liballoc/{Vec,String}: constify as_mut_* methods

Author: mammothbane

library/alloc/src/lib.rs

@@ -114,6 +114,8 @@
 #![feature(const_option)]
 #![feature(const_pin)]
 #![feature(const_size_of_val)]
+#![feature(const_slice_from_raw_parts_mut)]
+#![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_vec_string_slice)]
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]

library/alloc/src/string.rs

@@ -1098,8 +1098,9 @@ 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 {
-        unsafe { str::from_utf8_unchecked_mut(&mut self.vec) }
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_mut_str(&mut self) -> &mut str {
+        unsafe { str::from_utf8_unchecked_mut(self.vec.as_mut_slice()) }
     }
 
     /// Appends a given string slice onto the end of this `String`.
@@ -1788,7 +1789,8 @@ 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
     }
 

library/alloc/src/vec/mod.rs

@@ -1568,7 +1568,8 @@ impl<T, A: Allocator> Vec<T, A> {
     #[inline]
     #[stable(feature = "vec_as_slice", since = "1.7.0")]
     #[cfg_attr(not(test), rustc_diagnostic_item = "vec_as_mut_slice")]
-    pub fn as_mut_slice(&mut self) -> &mut [T] {
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_mut_slice(&mut self) -> &mut [T] {
         unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
     }
 
@@ -1684,9 +1685,10 @@ impl<T, A: Allocator> Vec<T, A> {
     /// [`as_mut_ptr`]: Vec::as_mut_ptr
     /// [`as_ptr`]: Vec::as_ptr
     #[stable(feature = "vec_as_ptr", since = "1.37.0")]
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
     #[rustc_never_returns_null_ptr]
     #[inline]
-    pub fn as_mut_ptr(&mut self) -> *mut T {
+    pub const fn as_mut_ptr(&mut self) -> *mut T {
         // We shadow the slice method of the same name to avoid going through
         // `deref_mut`, which creates an intermediate reference.
         self.buf.ptr()