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/librustc_borrowck/borrowck/check_loans.rs

@@ -557,12 +557,8 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                 if new_loan.loan_path.has_fork(&old_loan.loan_path) && common.is_some() {
                     let nl = self.bccx.loan_path_to_string(&common.unwrap());
                     let ol = nl.clone();
-                    let new_loan_msg = format!(" (via `{}`)",
-                                               self.bccx.loan_path_to_string(
-                                                   &new_loan.loan_path));
-                    let old_loan_msg = format!(" (via `{}`)",
-                                               self.bccx.loan_path_to_string(
-                                                   &old_loan.loan_path));
+                    let new_loan_msg = self.bccx.loan_path_to_string(&new_loan.loan_path);
+                    let old_loan_msg = self.bccx.loan_path_to_string(&old_loan.loan_path);
                     (nl, ol, new_loan_msg, old_loan_msg)
                 } else {
                     (self.bccx.loan_path_to_string(&new_loan.loan_path),

src/librustc_mir/borrow_check/error_reporting.rs

@@ -329,12 +329,8 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             "closure"
         };
 
-        let (desc_place, msg_place, msg_borrow) = self.describe_place_for_conflicting_borrow(
-            place, &issued_borrow.borrowed_place,
-        );
-        let via = |msg: String| if msg.is_empty() { msg } else { format!(" (via `{}`)", msg) };
-        let msg_place = via(msg_place);
-        let msg_borrow = via(msg_borrow);
+        let (desc_place, msg_place, msg_borrow, union_type_name) =
+            self.describe_place_for_conflicting_borrow(place, &issued_borrow.borrowed_place);
 
         let explanation = self.explain_why_borrow_contains_point(context, issued_borrow, None);
         let second_borrow_desc = if explanation.is_explained() {
@@ -516,6 +512,13 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             );
         }
 
+        if union_type_name != "" {
+            err.note(&format!(
+                "`{}` is a field of the union `{}`, so it overlaps the field `{}`",
+                msg_place, union_type_name, msg_borrow,
+            ));
+        }
+
         explanation
             .add_explanation_to_diagnostic(self.infcx.tcx, self.mir, &mut err, first_borrow_desc);
 
@@ -549,7 +552,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         &self,
         first_borrowed_place: &Place<'tcx>,
         second_borrowed_place: &Place<'tcx>,
-    ) -> (String, String, String) {
+    ) -> (String, String, String, String) {
         // Define a small closure that we can use to check if the type of a place
         // is a union.
         let is_union = |place: &Place<'tcx>| -> bool {
@@ -600,7 +603,12 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                                 .unwrap_or_else(|| "_".to_owned());
                             let desc_second = self.describe_place(second_borrowed_place)
                                 .unwrap_or_else(|| "_".to_owned());
-                            return Some((desc_base, desc_first, desc_second));
+
+                            // Also compute the name of the union type, eg. `Foo` so we
+                            // can add a helpful note with it.
+                            let ty = base.ty(self.mir, self.infcx.tcx).to_ty(self.infcx.tcx);
+
+                            return Some((desc_base, desc_first, desc_second, ty.to_string()));
                         },
                         _ => current = base,
                     }
@@ -612,7 +620,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 // only return the description of the first place.
                 let desc_place = self.describe_place(first_borrowed_place)
                     .unwrap_or_else(|| "_".to_owned());
-                (desc_place, "".to_string(), "".to_string())
+                (desc_place, "".to_string(), "".to_string(), "".to_string())
             })
     }
 

src/librustc_mir/util/borrowck_errors.rs

@@ -138,13 +138,15 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
         old_load_end_span: Option<Span>,
         o: Origin,
     ) -> DiagnosticBuilder<'cx> {
+        let via = |msg: &str|
+            if msg.is_empty() { msg.to_string() } else { format!(" (via `{}`)", msg) };
         let mut err = struct_span_err!(
             self,
             new_loan_span,
             E0499,
             "cannot borrow `{}`{} as mutable more than once at a time{OGN}",
             desc,
-            opt_via,
+            via(opt_via),
             OGN = o
         );
         if old_loan_span == new_loan_span {
@@ -164,11 +166,11 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
         } else {
             err.span_label(
                 old_loan_span,
-                format!("first mutable borrow occurs here{}", old_opt_via),
+                format!("first mutable borrow occurs here{}", via(old_opt_via)),
             );
             err.span_label(
                 new_loan_span,
-                format!("second mutable borrow occurs here{}", opt_via),
+                format!("second mutable borrow occurs here{}", via(opt_via)),
             );
             if let Some(old_load_end_span) = old_load_end_span {
                 err.span_label(old_load_end_span, "first borrow ends here");
@@ -292,27 +294,46 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
         old_load_end_span: Option<Span>,
         o: Origin,
     ) -> DiagnosticBuilder<'cx> {
+        let via = |msg: &str|
+            if msg.is_empty() { msg.to_string() } else { format!(" (via `{}`)", msg) };
         let mut err = struct_span_err!(
             self,
             span,
             E0502,
-            "cannot borrow `{}`{} as {} because {} is also borrowed as {}{}{OGN}",
+            "cannot borrow `{}`{} as {} because {} is also borrowed \
+             as {}{}{OGN}",
             desc_new,
-            msg_new,
+            via(msg_new),
             kind_new,
             noun_old,
             kind_old,
-            msg_old,
+            via(msg_old),
             OGN = o
         );
-        err.span_label(span, format!("{} borrow occurs here{}", kind_new, msg_new));
-        err.span_label(
-            old_span,
-            format!("{} borrow occurs here{}", kind_old, msg_old),
-        );
+
+        if msg_new == "" {
+            // If `msg_new` is empty, then this isn't a borrow of a union field.
+            err.span_label(span, format!("{} borrow occurs here", kind_new));
+            err.span_label(old_span, format!("{} borrow occurs here", kind_old));
+        } else {
+            // If `msg_new` isn't empty, then this a borrow of a union field.
+            err.span_label(
+                span,
+                format!(
+                    "{} borrow of `{}` -- which overlaps with `{}` -- occurs here",
+                    kind_new, msg_new, msg_old,
+                )
+            );
+            err.span_label(
+                old_span,
+                format!("{} borrow occurs here{}", kind_old, via(msg_old)),
+            );
+        }
+
         if let Some(old_load_end_span) = old_load_end_span {
             err.span_label(old_load_end_span, format!("{} borrow ends here", kind_old));
         }
+
         self.cancel_if_wrong_origin(err, o)
     }
 

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