Convert _d_arraycast to template

src/core/internal/string.d

@@ -15,7 +15,20 @@ nothrow:
 
 alias UnsignedStringBuf = char[20];
 
-char[] unsignedToTempString(ulong value, return scope char[] buf, uint radix = 10) @safe
+/**
+Converts an unsigned integer value to a string of characters.
+
+This implementation is a template so it can be used when compiling with -betterC.
+
+Params:
+    value = the unsigned integer value to convert
+    buf   = the pre-allocated buffer used to store the result
+    radix = the numeric base to use in the conversion (defaults to 10)
+
+Returns:
+    The unsigned integer value as a string of characters
+*/
+char[] unsignedToTempString()(ulong value, return scope char[] buf, uint radix = 10) @safe
 {
     if (radix < 2)
         // not a valid radix, just return an empty string
@@ -52,7 +65,19 @@ private struct TempStringNoAlloc
     alias get this;
 }
 
-auto unsignedToTempString(ulong value, uint radix = 10) @safe
+/**
+Converts an unsigned integer value to a string of characters.
+
+This implementation is a template so it can be used when compiling with -betterC.
+
+Params:
+    value = the unsigned integer value to convert
+    radix = the numeric base to use in the conversion (defaults to 10)
+
+Returns:
+    The unsigned integer value as a string of characters
+*/
+auto unsignedToTempString()(ulong value, uint radix = 10) @safe
 {
     TempStringNoAlloc result = void;
     result._len = unsignedToTempString(value, result._buf, radix).length & 0xff;

src/object.d

@@ -4881,3 +4881,101 @@ void __ArrayDtor(T)(T[] a)
     foreach_reverse (ref T e; a)
         e.__xdtor();
 }
+
+/**
+Used by `__ArrayCast` to emit a descriptive error message.
+
+It is a template so it can be used by `__ArrayCast` in -betterC
+builds.  It is separate from `__ArrayCast` to minimize code
+bloat.
+
+Params:
+    fromType = name of the type being cast from
+    fromSize = total size in bytes of the array being cast from
+    toType   = name of the type being cast o
+    toSize   = total size in bytes of the array being cast to
+ */
+private void onArrayCastError()(string fromType, size_t fromSize, string toType, size_t toSize) @trusted
+{
+    import core.internal.string : unsignedToTempString;
+    import core.stdc.stdlib : alloca;
+
+    const(char)[][8] msgComponents =
+    [
+        "Cannot cast `"
+        , fromType
+        , "` to `"
+        , toType
+        , "`; an array of size "
+        , unsignedToTempString(fromSize)
+        , " does not align on an array of size "
+        , unsignedToTempString(toSize)
+    ];
+
+    // convert discontiguous `msgComponents` to contiguous string on the stack
+    size_t length = 0;
+    foreach (m ; msgComponents)
+        length += m.length;
+
+    auto msg = (cast(char*)alloca(length))[0 .. length];
+
+    size_t index = 0;
+    foreach (m ; msgComponents)
+        foreach (c; m)
+            msg[index++] = c;
+
+    // first argument must evaluate to `false` at compile-time to maintain memory safety in release builds
+    assert(false, msg);
+}
+
+/**
+The compiler lowers expressions of `cast(TTo[])TFrom[]` to
+this implementation.
+
+Params:
+    from = the array to reinterpret-cast
+
+Returns:
+    `from` reinterpreted as `TTo[]`
+ */
+TTo[] __ArrayCast(TFrom, TTo)(TFrom[] from) @nogc pure @trusted
+{
+    const fromSize = from.length * TFrom.sizeof;
+    const toLength = fromSize / TTo.sizeof;
+
+    if ((fromSize % TTo.sizeof) != 0)
+    {
+        onArrayCastError(TFrom.stringof, fromSize, TTo.stringof, toLength * TTo.sizeof);
+    }
+
+    struct Array
+    {
+        size_t length;
+        void* ptr;
+    }
+    auto a = cast(Array*)&from;
+    a.length = toLength; // jam new length
+    return *cast(TTo[]*)a;
+}
+
+@safe @nogc pure nothrow unittest
+{
+    byte[int.sizeof * 3] b = cast(byte) 0xab;
+    int[] i;
+    short[] s;
+
+    i = __ArrayCast!(byte, int)(b);
+    assert(i.length == 3);
+    foreach (v; i)
+        assert(v == cast(int) 0xabab_abab);
+
+    s = __ArrayCast!(byte, short)(b);
+    assert(s.length == 6);
+    foreach (v; s)
+        assert(v == cast(short) 0xabab);
+
+    s = __ArrayCast!(int, short)(i);
+    assert(s.length == 6);
+    foreach (v; s)
+        assert(v == cast(short) 0xabab);
+}