attempt to clarify what the backtrace belongs to when there could be ambiguity

Author: RalfJung

src/tools/miri/src/diagnostics.rs

@@ -362,23 +362,26 @@ fn report_msg<'tcx>(
     }
 
     // Show note and help messages.
+    let mut extra_span = false;
     for (span_data, note) in &notes {
         if let Some(span_data) = span_data {
             err.span_note(span_data.span(), note);
+            extra_span = true;
         } else {
             err.note(note);
         }
     }
     for (span_data, help) in &helps {
         if let Some(span_data) = span_data {
             err.span_help(span_data.span(), help);
+            extra_span = true;
         } else {
             err.help(help);
         }
     }
     if notes.len() + helps.len() > 0 {
         // Add visual separator before backtrace.
-        err.note("BACKTRACE:");
+        err.note(if extra_span { "BACKTRACE (of the first span):" } else { "BACKTRACE:" });
     }
     // Add backtrace
     for (idx, frame_info) in stacktrace.iter().enumerate() {

src/tools/miri/tests/fail/box-cell-alias.stderr

@@ -19,7 +19,7 @@ help: <TAG> was later invalidated at offsets [0x0..0x1] by a Unique retag
    |
 LL |     let res = helper(val, ptr);
    |                      ^^^
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside `helper` at $DIR/box-cell-alias.rs:LL:CC
 note: inside `main`
   --> $DIR/box-cell-alias.rs:LL:CC

src/tools/miri/tests/fail/data_race/alloc_read_race.stderr

@@ -11,7 +11,7 @@ LL |             pointer.store(Box::into_raw(Box::new_uninit()), Ordering::Relax
    |                                         ^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/alloc_read_race.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/alloc_write_race.stderr

@@ -11,7 +11,7 @@ LL |                 .store(Box::into_raw(Box::<usize>::new_uninit()) as *mut us
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/alloc_write_race.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_read_na_write_race1.stderr

@@ -11,7 +11,7 @@ LL |             *(c.0 as *mut usize) = 32;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_read_na_write_race1.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_read_na_write_race2.stderr

@@ -11,7 +11,7 @@ LL |             atomic_ref.load(Ordering::SeqCst)
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_read_na_write_race2.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_write_na_read_race1.stderr

@@ -11,7 +11,7 @@ LL |             atomic_ref.store(32, Ordering::SeqCst)
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_write_na_read_race1.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_write_na_read_race2.stderr

@@ -11,7 +11,7 @@ LL |             let _val = *(c.0 as *mut usize);
    |                        ^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_write_na_read_race2.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_write_na_write_race1.stderr

@@ -11,7 +11,7 @@ LL |             *(c.0 as *mut usize) = 32;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_write_na_write_race1.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

src/tools/miri/tests/fail/data_race/atomic_write_na_write_race2.stderr

@@ -11,7 +11,7 @@ LL |             atomic_ref.store(64, Ordering::SeqCst);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
-   = note: BACKTRACE:
+   = note: BACKTRACE (of the first span):
    = note: inside closure at $DIR/atomic_write_na_write_race2.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace