field `u.a`
 
 error[E0503]: cannot use `u.b` because it was mutably borrowed
   --> $DIR/borrowck-union-borrow.rs:83:21
@@ -108,6 +112,8 @@ LL |             let rmb2 = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as
    |                        ^^^^^^^^ second mutable borrow occurs here (via `u.b`)
 LL |             drop(rma);
    |                  --- first borrow later used here
+   |
+   = note: `u.b` is a field of the union `U`, so it overlaps the field `u.a`
 
 error[E0506]: cannot assign to `u.b` because it is borrowed
   --> $DIR/borrowck-union-borrow.rs:94:13

src/test/ui/borrowck/borrowck-union-borrow.stderr

@@ -23,7 +23,7 @@ error[E0502]: cannot borrow `u` (via `u.b`) as mutable because `u` is also borro
 LL |             let ra = &u.a;
    |                       --- immutable borrow occurs here (via `u.a`)
 LL |             let rmb = &mut u.b; //~ ERROR cannot borrow `u` (via `u.b`) as mutable because `u` is also borrowed as immutable (via `u.a`)
-   |                            ^^^ mutable borrow occurs here (via `u.b`)
+   |                            ^^^ mutable borrow of `u.b` -- which overlaps with `u.a` -- occurs here
 LL |             drop(ra);
 LL |         }
    |         - immutable borrow ends here
@@ -80,7 +80,7 @@ error[E0502]: cannot borrow `u` (via `u.b`) as immutable because `u` is also bor
 LL |             let rma = &mut u.a;
    |                            --- mutable borrow occurs here (via `u.a`)
 LL |             let rb = &u.b; //~ ERROR cannot borrow `u` (via `u.b`) as immutable because `u` is also borrowed as mutable (via `u.a`)
-   |                       ^^^ immutable borrow occurs here (via `u.b`)
+   |                       ^^^ immutable borrow of `u.b` -- which overlaps with `u.a` -- occurs here
 LL |             drop(rma);
 LL |         }
    |         - mutable borrow ends here

src/test/ui/issues/issue-17263.ast.stderr

@@ -13,7 +13,7 @@ error[E0502]: cannot borrow `foo` (via `foo.b`) as immutable because `foo` is al
   --> $DIR/issue-17263.rs:21:32
    |
 LL |     let (c, d) = (&mut foo.a, &foo.b);
-   |                        -----   ^^^^^ immutable borrow occurs here (via `foo.b`)
+   |                        -----   ^^^^^ immutable borrow of `foo.b` -- which overlaps with `foo.a` -- occurs here
    |                        |
    |                        mutable borrow occurs here (via `foo.a`)
 ...

src/test/ui/issues/issue-45157.stderr

@@ -5,10 +5,12 @@ LL |         let mref = &mut u.s.a;
    |                    ---------- mutable borrow occurs here (via `u.s.a`)
 ...
 LL |         let nref = &u.z.c;
-   |                    ^^^^^^ immutable borrow occurs here (via `u.z.c`)
+   |                    ^^^^^^ immutable borrow of `u.z.c` -- which overlaps with `u.s.a` -- occurs here
 LL |         //~^ ERROR cannot borrow `u` (via `u.z.c`) as immutable because it is also borrowed as mutable (via `u.s.a`) [E0502]
 LL |         println!("{} {}", mref, nref)
    |                           ---- mutable borrow later used here
+   |
+   = note: `u.z.c` is a field of the union `U`, so it overlaps the field `u.s.a`
 
 error: aborting due to previous error
 

src/test/ui/nll/issue-57100.stderr

@@ -5,10 +5,12 @@ LL |         let mref = &mut r.r2_union.f3_union.s1_leaf.l1_u8;
    |                    -------------------------------------- mutable borrow occurs here (via `r.r2_union.f3_union.s1_leaf.l1_u8`)
 ...
 LL |         let nref = &r.r2_union.f3_union.s2_leaf.l1_u8;
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here (via `r.r2_union.f3_union.s2_leaf.l1_u8`)
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow of `r.r2_union.f3_union.s2_leaf.l1_u8` -- which overlaps with `r.r2_union.f3_union.s1_leaf.l1_u8` -- occurs here
 ...
 LL |         println!("{} {}", mref, nref)
    |                           ---- mutable borrow later used here
+   |
+   = note: `r.r2_union.f3_union.s2_leaf.l1_u8` is a field of the union `Second`, so it overlaps the field `r.r2_union.f3_union.s1_leaf.l1_u8`
 
