Auto merge of #52546 - nikomatsakis:issue-52050, r=pnkfelix

do not overwrite child def-id in place but rather remove/insert

When inserting a node N into the tree of impls, we sometimes find than an existing node C should be replaced with N. We used to overwrite C in place with the new def-id N -- but since the lists of def-ids are separated by simplified type, that could lead to N being inserted in the wrong place. This meant we might miss conflicts. We are now not trying to be so smart -- we remove C and then add N later.

Fixes #52050

r? @aturon -- do you still remember this code at all? :)

src/test/compile-fail/specialization/issue-52050.rs

@@ -0,0 +1,42 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(specialization)]
+
+// Regression test for #52050: when inserting the blanket impl `I`
+// into the tree, we had to replace the child node for `Foo`, which
+// led to the struture of the tree being messed up.
+
+use std::iter::Iterator;
+
+trait IntoPyDictPointer { }
+
+struct Foo { }
+
+impl Iterator for Foo {
+    type Item = ();
+    fn next(&mut self) -> Option<()> {
+        None
+    }
+}
+
+impl IntoPyDictPointer for Foo { }
+
+impl<I> IntoPyDictPointer for I
+where
+    I: Iterator,
+{
+}
+
+impl IntoPyDictPointer for () //~ ERROR conflicting implementations
+{
+}
+
+fn main() { }