Avoid ICE in drop recursion check in case of invalid drop impls

compiler/rustc_middle/src/ty/sty.rs

@@ -1312,6 +1312,7 @@ pub fn inputs(&self) -> Binder<'tcx, &'tcx [Ty<'tcx>]> {
         self.map_bound_ref_unchecked(|fn_sig| fn_sig.inputs())
     }
     #[inline]
+    #[track_caller]
     pub fn input(&self, index: usize) -> ty::Binder<'tcx, Ty<'tcx>> {
         self.map_bound_ref(|fn_sig| fn_sig.inputs()[index])
     }

compiler/rustc_mir_build/src/lints.rs

@@ -72,15 +72,14 @@ pub fn check_drop_recursion<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
             tcx.impl_of_method(def_id.to_def_id()).and_then(|def_id| tcx.impl_trait_ref(def_id))
         && let Some(drop_trait) = tcx.lang_items().drop_trait()
         && drop_trait == trait_ref.instantiate_identity().def_id
+        // avoid erroneous `Drop` impls from causing ICEs below
+        && let sig = tcx.fn_sig(def_id).instantiate_identity()
+        && sig.inputs().skip_binder().len() == 1
     {
         // It was. Now figure out for what type `Drop` is implemented and then
         // check for recursion.
-        if let ty::Ref(_, dropped_ty, _) = tcx
-            .liberate_late_bound_regions(
-                def_id.to_def_id(),
-                tcx.fn_sig(def_id).instantiate_identity().input(0),
-            )
-            .kind()
+        if let ty::Ref(_, dropped_ty, _) =
+            tcx.liberate_late_bound_regions(def_id.to_def_id(), sig.input(0)).kind()
         {
             check_recursion(tcx, body, RecursiveDrop { drop_for: *dropped_ty });
         }

tests/ui/drop/recursion-check-on-erroneous-impl.rs

@@ -0,0 +1,11 @@
+// can't use build-fail, because this also fails check-fail, but
+// the ICE from #120787 only reproduces on build-fail.
+// compile-flags: --emit=mir
+
+struct PrintOnDrop<'a>(&'a str);
+
+impl Drop for PrintOnDrop<'_> {
+    fn drop() {} //~ ERROR method `drop` has a `&mut self` declaration in the trait
+}
+
+fn main() {}

tests/ui/drop/recursion-check-on-erroneous-impl.stderr

@@ -0,0 +1,11 @@
+error[E0186]: method `drop` has a `&mut self` declaration in the trait, but not in the impl
+  --> $DIR/recursion-check-on-erroneous-impl.rs:8:5
+   |
+LL |     fn drop() {}
+   |     ^^^^^^^^^ expected `&mut self` in impl
+   |
+   = note: `drop` from trait: `fn(&mut Self)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0186`.