From a267e0905e8ed844ecad8e40d804adf662a2b11c Mon Sep 17 00:00:00 2001
From: Alex Butler <alexheretic@gmail.com>
Date: Sun, 14 Sep 2025 00:36:31 +0100
Subject: [PATCH] Optimise to_ascii_{upper,lower}case_smolstr

---
 lib/smol_str/src/lib.rs | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/lib/smol_str/src/lib.rs b/lib/smol_str/src/lib.rs
index d76f029dbe..ff25651f54 100644
--- a/lib/smol_str/src/lib.rs
+++ b/lib/smol_str/src/lib.rs
@@ -644,12 +644,36 @@ impl StrExt for str {
 
     #[inline]
     fn to_ascii_lowercase_smolstr(&self) -> SmolStr {
-        from_char_iter(self.chars().map(|c| c.to_ascii_lowercase()))
+        let len = self.len();
+        if len <= INLINE_CAP {
+            let mut buf = [0u8; INLINE_CAP];
+            buf[..len].copy_from_slice(self.as_bytes());
+            buf[..len].make_ascii_lowercase();
+            SmolStr(Repr::Inline {
+                // SAFETY: `len` is in bounds
+                len: unsafe { InlineSize::transmute_from_u8(len as u8) },
+                buf,
+            })
+        } else {
+            self.to_ascii_lowercase().into()
+        }
     }
 
     #[inline]
     fn to_ascii_uppercase_smolstr(&self) -> SmolStr {
-        from_char_iter(self.chars().map(|c| c.to_ascii_uppercase()))
+        let len = self.len();
+        if len <= INLINE_CAP {
+            let mut buf = [0u8; INLINE_CAP];
+            buf[..len].copy_from_slice(self.as_bytes());
+            buf[..len].make_ascii_uppercase();
+            SmolStr(Repr::Inline {
+                // SAFETY: `len` is in bounds
+                len: unsafe { InlineSize::transmute_from_u8(len as u8) },
+                buf,
+            })
+        } else {
+            self.to_ascii_uppercase().into()
+        }
     }
 
     #[inline]