Auto merge of #103214 - Nilstrieb:set-theory, r=petrochenkov

Use Set instead of Vec in transitive_relation

Helps with #103195. It doesn't fix the underlying quadraticness but it makes it _a lot_ faster to an extent where even doubling the amount of nested references still takes less than two seconds (50s on nightly).

I want to see whether this causes regressions (because the vec was usually quite small) or improvements (as lookup for bigger sets is now much faster) in real code.

compiler/rustc_data_structures/src/transitive_relation.rs

 use crate::frozen::Frozen;
-use crate::fx::FxIndexSet;
+use crate::fx::{FxHashSet, FxIndexSet};
 use rustc_index::bit_set::BitMatrix;
 use std::fmt::Debug;
 use std::hash::Hash;
@@ -16,7 +16,7 @@ pub struct TransitiveRelationBuilder<T> {
 
     // List of base edges in the graph. Require to compute transitive
     // closure.
-    edges: Vec<Edge>,
+    edges: FxHashSet<Edge>,
 }
 
 #[derive(Debug)]
@@ -52,10 +52,10 @@ fn default() -> Self {
     }
 }
 
-#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Debug, Hash)]
 struct Index(usize);
 
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 struct Edge {
     source: Index,
     target: Index,
@@ -99,9 +99,7 @@ pub fn add(&mut self, a: T, b: T) {
         let a = self.add_index(a);
         let b = self.add_index(b);
         let edge = Edge { source: a, target: b };
-        if !self.edges.contains(&edge) {
-            self.edges.push(edge);
-        }
+        self.edges.insert(edge);
     }
 
     /// Compute the transitive closure derived from the edges, and converted to