liballoc: introduce String, Vec const-slicing

This change `const`-qualifies many methods on Vec and String, notably
`as_slice`, `as_str`, `len`. These changes are made behind the unstable
feature flag `const_vec_string_slice` with the following tracking issue:

rust-lang#129041

Author: mammothbane

library/alloc/src/lib.rs

@@ -117,6 +117,7 @@
 #![feature(const_pin)]
 #![feature(const_refs_to_cell)]
 #![feature(const_size_of_val)]
+#![feature(const_vec_string_slice)]
 #![feature(core_intrinsics)]
 #![feature(deprecated_suggestion)]
 #![feature(deref_pure_trait)]

library/alloc/src/raw_vec.rs

@@ -293,7 +293,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
     /// be careful.
     #[inline]
-    pub fn ptr(&self) -> *mut T {
+    pub const fn ptr(&self) -> *mut T {
         self.inner.ptr()
     }
 
@@ -306,7 +306,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     ///
     /// This will always be `usize::MAX` if `T` is zero-sized.
     #[inline]
-    pub fn capacity(&self) -> usize {
+    pub const fn capacity(&self) -> usize {
         self.inner.capacity(size_of::<T>())
     }
 
@@ -501,8 +501,8 @@ impl<A: Allocator> RawVecInner<A> {
     }
 
     #[inline]
-    fn ptr<T>(&self) -> *mut T {
-        self.non_null::<T>().as_ptr()
+    const fn ptr<T>(&self) -> *mut T {
+        self.ptr.as_ptr() as _
     }
 
     #[inline]
@@ -511,7 +511,7 @@ impl<A: Allocator> RawVecInner<A> {
     }
 
     #[inline]
-    fn capacity(&self, elem_size: usize) -> usize {
+    const fn capacity(&self, elem_size: usize) -> usize {
         if elem_size == 0 { usize::MAX } else { self.cap.0 }
     }
 

library/alloc/src/string.rs

@@ -1006,7 +1006,8 @@ 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
     }
 
@@ -1022,8 +1023,9 @@ impl String {
     #[inline]
     #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
-    pub fn as_str(&self) -> &str {
-        self
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_str(&self) -> &str {
+        unsafe { str::from_utf8_unchecked(self.vec.as_slice()) }
     }
 
     /// Converts a `String` into a mutable string slice.
@@ -1042,7 +1044,7 @@ impl String {
     #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
     pub fn as_mut_str(&mut self) -> &mut str {
-        self
+        unsafe { str::from_utf8_unchecked_mut(&mut self.vec) }
     }
 
     /// Appends a given string slice onto the end of this `String`.
@@ -1112,7 +1114,8 @@ 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()
     }
 
@@ -1375,8 +1378,9 @@ 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.
@@ -1748,8 +1752,9 @@ 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()
     }
 
@@ -1767,7 +1772,8 @@ 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
     }
 
@@ -2484,7 +2490,7 @@ impl ops::Deref for String {
 
     #[inline]
     fn deref(&self) -> &str {
-        unsafe { str::from_utf8_unchecked(&self.vec) }
+        self.as_str()
     }
 }
 
@@ -2495,7 +2501,7 @@ 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()
     }
 }
 

library/alloc/src/vec/mod.rs

@@ -946,7 +946,8 @@ impl<T, A: Allocator> Vec<T, A> {
     /// ```
     #[inline]
     #[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.buf.capacity()
     }
 
@@ -1253,8 +1254,9 @@ impl<T, A: Allocator> Vec<T, A> {
     /// ```
     #[inline]
     #[stable(feature = "vec_as_slice", since = "1.7.0")]
-    pub fn as_slice(&self) -> &[T] {
-        self
+    #[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
+    pub const fn as_slice(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
     }
 
     /// Extracts a mutable slice of the entire vector.
@@ -1271,7 +1273,7 @@ impl<T, A: Allocator> Vec<T, A> {
     #[inline]
     #[stable(feature = "vec_as_slice", since = "1.7.0")]
     pub fn as_mut_slice(&mut self) -> &mut [T] {
-        self
+        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
     }
 
     /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
@@ -1326,9 +1328,10 @@ 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_ptr(&self) -> *const T {
+    pub const fn as_ptr(&self) -> *const T {
         // We shadow the slice method of the same name to avoid going through
         // `deref`, which creates an intermediate reference.
         self.buf.ptr()
@@ -2262,8 +2265,9 @@ impl<T, A: Allocator> Vec<T, A> {
     /// ```
     #[inline]
     #[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.len
     }
 
@@ -2279,7 +2283,8 @@ impl<T, A: Allocator> Vec<T, A> {
     /// assert!(!v.is_empty());
     /// ```
     #[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
     }
 
