Auto merge of rust-lang#2369 - RalfJung:reborrow, r=RalfJung

reborrow error: clarify that we are reborrowing *from* that tag

`@saethlin` I found the current message not entirely clear, so what do you think about this?

src/stacked_borrows/diagnostics.rs

@@ -140,7 +140,7 @@ impl AllocHistory {
         stack: &Stack,
     ) -> InterpError<'tcx> {
         let action = format!(
-            "trying to reborrow {derived_from:?} for {new_perm:?} permission at {alloc_id:?}[{offset:#x}]",
+            "trying to reborrow from {derived_from:?} for {new_perm:?} permission at {alloc_id:?}[{offset:#x}]",
             new_perm = new.perm(),
             offset = error_offset.bytes(),
         );

tests/fail/box-cell-alias.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/box-cell-alias.rs:LL:CC
    |
 LL |     unsafe { (*ptr).set(20) };
    |              ^^^^^^^^^^^^^^
    |              |
-   |              trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |              trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |              this error occurs as part of a reborrow at ALLOC[0x0..0x1]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/aliasing_mut3.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/aliasing_mut3.rs:LL:CC
    |
 LL | pub fn safe(_x: &mut i32, _y: &i32) {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
-   | trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   | trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    | this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/buggy_split_at_mut.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/buggy_split_at_mut.rs:LL:CC
    |
 LL |     let (a, b) = safe::split_at_mut(&mut array, 0);
    |          ^
    |          |
-   |          trying to reborrow <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |          trying to reborrow from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |          this error occurs as part of a reborrow at ALLOC[0x0..0x10]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/illegal_read7.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/illegal_read7.rs:LL:CC
    |
 LL |         let _val = *x.get_mut();
    |                     ^^^^^^^^^^^
    |                     |
-   |                     trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                     trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |                     this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/interior_mut1.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/interior_mut1.rs:LL:CC
    |
 LL |         let _val = *inner_shr.get();
    |                     ^^^^^^^^^^^^^^^
    |                     |
-   |                     trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                     trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |                     this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/interior_mut2.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/interior_mut2.rs:LL:CC
    |
 LL |         let _val = *inner_shr.get();
    |                     ^^^^^^^^^^^^^^^
    |                     |
-   |                     trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                     trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |                     this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/load_invalid_mut.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/load_invalid_mut.rs:LL:CC
    |
 LL |     let _val = *xref_in_mem;
    |                ^^^^^^^^^^^^
    |                |
-   |                trying to reborrow <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                trying to reborrow from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |                this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/load_invalid_shr.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/load_invalid_shr.rs:LL:CC
    |
 LL |     let _val = *xref_in_mem;
    |                ^^^^^^^^^^^^
    |                |
-   |                trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |                this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/pass_invalid_mut.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/pass_invalid_mut.rs:LL:CC
    |
 LL |     foo(xref);
    |         ^^^^
    |         |
-   |         trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |         trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/pass_invalid_shr.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/pass_invalid_shr.rs:LL:CC
    |
 LL |     foo(xref);
    |         ^^^^
    |         |
-   |         trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |         trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_mut.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_mut.rs:LL:CC
    |
 LL |     ret
    |     ^^^
    |     |
-   |     trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |     trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_mut_option.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_mut_option.rs:LL:CC
    |
 LL |         Some(_x) => {}
    |              ^^
    |              |
-   |              trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |              trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |              this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_mut_tuple.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_mut_tuple.rs:LL:CC
    |
 LL |     foo(&mut (1, 2)).0;
    |     ^^^^^^^^^^^^^^^^^^
    |     |
-   |     trying to reborrow <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |     trying to reborrow from <TAG> for Unique permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_shr.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_shr.rs:LL:CC
    |
 LL |     ret
    |     ^^^
    |     |
-   |     trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |     trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_shr_option.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_shr_option.rs:LL:CC
    |
 LL |         Some(_x) => {}
    |              ^^
    |              |
-   |              trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |              trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |              this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/return_invalid_shr_tuple.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> $DIR/return_invalid_shr_tuple.rs:LL:CC
    |
 LL |     foo(&mut (1, 2)).0;
    |     ^^^^^^^^^^^^^^^^^^
    |     |
-   |     trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |     trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |     this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/shared_rw_borrows_are_weak1.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   --> $DIR/shared_rw_borrows_are_weak1.rs:LL:CC
    |
 LL |         y.get_mut();
    |         ^^^^^^^^^^^
    |         |
-   |         trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |         trying to reborrow from <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
    |         this error occurs as part of a reborrow at ALLOC[0x0..0x4]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental

tests/fail/stacked_borrows/zst_slice.stderr

@@ -1,10 +1,10 @@
-error: Undefined Behavior: trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> RUSTLIB/core/src/slice/mod.rs:LL:CC
    |
 LL |         unsafe { &*index.get_unchecked(self) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                  |
-   |                  trying to reborrow <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |                  trying to reborrow from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
    |                  this error occurs as part of a reborrow at ALLOC[0x4..0x8]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental