Auto merge of #2537 - saethlin:dont-back-up-too-far, r=RalfJung

Don't back up past the caller when looking for an FnEntry span

Fixes https://github.com/rust-lang/miri/issues/2536

This adds a fix for the logic as well as a regression test. In the new test `tests/fail/stacked_borrows/fnentry_invalidation2.rs`, before this PR, we display this diagnostic:
```
help: <3278> was later invalidated at offsets [0x0..0xc] by a Unique FnEntry retag
  --> tests/fail/stacked_borrows/fnentry_invalidation2.rs:13:5
   |
13 |     inner(&mut t);
   |     ^^^^^^^^^^^^^
```
Which is very misleading. It is not this call itself, but what happens within the call that invalidates the tag we want. With this PR, we get:
```
help: <2798> was later invalidated at offsets [0x0..0xc] by a Unique FnEntry retag inside this call
  --> tests/fail/stacked_borrows/fnentry_invalidation2.rs:20:13
   |
20 |     let _ = t.sli.as_mut_ptr();
   |             ^^^^^^^^^^^^^^^^^^
```
Which is much better.

4 files changed, 51 insertions, 2 deletions

diff --git a/src/tools/miri/tests/fail/stacked_borrows/aliasing_mut3.stderr b/src/tools/miri/tests/fail/stacked_borrows/aliasing_mut3.stderr
index c2ea90f242a..eb6b01fc6b1 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/aliasing_mut3.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/aliasing_mut3.stderr
@@ -14,7 +14,7 @@ help: <TAG> was created by a SharedReadOnly retag at offsets [0x0..0x4]
    |
 LL |     safe_raw(xraw, xshr);
    |                    ^^^^
-help: <TAG> was later invalidated at offsets [0x0..0x4] by a Unique FnEntry retag
+help: <TAG> was later invalidated at offsets [0x0..0x4] by a Unique FnEntry retag inside this call
   --> $DIR/aliasing_mut3.rs:LL:CC
    |
 LL |     safe_raw(xraw, xshr);
diff --git a/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation.stderr b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation.stderr
index 653ceca8588..e81411bbdd8 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation.stderr
@@ -14,7 +14,7 @@ help: <TAG> was created by a SharedReadWrite retag at offsets [0x0..0x4]
    |
 LL |     let z = &mut x as *mut i32;
    |             ^^^^^^
-help: <TAG> was later invalidated at offsets [0x0..0x4] by a Unique FnEntry retag
+help: <TAG> was later invalidated at offsets [0x0..0x4] by a Unique FnEntry retag inside this call
   --> $DIR/fnentry_invalidation.rs:LL:CC
    |
 LL |     x.do_bad();
diff --git a/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.rs b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.rs
new file mode 100644
index 00000000000..dc51a8a8ac6
--- /dev/null
+++ b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.rs
@@ -0,0 +1,21 @@
+// Regression test for https://github.com/rust-lang/miri/issues/2536
+// This tests that we don't try to back too far up the stack when selecting a span to report.
+// We should display the as_mut_ptr() call as the location of the invalidation, not the call to
+// inner
+
+struct Thing<'a> {
+    sli: &'a mut [i32],
+}
+
+fn main() {
+    let mut t = Thing { sli: &mut [0, 1, 2] };
+    let ptr = t.sli.as_ptr();
+    inner(&mut t);
+    unsafe {
+        let _oof = *ptr; //~ ERROR: /read access .* tag does not exist in the borrow stack/
+    }
+}
+
+fn inner(t: &mut Thing) {
+    let _ = t.sli.as_mut_ptr();
+}
diff --git a/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.stderr b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.stderr
new file mode 100644
index 00000000000..d6d0084fa2a
--- /dev/null
+++ b/src/tools/miri/tests/fail/stacked_borrows/fnentry_invalidation2.stderr
@@ -0,0 +1,28 @@
+error: Undefined Behavior: attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+  --> $DIR/fnentry_invalidation2.rs:LL:CC
+   |
+LL |         let _oof = *ptr;
+   |                    ^^^^
+   |                    |
+   |                    attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+   |                    this error occurs as part of an access 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
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a SharedReadOnly retag at offsets [0x0..0xc]
+  --> $DIR/fnentry_invalidation2.rs:LL:CC
+   |
+LL |     let ptr = t.sli.as_ptr();
+   |               ^^^^^^^^^^^^^^
+help: <TAG> was later invalidated at offsets [0x0..0xc] by a Unique FnEntry retag inside this call
+  --> $DIR/fnentry_invalidation2.rs:LL:CC
+   |
+LL |     let _ = t.sli.as_mut_ptr();
+   |             ^^^^^^^^^^^^^^^^^^
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/fnentry_invalidation2.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+