Remove rayon dependency of cranelift-isle (#5101)

Using rayon adds a lot of dependencies to Cranelift. The total
unparallelized time the code that uses rayon takes is less than half a
second and it runs at compile time, so there is pretty much no benefit
to parallelizing it.

Cargo.lock

@@ -647,7 +647,6 @@ version = "0.90.0"
 dependencies = [
  "log",
  "miette",
- "rayon",
  "tempfile",
 ]
 

cranelift/isle/isle/Cargo.toml

@@ -11,7 +11,6 @@ version = "0.90.0"
 [dependencies]
 log = { workspace = true, optional = true }
 miette = { version = "5.1.0", optional = true }
-rayon = "^1.5"
 
 [dev-dependencies]
 tempfile = "3"

cranelift/isle/isle/src/overlap.rs

@@ -1,6 +1,5 @@
 //! Overlap detection for rules in ISLE.
 
-use rayon::prelude::*;
 use std::collections::hash_map::Entry;
 use std::collections::{HashMap, HashSet};
 
@@ -30,17 +29,6 @@ struct Errors {
 }
 
 impl Errors {
-    /// Merge together two Error graphs.
-    fn union(mut self, other: Self) -> Self {
-        for (id, edges) in other.nodes {
-            match self.nodes.entry(id) {
-                Entry::Occupied(entry) => entry.into_mut().extend(edges),
-                Entry::Vacant(entry) => _ = entry.insert(edges),
-            }
-        }
-        self
-    }
-
     /// Condense the overlap information down into individual errors. We iteratively remove the
     /// nodes from the graph with the highest degree, reporting errors for them and their direct
     /// connections. The goal with reporting errors this way is to prefer reporting rules that
@@ -145,11 +133,9 @@ fn check_overlaps(env: &TermEnv) -> Errors {
         }
     }
 
-    // Process rule pairs in parallel. Rayon makes this easy and we have independent bite-sized
-    // chunks of work, so we might as well take advantage of multiple CPUs if they're available.
     pairs
-        .into_par_iter()
-        .fold(Errors::default, |mut errs, (left, right)| {
+        .into_iter()
+        .fold(Errors::default(), |mut errs, (left, right)| {
             if left.rule.prio == right.rule.prio {
                 if check_overlap_pair(&left.pats, &right.pats) {
                     errs.add_edge(left.rule.id, right.rule.id);
@@ -157,7 +143,6 @@ fn check_overlaps(env: &TermEnv) -> Errors {
             }
             errs
         })
-        .reduce(Errors::default, Errors::union)
 }
 
 /// Check if two rules overlap in the inputs they accept.