Merge #8467

8467: Adds impl Deref assist r=jhgg a=jhgg

This PR adds a new `generate_deref` assist that automatically generates a deref impl for a given struct field.

Check out this gif:

![2021-04-11_00-33-33](https://user-images.githubusercontent.com/5489149/114296006-b38e1000-9a5d-11eb-9112-807c01b8fd0a.gif)

--

I have a few Q's:
 - [x] Should I write more tests, if so, what precisely should I test for?
 - [x] I have an inline question on line 65, can someone provide guidance? :) 
 - [x] I can implement this for `ast::TupleField` too. But should it be a separate assist fn, or should I try and jam both into the `generate_deref`?
 - [x] I want to follow this up with an assist on `impl $0Deref for T {` which would automatically generate a `DerefMut` impl that mirrors the Deref as well, however, I could probably use some pointers on how to do that, since I'll have to reach into the ast of `fn deref` to grab the field that it's referencing for the `DerefMut` impl. 

Co-authored-by: jake <jh@discordapp.com>

crates/ide_db/src/helpers.rs

@@ -113,6 +113,10 @@ impl FamousDefs<'_, '_> {
         self.find_module("core:iter")
     }
 
+    pub fn core_ops_Deref(&self) -> Option<Trait> {
+        self.find_trait("core:ops:Deref")
+    }
+
     fn find_trait(&self, path: &str) -> Option<Trait> {
         match self.find_def(path)? {
             hir::ScopeDef::ModuleDef(hir::ModuleDef::Trait(it)) => Some(it),

crates/ide_db/src/helpers/famous_defs_fixture.rs

@@ -112,6 +112,12 @@ pub mod ops {
         type Output;
         extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
     }
+
+    #[lang = "deref"]
+    pub trait Deref {
+        type Target: ?Sized;
+        fn deref(&self) -> &Self::Target;
+    }
 }
 
 pub mod option {
@@ -141,3 +147,5 @@ mod return_keyword {}
 
 /// Docs for prim_str
 mod prim_str {}
+
+pub use core::ops;