[Issue 24795] New: emplace mutates immutable data in @safe code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24795

          Issue ID: 24795
           Summary: emplace mutates immutable data in @safe code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

As of DMD 2.109.1, the example program below compiles and runs to completion
without errors.

The program uses core.lifetime.emplace to mutate immutable data in @safe code.
Since mutating immutable data results in undefined behavior, it is a bug for
the program to compile.

The cause of the bug is the use of @trusted in core.lifetime.emplace to cast
away type qualifiers from the target object.

---
import core.lifetime;

void example1() @safe
{
    const(int)* obj = new immutable(int)(123);
    assert(*obj == 123);
    emplace(obj, 456);
    assert(*obj == 456); // immutable object mutated!
}

void example2() @safe
{
    static class C
    {
        int n;
        this(int n) pure @safe { this.n = n; }
    }

    const(C) obj = new immutable(C)(123);
    assert(obj.n == 123);
    emplace(obj, 456);
    assert(obj.n == 456); // immutable object mutated!
}

void example3() @safe
{
    static struct S
    {
        int n;
        this(int n) pure @safe { this.n = n; }
    }

    const(S)* obj = new immutable(S)(123);
    assert(obj.n == 123);
    emplace(obj, 456);
    assert(obj.n == 456); // immutable object mutated!
}

void main() @safe
{
    example1();
    example2();
    example3();
}
---

--