Rollup merge of #3851 - phansch:refactor_trait_stuff, r=flip1995

Refactor: Extract `trait_ref_of_method` function

This pattern was used in three places after #3844, so I think it's worth moving it into `utils/mod.rs` and documenting it.

clippy_lints/src/utils/mod.rs

@@ -301,6 +301,33 @@ pub fn implements_trait<'a, 'tcx>(
         .enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
 }
 
+/// Get the `hir::TraitRef` of the trait the given method is implemented for
+///
+/// Use this if you want to find the `TraitRef` of the `Add` trait in this example:
+///
+/// ```rust
+/// struct Point(isize, isize);
+///
+/// impl std::ops::Add for Point {
+///     type Output = Self;
+///
+///     fn add(self, other: Self) -> Self {
+///         Point(0, 0)
+///     }
+/// }
+/// ```
+pub fn trait_ref_of_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> Option<TraitRef> {
+    // Get the implemented trait for the current function
+    let parent_impl = cx.tcx.hir().get_parent_item(hir_id);
+    if_chain! {
+        if parent_impl != hir::CRATE_HIR_ID;
+        if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
+        if let hir::ItemKind::Impl(_, _, _, _, trait_ref, _, _) = &item.node;
+        then { return trait_ref.clone(); }
+    }
+    None
+}
+
 /// Check whether this type implements Drop.
 pub fn has_drop<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: Ty<'tcx>) -> bool {
     match ty.ty_adt_def() {