From 2b52d65c7719b058bed004e666b010585e01a1c1 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Tue, 10 Jun 2025 21:09:35 +0000
Subject: [PATCH] Don't fold in Instantiate when there's nothing to fold

---
 compiler/rustc_type_ir/src/binder.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/compiler/rustc_type_ir/src/binder.rs b/compiler/rustc_type_ir/src/binder.rs
index 55c0a3bba9f2b..b744bcc40dbdf 100644
--- a/compiler/rustc_type_ir/src/binder.rs
+++ b/compiler/rustc_type_ir/src/binder.rs
@@ -622,6 +622,17 @@ impl<I: Interner, T: TypeFoldable<I>> ty::EarlyBinder<I, T> {
     where
         A: SliceLike<Item = I::GenericArg>,
     {
+        // Nothing to fold, so let's avoid visiting things and possibly re-hashing/equating
+        // them when interning. Perf testing found this to be a modest improvement.
+        // See: <https://github.com/rust-lang/rust/pull/142317>
+        if args.is_empty() {
+            debug_assert!(
+                !self.value.has_param(),
+                "{:?} has parameters, but no args were provided in instantiate",
+                self.value,
+            );
+            return self.value;
+        }
         let mut folder = ArgFolder { cx, args: args.as_slice(), binders_passed: 0 };
         self.value.fold_with(&mut folder)
     }