Make racy imperfectly overlapping atomic access unsupported instead of UB

Co-authored-by: Ralf Jung <[email protected]>

Author: cbeuw

src/concurrency/weak_memory.rs

@@ -431,8 +431,8 @@ pub(super) trait EvalContextExt<'mir, 'tcx: 'mir>:
                 && !alloc_clocks
                     .race_free_with_atomic(range, this.machine.data_race.as_ref().unwrap())
             {
-                throw_ub_format!(
-                    "racy imperfectly overlapping atomic access is not possible in the C++20 memory model"
+                throw_unsup_format!(
+                    "racy imperfectly overlapping atomic access is not possible in the C++20 memory model, and not supported by Miri's weak memory emulation"
                 );
             }
         }

tests/fail/weak_memory/racing_mixed_size.stderr

@@ -1,14 +1,13 @@
 warning: thread support is experimental: the scheduler is not preemptive, and can get stuck in spin loops.
          (see https://github.com/rust-lang/miri/issues/1388)
 
-error: Undefined Behavior: racy imperfectly overlapping atomic access is not possible in the C++20 memory model
+error: unsupported operation: racy imperfectly overlapping atomic access is not possible in the C++20 memory model, and not supported by Miri's weak memory emulation
   --> $DIR/racing_mixed_size.rs:LL:CC
    |
 LL |             std::intrinsics::atomic_load_relaxed(hi);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access is not possible in the C++20 memory model
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access is not possible in the C++20 memory model, and not supported by Miri's weak memory emulation
    |
-   = 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
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
 
    = note: inside closure at $DIR/racing_mixed_size.rs:LL:CC
 

tests/fail/weak_memory/racing_mixed_size_read.stderr

@@ -1,14 +1,13 @@
 warning: thread support is experimental: the scheduler is not preemptive, and can get stuck in spin loops.
          (see https://github.com/rust-lang/miri/issues/1388)
 
-error: Undefined Behavior: racy imperfectly overlapping atomic access is not possible in the C++20 memory model
+error: unsupported operation: racy imperfectly overlapping atomic access is not possible in the C++20 memory model, and not supported by Miri's weak memory emulation
   --> $DIR/racing_mixed_size_read.rs:LL:CC
    |
 LL |             std::intrinsics::atomic_load_relaxed(hi);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access is not possible in the C++20 memory model
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access is not possible in the C++20 memory model, and not supported by Miri's weak memory emulation
    |
-   = 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
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
 
    = note: inside closure at $DIR/racing_mixed_size_read.rs:LL:CC