Rollup merge of #66755 - mark-i-m:const-vec-new, r=ecstatic-morse

Remove a const-if-hack in RawVec

r? @ecstatic-morse

cc @Centril

src/liballoc/lib.rs

@@ -85,6 +85,7 @@
 #![feature(const_generic_impls_guard)]
 #![feature(const_generics)]
 #![feature(const_in_array_repeat_expressions)]
+#![feature(const_if_match)]
 #![feature(cow_is_borrowed)]
 #![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]

src/liballoc/raw_vec.rs

@@ -52,15 +52,12 @@ impl<T, A: Alloc> RawVec<T, A> {
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
     pub const fn new_in(a: A) -> Self {
-        // `!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 };
+        let cap = if mem::size_of::<T>() == 0 { core::usize::MAX } 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],
+            cap,
             a,
         }
     }
@@ -132,19 +129,7 @@ impl<T> RawVec<T, Global> {
     /// `RawVec` with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
     pub const fn new() -> Self {
-        // 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,
-        }
+        Self::new_in(Global)
     }
 
     /// Creates a `RawVec` (on the system heap) with exactly the