Rollup merge of #70535 - jonas-schievink:graph-refactor, r=nikomatsakis

Track the finalizing node in the specialization graph

Fixes https://github.com/rust-lang/rust/issues/70419
Fixes https://github.com/rust-lang/rust/issues/70442

r? @eddyb

src/test/ui/specialization/issue-70442.rs

@@ -0,0 +1,23 @@
+#![feature(specialization)]
+
+// check-pass
+
+trait Trait {
+    type Assoc;
+}
+
+impl<T> Trait for T {
+    default type Assoc = bool;
+}
+
+// This impl inherits the `Assoc` definition from above and "locks it in", or finalizes it, making
+// child impls unable to further specialize it. However, since the specialization graph didn't
+// correctly track this, we would refuse to project `Assoc` from this impl, even though that should
+// happen for items that are final.
+impl Trait for () {}
+
+fn foo<X: Trait<Assoc=bool>>() {}
+
+fn main() {
+    foo::<()>();  // `<() as Trait>::Assoc` is normalized to `bool` correctly
+}