fs: fill the destination buffer with 0s for MockFile::read() (#7596)

Author: martin-g

tokio/src/fs/mocks.rs

@@ -56,6 +56,13 @@ mock! {
 
 impl Read for MockFile {
     fn read(&mut self, dst: &mut [u8]) -> io::Result<usize> {
+        // Placate Miri.  Tokio will call this method with an uninitialized
+        // buffer, which is ok because std::io::Read::read implementations don't usually read
+        // from their input buffers.  But Mockall 0.12-0.13 will try to Debug::fmt the
+        // buffer, even if there is no failure, triggering an uninitialized data access alert from
+        // Miri.  Initialize the data here just to prevent those Miri alerts.
+        // This can be removed after upgrading to Mockall 0.14.
+        dst.fill(0);
         self.inner_read(dst)
     }
 }