allow implicit cast from `S` to `?&const S`

Allow implicit casts from container types to nullable const pointers to
said container type.  That is:

    fn f() void {
        const s = S {};
        g(s);   // Works.
        g(&s);  // So does this.
    }

    fn g(_: ?&const S) void {  // Nullable const pointer.
    }

Fixes #731.

1 files changed, 71 insertions, 0 deletions

diff --git a/test/cases/cast.zig b/test/cases/cast.zig
index 6ffb558174..dabf97a799 100644
--- a/test/cases/cast.zig
+++ b/test/cases/cast.zig
@@ -32,6 +32,77 @@ fn funcWithConstPtrPtr(x: &const &i32) void {
     **x += 1;
 }
 
+test "implicitly cast a container to a const pointer of it" {
+    const z = Struct(void) { .x = void{} };
+    assert(0 == @sizeOf(@typeOf(z)));
+    assert(void{} == Struct(void).pointer(z).x);
+    assert(void{} == Struct(void).pointer(&z).x);
+    assert(void{} == Struct(void).maybePointer(z).x);
+    assert(void{} == Struct(void).maybePointer(&z).x);
+    assert(void{} == Struct(void).maybePointer(null).x);
+    const s = Struct(u8) { .x = 42 };
+    assert(0 != @sizeOf(@typeOf(s)));
+    assert(42 == Struct(u8).pointer(s).x);
+    assert(42 == Struct(u8).pointer(&s).x);
+    assert(42 == Struct(u8).maybePointer(s).x);
+    assert(42 == Struct(u8).maybePointer(&s).x);
+    assert(0 == Struct(u8).maybePointer(null).x);
+    const u = Union { .x = 42 };
+    assert(42 == Union.pointer(u).x);
+    assert(42 == Union.pointer(&u).x);
+    assert(42 == Union.maybePointer(u).x);
+    assert(42 == Union.maybePointer(&u).x);
+    assert(0 == Union.maybePointer(null).x);
+    const e = Enum.Some;
+    assert(Enum.Some == Enum.pointer(e));
+    assert(Enum.Some == Enum.pointer(&e));
+    assert(Enum.Some == Enum.maybePointer(e));
+    assert(Enum.Some == Enum.maybePointer(&e));
+    assert(Enum.None == Enum.maybePointer(null));
+}
+
+fn Struct(comptime T: type) type {
+    return struct {
+        const Self = this;
+        x: T,
+
+        fn pointer(self: &const Self) Self {
+            return *self;
+        }
+
+        fn maybePointer(self: ?&const Self) Self {
+            const none = Self { .x = if (T == void) void{} else 0 };
+            return *(self ?? &none);
+        }
+    };
+}
+
+const Union = union {
+    x: u8,
+
+    fn pointer(self: &const Union) Union {
+        return *self;
+    }
+
+    fn maybePointer(self: ?&const Union) Union {
+        const none = Union { .x = 0 };
+        return *(self ?? &none);
+    }
+};
+
+const Enum = enum {
+    None,
+    Some,
+
+    fn pointer(self: &const Enum) Enum {
+        return *self;
+    }
+
+    fn maybePointer(self: ?&const Enum) Enum {
+        return *(self ?? &Enum.None);
+    }
+};
+
 test "explicit cast from integer to error type" {
     testCastIntToErr(error.ItBroke);
     comptime testCastIntToErr(error.ItBroke);