Added better error message for shared borrow treated as unique for purposes of lifetimes

compiler/rustc_mir/src/util/borrowck_errors.rs

@@ -68,9 +68,10 @@ impl<'cx, 'tcx> crate::borrow_check::MirBorrowckCtxt<'cx, 'tcx> {
             err.span_label(
                 new_loan_span,
                 format!(
-                    "mutable borrow starts here in previous \
-                     iteration of loop{}",
-                    opt_via
+                    "{}{} was mutably borrowed here in the previous iteration of the loop{}",
+                    desc,
+                    via(opt_via),
+                    opt_via,
                 ),
             );
             if let Some(old_load_end_span) = old_load_end_span {

src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr

@@ -23,7 +23,7 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
   --> $DIR/borrowck-mut-borrow-linear-errors.rs:12:30
    |
 LL |             _ => { addr.push(&mut x); }
-   |                              ^^^^^^ mutable borrow starts here in previous iteration of loop
+   |                              ^^^^^^ `x` was mutably borrowed here in the previous iteration of the loop
 
 error: aborting due to 3 previous errors
 

src/test/ui/borrowck/mut-borrow-in-loop.stderr

@@ -15,7 +15,7 @@ LL | impl<'a, T : 'a> FuncWrapper<'a, T> {
 LL |             (self.func)(arg)
    |             ------------^^^-
    |             |           |
-   |             |           mutable borrow starts here in previous iteration of loop
+   |             |           `*arg` was mutably borrowed here in the previous iteration of the loop
    |             argument requires that `*arg` is borrowed for `'a`
 
 error[E0499]: cannot borrow `*arg` as mutable more than once at a time
@@ -27,7 +27,7 @@ LL | impl<'a, T : 'a> FuncWrapper<'a, T> {
 LL |             (self.func)(arg)
    |             ------------^^^-
    |             |           |
-   |             |           mutable borrow starts here in previous iteration of loop
+   |             |           `*arg` was mutably borrowed here in the previous iteration of the loop
    |             argument requires that `*arg` is borrowed for `'a`
 
 error[E0499]: cannot borrow `*arg` as mutable more than once at a time
@@ -39,7 +39,7 @@ LL | impl<'a, T : 'a> FuncWrapper<'a, T> {
 LL |             (self.func)(arg)
    |             ------------^^^-
    |             |           |
-   |             |           mutable borrow starts here in previous iteration of loop
+   |             |           `*arg` was mutably borrowed here in the previous iteration of the loop
    |             argument requires that `*arg` is borrowed for `'a`
 
 error: aborting due to 3 previous errors; 1 warning emitted

src/test/ui/borrowck/two-phase-across-loop.stderr

@@ -2,7 +2,7 @@ error[E0499]: cannot borrow `foo` as mutable more than once at a time
   --> $DIR/two-phase-across-loop.rs:17:22
    |
 LL |         strings.push(foo.get_string());
-   |                      ^^^ mutable borrow starts here in previous iteration of loop
+   |                      ^^^ `foo` was mutably borrowed here in the previous iteration of the loop
 
 error: aborting due to previous error
 

src/test/ui/nll/closures-in-loops.stderr

@@ -15,7 +15,7 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
 LL |         v.push(|| x = String::new());
    |                ^^ - borrows occur due to use of `x` in closure
    |                |
-   |                mutable borrow starts here in previous iteration of loop
+   |                `x` was mutably borrowed here in the previous iteration of the loop
 
 error[E0524]: two closures require unique access to `x` at the same time
   --> $DIR/closures-in-loops.rs:20:16

