Rollup merge of #50233 - mark-i-m:const_vec, r=kennytm

Make `Vec::new` a `const fn`

`RawVec::empty/_in` are a hack. They're there because `if size_of::<T> == 0 { !0 } else { 0 }` is not allowed in `const` yet. However, because `RawVec` is unstable, the `empty/empty_in` constructors can be removed when #49146 is done...

src/liballoc/lib.rs

@@ -124,6 +124,7 @@
 #![feature(pointer_methods)]
 #![feature(inclusive_range_fields)]
 #![cfg_attr(stage0, feature(generic_param_attrs))]
+#![feature(rustc_const_unstable)]
 
 #![cfg_attr(not(test), feature(fn_traits, i128))]
 #![cfg_attr(test, feature(test))]

src/liballoc/raw_vec.rs

@@ -56,14 +56,16 @@ pub struct RawVec<T, A: Alloc = Global> {
 impl<T, A: Alloc> RawVec<T, A> {
     /// Like `new` but parameterized over the choice of allocator for
     /// the returned RawVec.
-    pub fn new_in(a: A) -> Self {
+    pub const fn new_in(a: A) -> Self {
         // !0 is usize::MAX. This branch should be stripped at compile time.
-        let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
+        // 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(),
-            cap,
+            // FIXME(mark-i-m): use `cap` when ifs are allowed in const
+            cap: [0, !0][(mem::size_of::<T>() == 0) as usize],
             a,
         }
     }
@@ -120,7 +122,7 @@ impl<T> RawVec<T, Global> {
     /// RawVec with capacity 0. If T has 0 size, then it makes a
     /// RawVec with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         Self::new_in(Global)
     }
 

src/liballoc/vec.rs

@@ -322,7 +322,8 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new() -> Vec<T> {
+    #[rustc_const_unstable(feature = "const_vec_new")]
+    pub const fn new() -> Vec<T> {
         Vec {
             buf: RawVec::new(),
             len: 0,

src/libcore/ptr.rs

@@ -2552,10 +2552,9 @@ impl<T: Sized> Unique<T> {
     /// This is useful for initializing types which lazily allocate, like
     /// `Vec::new` does.
     // FIXME: rename to dangling() to match NonNull?
-    pub fn empty() -> Self {
+    pub const fn empty() -> Self {
         unsafe {
-            let ptr = mem::align_of::<T>() as *mut T;
+            Unique::new_unchecked(mem::align_of::<T>() as *mut T)
-            Unique::new_unchecked(ptr)
         }
     }
 }

src/test/run-pass/vec-const-new.rs

@@ -0,0 +1,17 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that Vec::new() can be used for constants
+
+#![feature(const_vec_new)]
+
+const MY_VEC: Vec<usize> = Vec::new();
+
+pub fn main() {}