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:

#129041

Author: mammothbane

library/alloc/src/lib.rs

@@ -114,6 +114,7 @@
 #![feature(const_option)]
 #![feature(const_pin)]
 #![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

@@ -280,7 +280,7 @@ 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()
     }
 
@@ -293,7 +293,7 @@ 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>())
     }
 
@@ -488,8 +488,8 @@ 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]
@@ -498,7 +498,7 @@ 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

@@ -1057,7 +1057,8 @@ 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
     }
 
@@ -1073,8 +1074,9 @@ 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.
@@ -1093,7 +1095,7 @@ 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`.
@@ -1163,7 +1165,8 @@ 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()
     }
 
@@ -1426,8 +1429,9 @@ 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.
@@ -1799,8 +1803,9 @@ 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()
     }
 
@@ -1818,7 +1823,8 @@ 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
     }
 
@@ -2559,7 +2565,7 @@ impl ops::Deref for String {
 
     #[inline]
     fn deref(&self) -> &str {
-        unsafe { str::from_utf8_unchecked(&self.vec) }
+        self.as_str()
     }
 }
 
@@ -2570,7 +2576,7 @@ 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

@@ -1238,7 +1238,8 @@ 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()
     }
 
@@ -1545,8 +1546,9 @@ 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.
@@ -1563,7 +1565,7 @@ 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
@@ -1618,9 +1620,10 @@ 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()
@@ -2556,8 +2559,9 @@ 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
     }
 
@@ -2573,7 +2577,8 @@ 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
     }
 
@@ -3123,15 +3128,15 @@ 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/ui/consts/issue-94675.stderr

@@ -1,11 +1,3 @@
-error[E0015]: cannot call non-const fn `Vec::<u32>::len` in constant functions
-  --> $DIR/issue-94675.rs:11:27
-   |
-LL |         self.bar[0] = baz.len();
-   |                           ^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-
 error[E0015]: cannot call non-const operator in constant functions
   --> $DIR/issue-94675.rs:11:17
    |
@@ -20,6 +12,6 @@ help: add `#![feature(effects)]` to the crate attributes to enable
 LL + #![feature(effects)]
    |
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0015`.