Rollup merge of rust-lang#64500 - nnethercote:ObligForest-fixups, r=nikomatsakis

Various `ObligationForest` improvements

These commits make the code both nicer and faster.

r? @nikomatsakis

Author: Centril

src/librustc_data_structures/indexed_vec.rs

@@ -149,7 +149,7 @@ macro_rules! newtype_index {
 
             #[inline]
             $v const unsafe fn from_u32_unchecked(value: u32) -> Self {
-                unsafe { $type { private: value } }
+                $type { private: value }
             }
 
             /// Extracts the value of this index as an integer.

src/librustc_data_structures/obligation_forest/graphviz.rs

@@ -74,9 +74,9 @@ impl<'a, O: ForestObligation + 'a> dot::GraphWalk<'a> for &'a ObligationForest<O
             .flat_map(|i| {
                 let node = &self.nodes[i];
 
-                node.parent.iter().map(|p| p.get())
-                    .chain(node.dependents.iter().map(|p| p.get()))
-                    .map(move |p| (p, i))
+                node.parent.iter()
+                    .chain(node.dependents.iter())
+                    .map(move |p| (p.index(), i))
             })
             .collect()
     }