Small Fix && cargo fmt

os/src/fs/stdio.rs

@@ -1,5 +1,5 @@
 use super::File;
-use crate::mm::{UserBuffer};
+use crate::mm::UserBuffer;
 use crate::sbi::console_getchar;
 use crate::task::suspend_current_and_run_next;
 
@@ -8,8 +8,12 @@ pub struct Stdin;
 pub struct Stdout;
 
 impl File for Stdin {
-    fn readable(&self) -> bool { true }
-    fn writable(&self) -> bool { false }
+    fn readable(&self) -> bool {
+        true
+    }
+    fn writable(&self) -> bool {
+        false
+    }
     fn read(&self, mut user_buf: UserBuffer) -> usize {
         assert_eq!(user_buf.len(), 1);
         // busy loop
@@ -24,7 +28,9 @@ impl File for Stdin {
             }
         }
         let ch = c as u8;
-        unsafe { user_buf.buffers[0].as_mut_ptr().write_volatile(ch); }
+        unsafe {
+            user_buf.buffers[0].as_mut_ptr().write_volatile(ch);
+        }
         1
     }
     fn write(&self, _user_buf: UserBuffer) -> usize {
@@ -33,9 +39,13 @@ impl File for Stdin {
 }
 
 impl File for Stdout {
-    fn readable(&self) -> bool { false }
-    fn writable(&self) -> bool { true }
-    fn read(&self, _user_buf: UserBuffer) -> usize{
+    fn readable(&self) -> bool {
+        false
+    }
+    fn writable(&self) -> bool {
+        true
+    }
+    fn read(&self, _user_buf: UserBuffer) -> usize {
         panic!("Cannot read from stdout!");
     }
     fn write(&self, user_buf: UserBuffer) -> usize {
@@ -44,4 +54,4 @@ impl File for Stdout {
         }
         user_buf.len()
     }
-}
+}