Add std.typecons.Rebindable!struct

std/typecons.d

@@ -1666,11 +1666,11 @@ Convenience function for creating a $(D Rebindable) using automatic type
 inference.
 
 Params:
-    obj = A reference to an object, interface, associative array, or an array slice
-          to initialize the `Rebindable` with.
+    obj = A reference to an object, interface, associative array, or an array slice.
+    s = Struct instance.
 
 Returns:
-    A newly constructed `Rebindable` initialized with the given reference.
+    A newly constructed `Rebindable` initialized with the given data.
 */
 Rebindable!T rebindable(T)(T obj)
     if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)
@@ -1680,6 +1680,17 @@ Rebindable!T rebindable(T)(T obj)
     return ret;
 }
 
+/// ditto
+Rebindable!S rebindable(S)(S s)
+if (is(S == struct) && !isInstanceOf!(Rebindable, S))
+{
+    // workaround for rebindableS = rebindable(s)
+    static if (isMutable!S)
+        return s;
+    else
+        return Rebindable!S(s);
+}
+
 /**
 This function simply returns the $(D Rebindable) object passed in.  It's useful
 in generic programming cases when a given object may be either a regular
@@ -1787,6 +1798,220 @@ unittest
     assert(rebindable(pr3341_aa)[321] == 543);
 }
 
+/** Models safe reassignment of otherwise constant struct instances.
+ *
+ * A struct with a field of reference type cannot be assigned to a constant
+ * struct of the same type. `Rebindable!(const S)` allows assignment to
+ * a `const S` while enforcing only constant access to its fields.
+ *
+ * `Rebindable!(immutable S)` does the same but field access may create a
+ * temporary copy of `S` in order to enforce _true immutability.
+ */
+template Rebindable(S)
+if (is(S == struct))
+{
+    static if (isMutable!S)
+        alias Rebindable = S;
+    else
+    struct Rebindable
+    {
+    private:
+        // mutPayload's pointers must be treated as tail const
+        void[S.sizeof] mutPayload;
+
+        void emplace(ref S s)
+        {
+            import std.conv : emplace;
+            static if (__traits(compiles, () @safe {S tmp = s;}))
+                () @trusted {emplace!S(mutPayload, s);}();
+            else
+                emplace!S(mutPayload, s);
+        }
+
+    public:
+        this()(auto ref S s)
+        {
+            emplace(s);
+        }
+
+        // immutable S cannot be passed to auto ref S above
+        static if (!is(S == immutable))
+        this()(immutable S s)
+        {
+            emplace(s);
+        }
+
+        void opAssign()(auto ref S s)
+        {
+            movePayload;
+            emplace(s);
+        }
+
+        static if (!is(S == immutable))
+        void opAssign()(immutable S s)
+        {
+            movePayload;
+            emplace(s);
+        }
+
+        void opAssign(Rebindable other)
+        {
+            this = other.trustedPayload;
+        }
+
+        // must not escape when S is immutable
+        private
+        ref trustedPayload() @trusted
+        {
+            return *cast(S*)mutPayload.ptr;
+        }
+
+        static if (!is(S == immutable))
+        ref S Rebindable_getRef() @property
+        {
+            // payload exposed as const ref when S is const
+            return trustedPayload;
+        }
+
+        static if (is(S == immutable))
+        S Rebindable_get() @property
+        {
+            // we return a copy for immutable S
+            return trustedPayload;
+        }
+
+        static if (is(S == immutable))
+            alias Rebindable_get this;
+        else
+            alias Rebindable_getRef this;
+
+        private
+        auto movePayload() @trusted
+        {
+            import std.algorithm : move;
+            return cast(S)move(*cast(Unqual!S*)mutPayload.ptr);
+        }
+
+        ~this()
+        {
+            // call destructor with proper constness
+            movePayload;
+        }
+    }
+}
+
+///
+@safe unittest
+{
+    static struct S
+    {
+        int* ptr;
+    }
+    S s = S(new int);
+
+    const cs = s;
+    // Can't assign s.ptr to cs.ptr
+    static assert(!__traits(compiles, {s = cs;}));
+
+    Rebindable!(const S) rs = s;
+    assert(rs.ptr is s.ptr);
+    // rs.ptr is const
+    static assert(!__traits(compiles, {rs.ptr = null;}));
+
+    // Can't assign s.ptr to rs.ptr
+    static assert(!__traits(compiles, {s = rs;}));
+
+    const S cs2 = rs;
+    // Rebind rs
+    rs = cs2;
+    rs = S();
+    assert(rs.ptr is null);
+}
+
+// Test Rebindable!immutable
+@safe unittest
+{
+    static struct S
+    {
+        int* ptr;
+    }
+    S s = S(new int);
+
+    Rebindable!(immutable S) ri = S(new int);
+    assert(ri.ptr !is null);
+    static assert(!__traits(compiles, {ri.ptr = null;}));
+
+    // ri is not compatible with mutable S
+    static assert(!__traits(compiles, {s = ri;}));
+    static assert(!__traits(compiles, {ri = s;}));
+
+    auto ri2 = ri;
+    assert(ri2.ptr == ri.ptr);
+
+    const S cs3 = ri;
+    static assert(!__traits(compiles, {ri = cs3;}));
+
+    immutable S si = ri;
+    // Rebind ri
+    ri = si;
+    ri = S();
+    assert(ri.ptr is null);
+
+    // Test RB!immutable -> RB!const
+    Rebindable!(const S) rc = ri;
+    assert(rc.ptr is null);
+    ri = S(new int);
+    rc = ri;
+    assert(rc.ptr !is null);
+
+    // test rebindable, opAssign
+    rc.destroy;
+    assert(rc.ptr is null);
+    rc = rebindable(cs3);
+    rc = rebindable(si);
+    assert(rc.ptr !is null);
+
+    ri.destroy;
+    assert(ri.ptr is null);
+    ri = rebindable(si);
+    assert(ri.ptr !is null);
+}
+
+// Test Rebindable!mutable
+@safe unittest
+{
+    static struct S
+    {
+        int* ptr;
+    }
+    S s;
+
+    Rebindable!S rs = s;
+    static assert(is(typeof(rs) == S));
+    rs = rebindable(S());
+}
+
+// Test disabled default ctor
+unittest
+{
+    static struct ND
+    {
+        int i;
+        @disable this();
+        this(int i) inout {this.i = i;}
+    }
+    static assert(!__traits(compiles, Rebindable!ND()));
+
+    Rebindable!(const ND) rb = const ND(1);
+    assert(rb.i == 1);
+    rb = immutable ND(2);
+    assert(rb.i == 2);
+    rb = rebindable(const ND(3));
+    assert(rb.i == 3);
+    static assert(!__traits(compiles, rb.i++));
+}
+
+
 /**
     Similar to $(D Rebindable!(T)) but strips all qualifiers from the reference as
     opposed to just constness / immutability. Primary intended use case is with