foo.rs

struct Foo {                      
  foo : &int,                     
}                                 
                                  
impl Foo {                        
  fn a(&mut self) {}              
}                                 
                                  
fn main() {                       
  let mut l = Foo { foo: &4 };    
  l.a();                          
  l.a();                          
}                                 

gistfile1.txt

$ rustc --test foo.rs                                               
foo.rs:12:2: 12:3 error: loan of mutable local variable as mutable conflicts with prior loan
foo.rs:12   l.a();
            ^
foo.rs:11:2: 11:3 note: prior loan as mutable granted here
foo.rs:11   l.a();
            ^
error: aborting due to previous error