Make Vec::new a const fn

src/liballoc/raw_vec.rs

@@ -54,6 +54,7 @@ pub struct RawVec<T, A: Alloc = Global> {
 }
 
 impl<T, A: Alloc> RawVec<T, A> {
+    // FIXME: this should be made `const` when `if` statements are allowed
     /// Like `new` but parameterized over the choice of allocator for
     /// the returned RawVec.
     pub fn new_in(a: A) -> Self {
@@ -68,6 +69,17 @@ impl<T, A: Alloc> RawVec<T, A> {
         }
     }
 
+    // FIXME: this should removed when `new_in` can be made `const`
+    /// Like `empty` but parametrized over the choice of allocator for the returned `RawVec`.
+    pub const fn empty_in(a: A) -> Self {
+        // Unique::empty() doubles as "unallocated" and "zero-sized allocation"
+        RawVec {
+            ptr: Unique::empty(),
+            cap: 0,
+            a,
+        }
+    }
+
     /// Like `with_capacity` but parameterized over the choice of
     /// allocator for the returned RawVec.
     #[inline]
@@ -124,6 +136,13 @@ impl<T> RawVec<T, Global> {
         Self::new_in(Global)
     }
 
+    // FIXME: this should removed when `new` can be made `const`
+    /// Create a `RawVec` with capcity 0 (on the system heap), regardless of `T`, without
+    /// allocating.
+    pub const fn empty() -> Self {
+        Self::empty_in(Global)
+    }
+
     /// Creates a RawVec (on the system heap) with exactly the
     /// capacity and alignment requirements for a `[T; cap]`. This is
     /// equivalent to calling RawVec::new when `cap` is 0 or T is

src/liballoc/vec.rs

@@ -324,7 +324,7 @@ impl<T> Vec<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> Vec<T> {
         Vec {
-            buf: RawVec::new(),
+            buf: RawVec::empty(),
             len: 0,
         }
     }

src/libcore/ptr.rs

@@ -2551,10 +2551,9 @@ 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(ptr)
+            Unique::new_unchecked(mem::align_of::<T>() as *mut T)
         }
     }
 }

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

@@ -0,0 +1,15 @@
+// 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
+
+const MY_VEC: Vec<usize> = Vec::new();
+
+pub fn main() {}