Don't ICE in closure arg borrow suggestion

src/test/ui/suggestions/enum-variant-arg-mismatch.rs

@@ -0,0 +1,10 @@
+pub enum Sexpr<'a> {
+    Ident(&'a str),
+}
+
+fn map<'a, F: Fn(String) -> Sexpr<'a>>(f: F) {}
+
+fn main() {
+    map(Sexpr::Ident);
+    //~^ ERROR type mismatch in function arguments
+}

src/test/ui/suggestions/enum-variant-arg-mismatch.stderr

@@ -0,0 +1,22 @@
+error[E0631]: type mismatch in function arguments
+  --> $DIR/enum-variant-arg-mismatch.rs:8:9
+   |
+LL |     Ident(&'a str),
+   |     ----- found signature defined here
+...
+LL |     map(Sexpr::Ident);
+   |     --- ^^^^^^^^^^^^ expected due to this
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: expected function signature `fn(String) -> _`
+              found function signature `fn(&str) -> _`
+note: required by a bound in `map`
+  --> $DIR/enum-variant-arg-mismatch.rs:5:15
+   |
+LL | fn map<'a, F: Fn(String) -> Sexpr<'a>>(f: F) {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `map`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0631`.