 error[E0502]: cannot borrow `r.r2_union` (via `r.r2_union.f1_leaf.l1_u8`) as immutable because it is also borrowed as mutable (via `r.r2_union.f2_leaf.l1_u8`)
   --> $DIR/issue-57100.rs:62:20
@@ -17,10 +19,12 @@ LL |         let mref = &mut r.r2_union.f2_leaf.l1_u8;
    |                    ----------------------------- mutable borrow occurs here (via `r.r2_union.f2_leaf.l1_u8`)
 ...
 LL |         let nref = &r.r2_union.f1_leaf.l1_u8;
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here (via `r.r2_union.f1_leaf.l1_u8`)
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ immutable borrow of `r.r2_union.f1_leaf.l1_u8` -- which overlaps with `r.r2_union.f2_leaf.l1_u8` -- occurs here
 ...
 LL |         println!("{} {}", mref, nref)
    |                           ---- mutable borrow later used here
+   |
+   = note: `r.r2_union.f1_leaf.l1_u8` is a field of the union `First`, so it overlaps the field `r.r2_union.f2_leaf.l1_u8`
 
 error: aborting due to 2 previous errors
 

src/test/ui/union/union-borrow-move-parent-sibling.nll.stderr

@@ -4,9 +4,11 @@ error[E0502]: cannot borrow `u` (via `u.y`) as immutable because it is also borr
 LL |     let a = &mut u.x.0;
    |             ---------- mutable borrow occurs here (via `u.x.0`)
 LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
-   |             ^^^^ immutable borrow occurs here (via `u.y`)
+   |             ^^^^ immutable borrow of `u.y` -- which overlaps with `u.x.0` -- occurs here
 LL |     use_borrow(a);
    |                - mutable borrow later used here
+   |
+   = note: `u.y` is a field of the union `U`, so it overlaps the field `u.x.0`
 
 error[E0382]: use of moved value: `u`
   --> $DIR/union-borrow-move-parent-sibling.rs:22:13
@@ -24,9 +26,11 @@ error[E0502]: cannot borrow `u` (via `u.y`) as immutable because it is also borr
 LL |     let a = &mut (u.x.0).0;
    |             -------------- mutable borrow occurs here (via `u.x.0.0`)
 LL |     let b = &u.y; //~ ERROR cannot borrow `u.y`
-   |             ^^^^ immutable borrow occurs here (via `u.y`)
+   |             ^^^^ immutable borrow of `u.y` -- which overlaps with `u.x.0.0` -- occurs here
 LL |     use_borrow(a);
    |                - mutable borrow later used here
+   |
+   = note: `u.y` is a field of the union `U`, so it overlaps the field `u.x.0.0`
 
 error[E0382]: use of moved value: `u`
   --> $DIR/union-borrow-move-parent-sibling.rs:35:13
@@ -44,9 +48,11 @@ error[E0502]: cannot borrow `u` (via `u.x`) as immutable because it is also borr
 LL |     let a = &mut *u.y;
    |             --------- mutable borrow occurs here (via `*u.y`)
 LL |     let b = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
-   |             ^^^^ immutable borrow occurs here (via `u.x`)
+   |             ^^^^ immutable borrow of `u.x` -- which overlaps with `*u.y` -- occurs here
 LL |     use_borrow(a);
    |                - mutable borrow later used here
+   |
+   = note: `u.x` is a field of the union `U`, so it overlaps the field `*u.y`
 
 error[E0382]: use of moved value: `u`
   --> $DIR/union-borrow-move-parent-sibling.rs:48:13

src/test/ui/union/union-borrow-move-parent-sibling.stderr

@@ -46,7 +46,7 @@ error[E0502]: cannot borrow `u` (via `u.x`) as immutable because `u` is also bor
 LL |     let a = &mut *u.y;
    |                  ---- mutable borrow occurs here (via `*u.y`)
 LL |     let b = &u.x; //~ ERROR cannot borrow `u` (via `u.x`)
-   |              ^^^ immutable borrow occurs here (via `u.x`)
+   |              ^^^ immutable borrow of `u.x` -- which overlaps with `*u.y` -- occurs here
 LL |     use_borrow(a);
 LL | }
    | - mutable borrow ends here