When a local binding shadows a fn, point at fn def in call failure

When a local binding shadows a function that is then called, this local
binding will cause an E0618 error. We now point not only at the binding
definition, but also at the locally defined function of the same name.

```
error[E0618]: expected function, found `&str`
  --> $DIR/issue-22468.rs:3:13
   |
LL |     let foo = "bar";
   |         --- `foo` has type `&str`
LL |     let x = foo("baz");
   |             ^^^-------
   |             |
   |             call expression requires function
...
LL | fn foo(file: &str) -> bool {
   | -------------------------- this function of the same name is avalable here, but it shadowed by the local binding of the same name
```

Fix rust-lang#53841

Author: estebank

compiler/rustc_hir_typeck/src/callee.rs

@@ -625,6 +625,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             );
         }
 
+        if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = callee_expr.kind
+            && let Res::Local(_) = path.res
+            && let [segment] = &path.segments[..]
+        {
+            for id in self.tcx.hir().items() {
+                if let Some(node) = self.tcx.hir().get_if_local(id.owner_id.into())
+                    && let hir::Node::Item(item) = node
+                    && let hir::ItemKind::Fn(..) = item.kind
+                    && item.ident.name == segment.ident.name
+                {
+                    err.span_label(
+                        self.tcx.def_span(id.owner_id),
+                        "this function of the same name is available here, but it's shadowed by \
+                         the local binding",
+                    );
+                }
+            }
+        }
+
         let mut inner_callee_path = None;
         let def = match callee_expr.kind {
             hir::ExprKind::Path(ref qpath) => {

tests/ui/issues/issue-22468.stderr

@@ -7,6 +7,9 @@ LL |     let x = foo("baz");
    |             ^^^-------
    |             |
    |             call expression requires function
+...
+LL | fn foo(file: &str) -> bool {
+   | -------------------------- this function of the same name is available here, but it's shadowed by the local binding
 
 error: aborting due to previous error