Fix index out of bounds in suggest_trait_fn_ty_for_impl_fn_infer

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -3317,10 +3317,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             tcx,
             trait_ref.substs.extend_to(tcx, assoc.def_id, |param, _| tcx.mk_param_from_def(param)),
         );
+        let fn_sig = tcx.liberate_late_bound_regions(fn_hir_id.expect_owner().to_def_id(), fn_sig);
 
-        let ty = if let Some(arg_idx) = arg_idx { fn_sig.input(arg_idx) } else { fn_sig.output() };
+        Some(if let Some(arg_idx) = arg_idx {
-
+            *fn_sig.inputs().get(arg_idx)?
-        Some(tcx.liberate_late_bound_regions(fn_hir_id.expect_owner().to_def_id(), ty))
+        } else {
+            fn_sig.output()
+        })
     }
 
     #[instrument(level = "trace", skip(self, generate_err))]

tests/ui/suggestions/bad-infer-in-trait-impl.rs

@@ -0,0 +1,10 @@
+trait Foo {
+    fn bar();
+}
+
+impl Foo for () {
+    fn bar(s: _) {}
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
+}
+
+fn main() {}

tests/ui/suggestions/bad-infer-in-trait-impl.stderr

@@ -0,0 +1,14 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
+  --> $DIR/bad-infer-in-trait-impl.rs:6:15
+   |
+LL |     fn bar(s: _) {}
+   |               ^ not allowed in type signatures
+   |
+help: use type parameters instead
+   |
+LL |     fn bar<T>(s: T) {}
+   |           +++    ~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.