thread name handling that actually makes sense

Author: beepster4096

src/shims/windows/dlsym.rs

@@ -122,7 +122,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     _ => this.invalid_handle("SetThreadDescription")?,
                 };
 
-                this.set_thread_name_wide(thread, name);
+                this.set_thread_name_wide(thread, &name);
 
                 this.write_null(dest)?;
             }

src/thread.rs

@@ -803,12 +803,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     }
 
     #[inline]
-    fn set_thread_name_wide(&mut self, thread: ThreadId, new_thread_name: Vec<u16>) {
+    fn set_thread_name_wide(&mut self, thread: ThreadId, new_thread_name: &[u16]) {
         let this = self.eval_context_mut();
-        this.machine.threads.set_thread_name(
-            thread,
-            new_thread_name.into_iter().flat_map(u16::to_ne_bytes).collect(),
-        );
+
+        // `GetThreadDescription` isn't implemented, so being lossy is okay.
+        // This is only read by diagnostics, which already use `from_utf8_lossy`.
+        this.machine
+            .threads
+            .set_thread_name(thread, String::from_utf16_lossy(new_thread_name).into_bytes());
     }
 
     #[inline]