Auto merge of #18267 - lnicola:sync-from-rust, r=lnicola

minor: Sync from downstream

1 files changed, 25 insertions, 13 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index ee878e879e9..82dbf030608 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1059,7 +1059,8 @@ impl String {
     #[inline]
     #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn into_bytes(self) -> Vec<u8> {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn into_bytes(self) -> Vec<u8> {
         self.vec
     }
 
@@ -1076,8 +1077,11 @@ impl String {
     #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
     #[cfg_attr(not(test), rustc_diagnostic_item = "string_as_str")]
-    pub fn as_str(&self) -> &str {
-        self
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_str(&self) -> &str {
+        // SAFETY: String contents are stipulated to be valid UTF-8, invalid contents are an error
+        // at construction.
+        unsafe { str::from_utf8_unchecked(self.vec.as_slice()) }
     }
 
     /// Converts a `String` into a mutable string slice.
@@ -1096,8 +1100,11 @@ impl String {
     #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
     #[cfg_attr(not(test), rustc_diagnostic_item = "string_as_mut_str")]
-    pub fn as_mut_str(&mut self) -> &mut str {
-        self
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_mut_str(&mut self) -> &mut str {
+        // SAFETY: String contents are stipulated to be valid UTF-8, invalid contents are an error
+        // at construction.
+        unsafe { str::from_utf8_unchecked_mut(self.vec.as_mut_slice()) }
     }
 
     /// Appends a given string slice onto the end of this `String`.
@@ -1168,7 +1175,8 @@ impl String {
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn capacity(&self) -> usize {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn capacity(&self) -> usize {
         self.vec.capacity()
     }
 
@@ -1431,8 +1439,9 @@ impl String {
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn as_bytes(&self) -> &[u8] {
-        &self.vec
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_bytes(&self) -> &[u8] {
+        self.vec.as_slice()
     }
 
     /// Shortens this `String` to the specified length.
@@ -1784,7 +1793,8 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
         &mut self.vec
     }
 
@@ -1805,8 +1815,9 @@ impl String {
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
     #[rustc_confusables("length", "size")]
-    pub fn len(&self) -> usize {
+    pub const fn len(&self) -> usize {
         self.vec.len()
     }
 
@@ -1824,7 +1835,8 @@ impl String {
     #[inline]
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn is_empty(&self) -> bool {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
@@ -2589,7 +2601,7 @@ impl ops::Deref for String {
 
     #[inline]
     fn deref(&self) -> &str {
-        unsafe { str::from_utf8_unchecked(&self.vec) }
+        self.as_str()
     }
 }
 
@@ -2600,7 +2612,7 @@ unsafe impl ops::DerefPure for String {}
 impl ops::DerefMut for String {
     #[inline]
     fn deref_mut(&mut self) -> &mut str {
-        unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
+        self.as_mut_str()
     }
 }