From aa493d0b6020f0f166d2e3a2628761b7428a12a1 Mon Sep 17 00:00:00 2001
From: Zachary S <zasample18+github@gmail.com>
Date: Wed, 23 Oct 2024 14:22:56 -0500
Subject: [PATCH] const fn str::split_at*

---
 library/core/src/lib.rs     |  2 ++
 library/core/src/str/mod.rs | 35 +++++++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index e323e88f261..8e735bb96ee 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -196,7 +196,9 @@
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(cfg_ub_checks)]
 #![feature(const_for)]
+#![feature(const_is_char_boundary)]
 #![feature(const_precise_live_drops)]
+#![feature(const_str_split_at)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
 #![feature(doc_cfg)]
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index e5b8e015aea..f5c39af1d53 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -640,7 +640,8 @@ pub unsafe fn slice_mut_unchecked(&mut self, begin: usize, end: usize) -> &mut s
     #[inline]
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
-    pub fn split_at(&self, mid: usize) -> (&str, &str) {
+    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    pub const fn split_at(&self, mid: usize) -> (&str, &str) {
         match self.split_at_checked(mid) {
             None => slice_error_fail(self, 0, mid),
             Some(pair) => pair,
@@ -680,7 +681,8 @@ pub fn split_at(&self, mid: usize) -> (&str, &str) {
     #[inline]
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
-    pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
+    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    pub const fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
             // SAFETY: just checked that `mid` is on a char boundary.
@@ -719,11 +721,12 @@ pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
     #[inline]
     #[must_use]
     #[stable(feature = "split_at_checked", since = "1.80.0")]
-    pub fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)> {
+    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    pub const fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)> {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
             // SAFETY: just checked that `mid` is on a char boundary.
-            Some(unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) })
+            Some(unsafe { self.split_at_unchecked(mid) })
         } else {
             None
         }
@@ -759,7 +762,9 @@ pub fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)> {
     #[inline]
     #[must_use]
     #[stable(feature = "split_at_checked", since = "1.80.0")]
-    pub fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)> {
+    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    #[rustc_allow_const_fn_unstable(const_is_char_boundary)]
+    pub const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)> {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
             // SAFETY: just checked that `mid` is on a char boundary.
@@ -775,7 +780,25 @@ pub fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str
     ///
     /// The caller must ensure that `mid` is a valid byte offset from the start
     /// of the string and falls on the boundary of a UTF-8 code point.
-    unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
+    const unsafe fn split_at_unchecked(&self, mid: usize) -> (&str, &str) {
+        let len = self.len();
+        let ptr = self.as_ptr();
+        // SAFETY: caller guarantees `mid` is on a char boundary.
+        unsafe {
+            (
+                from_utf8_unchecked(slice::from_raw_parts(ptr, mid)),
+                from_utf8_unchecked(slice::from_raw_parts(ptr.add(mid), len - mid)),
+            )
+        }
+    }
+
+    /// Divides one string slice into two at an index.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `mid` is a valid byte offset from the start
+    /// of the string and falls on the boundary of a UTF-8 code point.
+    const unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
         let len = self.len();
         let ptr = self.as_mut_ptr();
         // SAFETY: caller guarantees `mid` is on a char boundary.