turn the nested if-let expressions into an if-let-chain

Author: alpaylan

library/alloc/src/str.rs

@@ -348,10 +348,10 @@ impl str {
             Some(Utf8Pattern::CharPattern(c)) => c.as_ascii().map(|ascii_char| ascii_char.to_u8()),
             _ => None,
         } {
-            if let Some(sref) = from_byte.as_ascii().map(|ascii_char| f(ascii_char.as_str())) {
-                if let [to_byte] = sref.as_ref().as_bytes() {
-                    return unsafe { replace_ascii(self.as_bytes(), from_byte, *to_byte) };
-                }
+            if let Some(sref) = from_byte.as_ascii().map(|ascii_char| f(ascii_char.as_str()))
+                && let [to_byte] = sref.as_ref().as_bytes()
+            {
+                return unsafe { replace_ascii(self.as_bytes(), from_byte, *to_byte) };
             }
         }