Formatting `use` statements sometimes leaves behind spaces and newlines

Formatting the following code with cargo fmt leaves behind a space and curly braces.

Before:

use core::{
    slice,
    
    fmt::{},
};

(there are 4 spaces on the seemingly empty line)

After:

use core::{ slice};

Expected:

use core::slice;

Here a newline is left behind and rustfmt reports an internal error.

Before:

use core::{
    
    fmt::{Debug, Display},
    slice::{},
};

(there are again 4 spaces on the seemingly empty line)

After:

use core::{
    
    fmt::{Debug, Display},
};

(there are again 4 spaces on the seemingly empty line, rustfmt didn't touch them)

Expected:

use core::fmt::{Debug, Display};

Command output:

$ cargo fmt -- --check file.rs 
Diff in /path/to/file.rs at line 1:
 use core::{
     
     fmt::{Debug, Display},
-    slice::{},
 };
 
error[internal]: left behind trailing whitespace
 --> /path/to/file.rs:2:2:1
  |
2 |     
  | ^^^^
  |

warning: rustfmt has failed to format. See previous 1 errors.

Start with the reproducer in file.rs and run cargo fmt -- --check file.rs to observe the leftover whitespace, braces, and internal error. Trace the rustfmt handling of these use statements, then verify that empty imports are removed, whitespace is cleaned up, and the expected compact import formatting is produced without errors.