Wording improvements

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

Author: cbeuw

src/concurrency/weak_memory.rs

@@ -417,7 +417,9 @@ pub(super) trait EvalContextExt<'mir, 'tcx: 'mir>:
                 && !alloc_clocks
                     .read_race_free_with_atomic(range, this.machine.data_race.as_ref().unwrap())
             {
-                throw_ub_format!("racy imperfectly overlapping atomic access");
+                throw_ub_format!(
+                    "racy imperfectly overlapping atomic access is not possible in the C++20 memory model"
+                );
             }
         }
         Ok(())

src/machine.rs

@@ -743,8 +743,8 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         if let Some(weak_memory) = &alloc_extra.weak_memory {
             if !machine.data_race.as_ref().unwrap().ongoing_atomic_access() {
                 // This is a non-atomic access. And if we are accessing a previously atomically
-                // accessed location without racing with them, then the location no longer needs
-                // to exhibit weak-memory behaviours until a fresh atomic access happens
+                // accessed location without racing with them, then the location no longer
+                // exhibits weak-memory behaviors until a fresh atomic access happens.
                 weak_memory.destroy_atomicity(range);
             }
         }

tests/compile-fail/weak_memory/racing_mixed_size.stderr

@@ -1,11 +1,11 @@
 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
+error: Undefined Behavior: racy imperfectly overlapping atomic access is not possible in the C++20 memory model
   --> $DIR/racing_mixed_size.rs:LL:CC
    |
 LL |             std::intrinsics::atomic_load_relaxed(hi);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ racy imperfectly overlapping atomic access is not possible in the C++20 memory model
    |
    = 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

tests/run-pass/weak_memory/extra_cpp_unsafe.rs

@@ -62,7 +62,7 @@ fn racing_mixed_size_read() {
     j2.join().unwrap();
 }
 
-// And the combination of both of above
+// And we allow the combination of both of the above.
 fn racing_mixed_atomicity_and_size_read() {
     let x = static_atomic(u32::from_be(0xabbafafa));