@@ -2828,15 +2833,15 @@ impl<T, A: Allocator> ops::Deref for Vec<T, A> {
 
     #[inline]
     fn deref(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
+        self.as_slice()
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
     #[inline]
     fn deref_mut(&mut self) -> &mut [T] {
-        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
+        self.as_mut_slice()
     }
 }
 

tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir

@@ -5,67 +5,48 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
     let mut _0: &[u8];
     scope 1 (inlined <Vec<u8> as Deref>::deref) {
         debug self => _1;
-        let mut _7: usize;
-        scope 2 (inlined Vec::<u8>::as_ptr) {
+        scope 2 (inlined Vec::<u8>::as_slice) {
             debug self => _1;
-            let mut _2: &alloc::raw_vec::RawVec<u8>;
-            scope 3 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
-                debug self => _2;
-                let mut _3: &alloc::raw_vec::RawVecInner;
-                scope 4 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
-                    debug self => _3;
-                    let mut _6: std::ptr::NonNull<u8>;
-                    scope 5 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
+            let mut _6: usize;
+            scope 3 (inlined Vec::<u8>::as_ptr) {
+                debug self => _1;
+                let mut _2: &alloc::raw_vec::RawVec<u8>;
+                scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
+                    debug self => _2;
+                    let mut _3: &alloc::raw_vec::RawVecInner;
+                    scope 5 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
                         debug self => _3;
                         let mut _4: std::ptr::NonNull<u8>;
-                        scope 6 (inlined Unique::<u8>::cast::<u8>) {
+                        scope 6 (inlined Unique::<u8>::as_ptr) {
                             debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
                             debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 7 (inlined NonNull::<u8>::cast::<u8>) {
+                            scope 7 (inlined NonNull::<u8>::as_ptr) {
                                 debug self => _4;
-                                scope 8 (inlined NonNull::<u8>::as_ptr) {
-                                    debug self => _4;
-                                    let mut _5: *const u8;
-                                }
-                            }
-                        }
-                        scope 9 (inlined #[track_caller] <Unique<u8> as Into<NonNull<u8>>>::into) {
-                            debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _6;
-                            debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 10 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
-                                debug ((unique: Unique<u8>).0: std::ptr::NonNull<u8>) => _6;
-                                debug ((unique: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                scope 11 (inlined Unique::<u8>::as_non_null_ptr) {
-                                    debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _6;
-                                    debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                }
+                                let mut _5: *const u8;
                             }
                         }
                     }
-                    scope 12 (inlined NonNull::<u8>::as_ptr) {
-                        debug self => _6;
-                    }
                 }
             }
-        }
-        scope 13 (inlined std::slice::from_raw_parts::<'_, u8>) {
-            debug data => _5;
-            debug len => _7;
-            let _8: *const [u8];
-            scope 14 (inlined core::ub_checks::check_language_ub) {
-                scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
-                }
-            }
-            scope 16 (inlined std::mem::size_of::<u8>) {
-            }
-            scope 17 (inlined align_of::<u8>) {
-            }
-            scope 18 (inlined slice_from_raw_parts::<u8>) {
+            scope 8 (inlined std::slice::from_raw_parts::<'_, u8>) {
                 debug data => _5;
-                debug len => _7;
-                scope 19 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
-                    debug data_pointer => _5;
-                    debug metadata => _7;
+                debug len => _6;
+                let _7: *const [u8];
+                scope 9 (inlined core::ub_checks::check_language_ub) {
+                    scope 10 (inlined core::ub_checks::check_language_ub::runtime) {
+                    }
+                }
+                scope 11 (inlined std::mem::size_of::<u8>) {
+                }
+                scope 12 (inlined align_of::<u8>) {
+                }
+                scope 13 (inlined slice_from_raw_parts::<u8>) {
+                    debug data => _5;
+                    debug len => _6;
+                    scope 14 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
+                        debug data_pointer => _5;
+                        debug metadata => _6;
+                    }
                 }
             }
         }
@@ -76,22 +57,19 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
         _2 = &((*_1).0: alloc::raw_vec::RawVec<u8>);
         StorageLive(_3);
         _3 = &(((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner);
-        StorageLive(_6);
         StorageLive(_4);
         _4 = copy (((((*_1).0: alloc::raw_vec::RawVec<u8>).0: alloc::raw_vec::RawVecInner).0: std::ptr::Unique<u8>).0: std::ptr::NonNull<u8>);
         _5 = copy (_4.0: *const u8);
-        _6 = NonNull::<u8> { pointer: copy _5 };
         StorageDead(_4);
-        StorageDead(_6);
         StorageDead(_3);
         StorageDead(_2);
+        StorageLive(_6);
+        _6 = copy ((*_1).1: usize);
         StorageLive(_7);
-        _7 = copy ((*_1).1: usize);
-        StorageLive(_8);
-        _8 = *const [u8] from (copy _5, copy _7);
-        _0 = &(*_8);
-        StorageDead(_8);
+        _7 = *const [u8] from (copy _5, copy _6);
+        _0 = &(*_7);
         StorageDead(_7);
+        StorageDead(_6);
         return;
     }
 }