src/test/ui/nll/issue-62007-assign-const-index.stderr

@@ -5,7 +5,7 @@ LL | fn to_refs<T>(mut list: [&mut List<T>; 2]) -> Vec<&mut T> {
    |                          - let's call the lifetime of this reference `'1`
 ...
 LL |         result.push(&mut list[0].value);
-   |                     ^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
+   |                     ^^^^^^^^^^^^^^^^^^ `list[_].value` was mutably borrowed here in the previous iteration of the loop
 ...
 LL |             return result;
    |                    ------ returning this value requires that `list[_].value` is borrowed for `'1`
@@ -19,7 +19,7 @@ LL | fn to_refs<T>(mut list: [&mut List<T>; 2]) -> Vec<&mut T> {
 LL |         if let Some(n) = list[0].next.as_mut() {
    |                          ^^^^^^^^^^^^---------
    |                          |
-   |                          mutable borrow starts here in previous iteration of loop
+   |                          `list[_].next` was mutably borrowed here in the previous iteration of the loop
    |                          argument requires that `list[_].next` is borrowed for `'1`
 
 error: aborting due to 2 previous errors

src/test/ui/nll/issue-62007-assign-differing-fields.stderr

@@ -5,7 +5,7 @@ LL | fn to_refs<'a, T>(mut list: (&'a mut List<T>, &'a mut List<T>)) -> Vec<&'a
    |            -- lifetime `'a` defined here
 ...
 LL |         result.push(&mut (list.0).value);
-   |                     ^^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
+   |                     ^^^^^^^^^^^^^^^^^^^ `list.0.value` was mutably borrowed here in the previous iteration of the loop
 ...
 LL |             return result;
    |                    ------ returning this value requires that `list.0.value` is borrowed for `'a`
@@ -19,7 +19,7 @@ LL | fn to_refs<'a, T>(mut list: (&'a mut List<T>, &'a mut List<T>)) -> Vec<&'a
 LL |         if let Some(n) = (list.0).next.as_mut() {
    |                          ^^^^^^^^^^^^^---------
    |                          |
-   |                          mutable borrow starts here in previous iteration of loop
+   |                          `list.0.next` was mutably borrowed here in the previous iteration of the loop
    |                          argument requires that `list.0.next` is borrowed for `'a`
 
 error: aborting due to 2 previous errors

src/test/ui/nll/polonius/assignment-to-differing-field.stderr

@@ -5,7 +5,7 @@ LL | fn assignment_to_field_projection<'a, T>(
    |                                   -- lifetime `'a` defined here
 ...
 LL |         result.push(&mut (list.0).value);
-   |                     ^^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
+   |                     ^^^^^^^^^^^^^^^^^^^ `list.0.value` was mutably borrowed here in the previous iteration of the loop
 ...
 LL |             return result;
    |                    ------ returning this value requires that `list.0.value` is borrowed for `'a`
@@ -19,7 +19,7 @@ LL | fn assignment_to_field_projection<'a, T>(
 LL |         if let Some(n) = (list.0).next.as_mut() {
    |                          ^^^^^^^^^^^^^---------
    |                          |
-   |                          mutable borrow starts here in previous iteration of loop
+   |                          `list.0.next` was mutably borrowed here in the previous iteration of the loop
    |                          argument requires that `list.0.next` is borrowed for `'a`
 
 error[E0499]: cannot borrow `list.0.0.0.0.0.value` as mutable more than once at a time
@@ -29,7 +29,7 @@ LL | fn assignment_through_projection_chain<'a, T>(
    |                                        -- lifetime `'a` defined here
 ...
 LL |         result.push(&mut ((((list.0).0).0).0).0.value);
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `list.0.0.0.0.0.value` was mutably borrowed here in the previous iteration of the loop
 ...
 LL |             return result;
    |                    ------ returning this value requires that `list.0.0.0.0.0.value` is borrowed for `'a`
@@ -43,7 +43,7 @@ LL | fn assignment_through_projection_chain<'a, T>(
 LL |         if let Some(n) = ((((list.0).0).0).0).0.next.as_mut() {
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^---------
    |                          |
-   |                          mutable borrow starts here in previous iteration of loop
+   |                          `list.0.0.0.0.0.next` was mutably borrowed here in the previous iteration of the loop
    |                          argument requires that `list.0.0.0.0.0.next` is borrowed for `'a`
 
 error: aborting due to 4 previous errors