How to implement a graph (in our case, the call graph) idiomatically?

[jiezhoucs]

The current implementation of the CallGraphNode

SURust/compiler/rustc_mir_transform/src/sandbox/wpa.rs

Line 15 in 5e7efdb

struct CallGraphNode<'a> {

simply uses summarize_fn::FnID to represent a function's callers and callees. It works fine, but more naturally, we should directly use CallGraphNode instead of FnID, which means something like

struct CallGraphNode<'a> {                                                         
    fn_id: FnID,                                                                   
    crate_name: &'a str,                                                           
    fn_name: &'a str,                                                              
    callees: FxHashSet<Rc<RefCell<CallGraphNode<'a>>>>,                            
    callers: FxHashSet<Rc<RefCell<CallGraphNode<'a>>>>,                            
} 

However, there seems to be no idiomatic way to implement this. This CallGraphNode cannot derive Hash. Simply implementing Hash for it as follows does not work:

impl Hash for CallGraphNode<'_> {                                                  
    fn hash<H: Hasher>(&self, state: &mut H) {                                     
        self.fn_id.hash(state);                                                                                       
    }                                                                              
}

We still got unsatisfied traits bounds forLrc<RefCell<CallGraphNode<'_>>>: Hash and "the trait Hash is not implemented for RefCell<CallGraphNode<'a>>".

It seems that this can be "solved" by adding a wrapper to CallGraphNode, and implement Hash for this wrapper. However, this is not elegant as there would be unnecessary layers.

References:
HashMap keyed by Rc<RefCell<..>>
Hash based on address (not value) of Rc
Set of Rc where T isn't Hash or Ord?
Hash trait does not work for Rc<RefCell> in enum