infer ElementType for ranges in makeArray and makeSlice

Author: wilzbach

std/experimental/allocator/package.d

@@ -737,13 +737,21 @@ unittest
     test(theAllocator);
 }
 
+/// Ditto
+Unqual!(ElementEncodingType!R)[] makeArray(Allocator, R)(auto ref Allocator alloc, R range)
+if (isInputRange!R && !isInfinite!R)
+{
+    alias T = Unqual!(ElementEncodingType!R);
+    return makeArray!(T, Allocator, R)(alloc, range);
+}
+
 /// Ditto
 T[] makeArray(T, Allocator, R)(auto ref Allocator alloc, R range)
 if (isInputRange!R && !isInfinite!R)
 {
     static if (isForwardRange!R || hasLength!R)
     {
-        static if (hasLength!R)
+        static if (hasLength!R || isNarrowString!R)
             immutable length = range.length;
         else
             immutable length = range.save.walkLength;
@@ -763,10 +771,18 @@ if (isInputRange!R && !isInfinite!R)
             alloc.deallocate(m);
         }
 
-        for (; !range.empty; range.popFront, ++i)
+        import std.conv : emplace;
+        static if (isNarrowString!R || isRandomAccessRange!R)
         {
-            import std.conv : emplace;
-            cast(void) emplace!T(result.ptr + i, range.front);
+            foreach (j; 0 .. range.length)
+            {
+                cast(void) emplace!T(result.ptr + i++, range[j]);
+            }
+        }
+        else
+        {
+            for (; !range.empty; range.popFront, ++i)
+                cast(void) emplace!T(result.ptr + i, range.front);
         }
 
         return result;
@@ -826,12 +842,40 @@ unittest
         a = alloc.makeArray!long([5, 42]);
         assert(a.length == 2);
         assert(a == [ 5, 42]);
+
+        // we can also infer the type
+        auto b = alloc.makeArray([4.0, 2.0]);
+        static assert(is(typeof(b) == double[]));
+        assert(b == [4.0, 2.0]);
     }
     import std.experimental.allocator.gc_allocator : GCAllocator;
     test(GCAllocator.instance);
     test(theAllocator);
 }
 
+// infer types for strings
+unittest
+{
+    void test(A)(auto ref A alloc)
+    {
+        auto c = alloc.makeArray("fooπ😜");
+        static assert(is(typeof(c) == char[]));
+        assert(c == "fooπ😜");
+
+        auto d = alloc.makeArray("fooπ😜"d);
+        static assert(is(typeof(d) == dchar[]));
+        assert(d == "fooπ😜");
+
+        auto w = alloc.makeArray("fooπ😜"w);
+        static assert(is(typeof(w) == wchar[]));
+        assert(w == "fooπ😜");
+    }
+
+    import std.experimental.allocator.gc_allocator : GCAllocator;
+    test(GCAllocator.instance);
+    test(theAllocator);
+}
+
 version(unittest)
 {
     private struct ForcedInputRange

std/experimental/ndslice/slice.d

@@ -642,6 +642,16 @@ auto makeSlice(T,
     return .makeSlice!(T, replaceArrayWithPointer, Allocator)(alloc, [lengths]);
 }
 
+/// ditto
+auto makeSlice(
+    Flag!`replaceArrayWithPointer` replaceArrayWithPointer = Yes.replaceArrayWithPointer,
+    Allocator,
+    size_t N, Range)(auto ref Allocator alloc, auto ref Slice!(N, Range) slice)
+{
+    alias T = Unqual!(slice.DeepElemType);
+    return makeSlice!(T, replaceArrayWithPointer)(alloc, slice);
+}
+
 /// ditto
 auto makeSlice(T,
     Flag!`replaceArrayWithPointer` replaceArrayWithPointer = Yes.replaceArrayWithPointer,
@@ -701,7 +711,7 @@ auto makeSlice(T,
 
     // makes duplicate using `makeSlice`
     tup.slice[0, 0, 0] = 3;
-    auto dup = makeSlice!int(Mallocator.instance, tup.slice);
+    auto dup = makeSlice(Mallocator.instance, tup.slice);
     assert(dup.slice == tup.slice);
 
     Mallocator.instance.dispose(tup.array);
@@ -714,12 +724,25 @@ auto makeSlice(T,
     import std.experimental.allocator;
     import std.experimental.allocator.mallocator;
 
-    auto tup = makeSlice!int(Mallocator.instance, [2, 3, 4], 10);
+    auto tup = makeSlice(Mallocator.instance, [2, 3, 4], 10);
     auto slice = tup.slice;
     assert(slice[1, 1, 1] == 10);
     Mallocator.instance.dispose(tup.array);
 }
 
+
+@nogc unittest
+{
+    import std.experimental.allocator;
+    import std.experimental.allocator.mallocator;
+
+    // cast to your own type
+    auto tup = makeSlice!double(Mallocator.instance, [2, 3, 4], 10);
+    auto slice = tup.slice;
+    assert(slice[1, 1, 1] == 10.0);
+    Mallocator.instance.dispose(tup.array);
+}
+
 /++
 Creates a common n-dimensional array from a slice.
 Params: