std.enums: fix EnumIndexer branch quota

It's quite silly to have this override which nonetheless makes
assumptions about the input type. Encode the actual complexity of the
sort.

Also, simplify the sorting logic, and fix a bug (grab min and max
*after* the sort, not *before*!)

Author: mlugg

lib/std/enums.zig

@@ -1317,9 +1317,9 @@ test "EnumSet non-exhaustive" {
 }
 
 pub fn EnumIndexer(comptime E: type) type {
-    // Assumes that the enum fields are sorted in ascending order (optimistic).
-    // Unsorted enums may require the user to manually increase the quota.
-    @setEvalBranchQuota(3 * @typeInfo(E).@"enum".fields.len + eval_branch_quota_cushion);
+    // n log n for `std.mem.sortUnstable` call below.
+    const fields_len = @typeInfo(E).@"enum".fields.len;
+    @setEvalBranchQuota(3 * fields_len * std.math.log2(@max(fields_len, 1)) + eval_branch_quota_cushion);
 
     if (!@typeInfo(E).@"enum".is_exhaustive) {
         const BackingInt = @typeInfo(E).@"enum".tag_type;
@@ -1354,10 +1354,6 @@ pub fn EnumIndexer(comptime E: type) type {
         };
     }
 
-    const const_fields = @typeInfo(E).@"enum".fields;
-    var fields = const_fields[0..const_fields.len].*;
-    const fields_len = fields.len;
-
     if (fields_len == 0) {
         return struct {
             pub const Key = E;
@@ -1373,22 +1369,17 @@ pub fn EnumIndexer(comptime E: type) type {
         };
     }
 
-    const min = fields[0].value;
-    const max = fields[fields.len - 1].value;
-
-    const SortContext = struct {
-        fields: []EnumField,
+    var fields: [fields_len]EnumField = @typeInfo(E).@"enum".fields[0..].*;
 
-        pub fn lessThan(comptime ctx: @This(), comptime a: usize, comptime b: usize) bool {
-            return ctx.fields[a].value < ctx.fields[b].value;
+    std.mem.sortUnstable(EnumField, &fields, {}, struct {
+        fn lessThan(ctx: void, lhs: EnumField, rhs: EnumField) bool {
+            ctx;
+            return lhs.value < rhs.value;
         }
+    }.lessThan);
 
-        pub fn swap(comptime ctx: @This(), comptime a: usize, comptime b: usize) void {
-            return std.mem.swap(EnumField, &ctx.fields[a], &ctx.fields[b]);
-        }
-    };
-    std.sort.insertionContext(0, fields_len, SortContext{ .fields = &fields });
-
+    const min = fields[0].value;
+    const max = fields[fields_len - 1].value;
     if (max - min == fields.len - 1) {
         return struct {
             pub const Key = E;
@@ -1538,6 +1529,29 @@ test "EnumIndexer empty" {
     try testing.expectEqual(0, Indexer.count);
 }
 
+test "EnumIndexer large dense unsorted" {
+    @setEvalBranchQuota(500_000); // many `comptimePrint`s
+    // Make an enum with 500 fields with values in *descending* order.
+    const E = @Type(.{ .@"enum" = .{
+        .tag_type = u32,
+        .fields = comptime fields: {
+            var fields: [500]EnumField = undefined;
+            for (&fields, 0..) |*f, i| f.* = .{
+                .name = std.fmt.comptimePrint("f{d}", .{i}),
+                .value = 500 - i,
+            };
+            break :fields &fields;
+        },
+        .decls = &.{},
+        .is_exhaustive = true,
+    } });
+    const Indexer = EnumIndexer(E);
+    try testing.expectEqual(E.f0, Indexer.keyForIndex(499));
+    try testing.expectEqual(E.f499, Indexer.keyForIndex(0));
+    try testing.expectEqual(499, Indexer.indexOf(.f0));
+    try testing.expectEqual(0, Indexer.indexOf(.f499));
+}
+
 test values {
     const E = enum {
         X,