`collapsible_if` false positive with RefCell borrow

Lint name: collapsible_if

I tried this code:

use std::cell::RefCell;

struct Test {
    a: usize,
    b: RefCell<usize>,
}

fn main() {
    let mut t = Test {
        a: 1,
        b: RefCell::new(2),
    };
                                   
    if *t.b.borrow() == 0 {    
        if (&mut t).a == 5 {       
            println!("a");         
        }                          
    }                              
}                                  

Clippy suggests this change which does not compile because it requires borrowing t as mutable while it is already borrowed as immutable.

 if *t.b.borrow() == 0 && (&mut t).a == 5 {
     println!("a");
 }

Meta

• cargo clippy -V: clippy 0.1.54 (a178d03 2021-07-26)
• rustc -Vv:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-unknown-linux-gnu
release: 1.54.0
LLVM version: 12.0.1

Start by reproducing the collapsible_if warning with the Rust code in the issue and cargo clippy. Trace the collapsible_if lint and its existing tests, then add coverage for the RefCell borrow case. Done means Clippy no longer suggests a transformation that fails to compile.