Rollup merge of rust-lang#64028 - Centril:stabilize-alloc-new-2, r=alexcrichton

Stabilize `Vec::new` and `String::new` as `const fn`s

Closes rust-lang#64022.

r? @oli-obk

Author: Centril

src/liballoc/lib.rs

@@ -117,7 +117,7 @@
 #![feature(allocator_internals)]
 #![feature(on_unimplemented)]
 #![feature(rustc_const_unstable)]
-#![feature(const_vec_new)]
+#![cfg_attr(bootstrap, feature(const_vec_new))]
 #![feature(slice_partition_dedup)]
 #![feature(maybe_uninit_extra, maybe_uninit_slice)]
 #![feature(alloc_layout_extra)]

src/liballoc/raw_vec.rs

@@ -113,13 +113,38 @@ impl<T, A: Alloc> RawVec<T, A> {
 }
 
 impl<T> RawVec<T, Global> {
+    /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
+    /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
+    ///
+    /// If you change `RawVec<T>::new` or dependencies, please take care to not
+    /// introduce anything that would truly violate `min_const_fn`.
+    ///
+    /// NOTE: We could avoid this hack and check conformance with some
+    /// `#[rustc_force_min_const_fn]` attribute which requires conformance
+    /// with `min_const_fn` but does not necessarily allow calling it in
+    /// `stable(...) const fn` / user code not enabling `foo` when
+    /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
+    pub const NEW: Self = Self::new();
+
     /// Creates the biggest possible `RawVec` (on the system heap)
     /// without allocating. If `T` has positive size, then this makes a
     /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
     /// `RawVec` with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
     pub const fn new() -> Self {
-        Self::new_in(Global)
+        // FIXME(Centril): Reintegrate this with `fn new_in` when we can.
+
+        // `!0` is `usize::MAX`. This branch should be stripped at compile time.
+        // FIXME(mark-i-m): use this line when `if`s are allowed in `const`:
+        //let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
+
+        // `Unique::empty()` doubles as "unallocated" and "zero-sized allocation".
+        RawVec {
+            ptr: Unique::empty(),
+            // FIXME(mark-i-m): use `cap` when ifs are allowed in const
+            cap: [0, !0][(mem::size_of::<T>() == 0) as usize],
+            a: Global,
+        }
     }
 
     /// Creates a `RawVec` (on the system heap) with exactly the

src/liballoc/string.rs

@@ -369,7 +369,7 @@ impl String {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_string_new")]
+    #[cfg_attr(bootstrap, rustc_const_unstable(feature = "const_string_new"))]
     pub const fn new() -> String {
         String { vec: Vec::new() }
     }

src/liballoc/vec.rs

@@ -314,10 +314,10 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_vec_new")]
+    #[cfg_attr(bootstrap, rustc_const_unstable(feature = "const_vec_new"))]
     pub const fn new() -> Vec<T> {
         Vec {
-            buf: RawVec::new(),
+            buf: RawVec::NEW,
             len: 0,
         }
     }

src/test/ui/collections-const-new.rs

@@ -1,15 +1,11 @@
-// run-pass
+// check-pass
 
-#![allow(dead_code)]
 // Test several functions can be used for constants
 // 1. Vec::new()
 // 2. String::new()
 
-#![feature(const_vec_new)]
-#![feature(const_string_new)]
-
 const MY_VEC: Vec<usize> = Vec::new();
 
 const MY_STRING: String = String::new();
 
-pub fn main() {}
+fn main() {}

src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs

@@ -14,8 +14,9 @@ trait Bar<T, U: Foo<T>> {
 impl Foo<u32> for () {
     const X: u32 = 42;
 }
+
 impl Foo<Vec<u32>> for String {
-    const X: Vec<u32> = Vec::new(); //~ ERROR not yet stable as a const fn
+    const X: Vec<u32> = Vec::new();
 }
 
 impl Bar<u32, ()> for () {}

src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr

@@ -4,13 +4,5 @@ error[E0493]: destructors cannot be evaluated at compile-time
 LL |     const F: u32 = (U::X, 42).1;
    |                    ^^^^^^^^^^ constants cannot evaluate destructors
 
-error: `std::vec::Vec::<T>::new` is not yet stable as a const fn
-  --> $DIR/feature-gate-unleash_the_miri_inside_of_you.rs:18:25
-   |
-LL |     const X: Vec<u32> = Vec::new();
-   |                         ^^^^^^^^^^
-   |
-   = help: add `#![feature(const_vec_new)]` to the crate attributes to enable
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error