Auto merge of #102080 - yanchen4791:issue-99824-fix, r=cjgillot

Fix missing explanation of where the borrowed reference is used when the same borrow occurs multiple times due to loop iterations

Fix #99824.

Problem of the issue:
If a borrow occurs in a loop, the borrowed reference could be invalidated at the same place at next iteration of the loop. When this happens, the point where the borrow occurs is the same as the intervening point that might invalidate the reference in the loop. This causes a problem for the current code finding the point where the resulting reference is used, so that the explanation of the cause will be missing. As the second point of "explain all errors in terms of three points" (see [leveraging intuition framing errors in terms of points"](https://rust-lang.github.io/rfcs/2094-nll.html#leveraging-intuition-framing-errors-in-terms-of-points), this explanation is very helpful for user to understand the error.

In the current implementation, the searching region for finding the location where the borrowed reference is used is limited to between the place where the borrow occurs and the place where the reference is invalidated. If those two places happen to be the same, which indicates that the borrow and invalidation occur at the same place in a loop, the search will fail.

One solution to the problem is when these two places are the same,  find the terminator of the loop, and then use the location of the loop terminator instead of the location of the borrow for the region to find the place where the borrowed reference is used.

3 files changed, 18 insertions, 8 deletions

diff --git a/src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr b/src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr
index 15ac737606d..d2b845619c7 100644
--- a/src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr
+++ b/src/test/ui/borrowck/borrowck-mut-borrow-linear-errors.stderr
@@ -25,7 +25,10 @@ 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); }
-   |                              ^^^^^^ `x` was mutably borrowed here in the previous iteration of the loop
+   |                    ----------^^^^^^-
+   |                    |         |
+   |                    |         `x` was mutably borrowed here in the previous iteration of the loop
+   |                    first borrow used here, in later iteration of loop
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/borrowck/two-phase-across-loop.stderr b/src/test/ui/borrowck/two-phase-across-loop.stderr
index 95896c6bbf9..22f9b39dfee 100644
--- a/src/test/ui/borrowck/two-phase-across-loop.stderr
+++ b/src/test/ui/borrowck/two-phase-across-loop.stderr
@@ -2,7 +2,10 @@ 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());
-   |                      ^^^^^^^^^^^^^^^^ `foo` was mutably borrowed here in the previous iteration of the loop
+   |         -------------^^^^^^^^^^^^^^^^-
+   |         |            |
+   |         |            `foo` was mutably borrowed here in the previous iteration of the loop
+   |         first borrow used here, in later iteration of loop
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/closures-in-loops.stderr b/src/test/ui/nll/closures-in-loops.stderr
index 2be0460df1f..1c1a31d356d 100644
--- a/src/test/ui/nll/closures-in-loops.stderr
+++ b/src/test/ui/nll/closures-in-loops.stderr
@@ -13,17 +13,21 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
   --> $DIR/closures-in-loops.rs:13:16
    |
 LL |         v.push(|| x = String::new());
-   |                ^^ - borrows occur due to use of `x` in closure
-   |                |
-   |                `x` was mutably borrowed here in the previous iteration of the loop
+   |         -------^^-------------------
+   |         |      |  |
+   |         |      |  borrows occur due to use of `x` in closure
+   |         |      `x` was mutably borrowed here in the previous iteration of the loop
+   |         first borrow used here, in later iteration of loop
 
 error[E0524]: two closures require unique access to `x` at the same time
   --> $DIR/closures-in-loops.rs:20:16
    |
 LL |         v.push(|| *x = String::new());
-   |                ^^ -- borrows occur due to use of `x` in closure
-   |                |
-   |                closures are constructed here in different iterations of loop
+   |         -------^^--------------------
+   |         |      |  |
+   |         |      |  borrows occur due to use of `x` in closure
+   |         |      closures are constructed here in different iterations of loop
+   |         first borrow used here, in later iteration of loop
 
 error: aborting due to 3 previous errors