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digitalmars.D.bugs - [Issue 4539] New: Refuse assignment to string literal

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4539

           Summary: Refuse assignment to string literal
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: accepts-invalid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



dmd 2.047 compiles this code, but string literals can't be lvalues:

void main() {
    "hello" = "red";
}

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Andrej Mitrovic <andrej.mitrovich gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |andrej.mitrovich gmail.com



07:27:57 PDT ---
I just found this by accident on the compiler docs page:

http://www.digitalmars.com/d/2.0/dmd-windows.html

"
Differences between Windows and Linux versions

    * String literals are read-only under Linux. Attempting to write to them
will cause a segment violation.
"

Maybe this is relevant?

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I think that's not relevant, I think that refers to changing the content of a
variable initialized with a string literal (plus cast to mutable).

But the code I have shown is meaningless, because a string can't be a lvalue.
It's like doing:

void main() {
    5 = 7;
}

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09:18:53 PDT ---
Yeah, it does not make sense at all. But it gets worse:

import std.stdio;

void main() {
    "hello" = "red";
    string x = "hello";
    writeln(x);
}

Writes "red"

That's not good. :)

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
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                 CC|                            |clugdbug yahoo.com.au
           Severity|normal                      |major





 Yeah, it does not make sense at all. But it gets worse:
 
 import std.stdio;
 
 void main() {
     "hello" = "red";
     string x = "hello";
     writeln(x);
 }
 
 Writes "red"
 
 That's not good. :)


Ouch!

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nfxjfg gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |nfxjfg gmail.com



This segfaults on the far superior Linux (dmd v2.046), so yes, it's probably
because OPTLINK can't do read-only sections.

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A related bug found by denis spir:

auto p = &"hello";

String literals aren't lvalues, so you can't take their address. Just as you
can't tale the address of a decimal literal:

auto q = &(1);

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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
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           Keywords|                            |patch
                 CC|                            |k.hara.pg gmail.com



Patch posted:
https://github.com/D-Programming-Language/dmd/pull/46

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kennytm gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla kyllingen.net



*** Issue 4309 has been marked as a duplicate of this issue. ***

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla digitalmars.com



22:55:03 PDT ---


 Patch posted:
 https://github.com/D-Programming-Language/dmd/pull/46


See github for problems with the patch.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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---
I think the main problem is that dmd treats string literal as lvalue.
Lvalue can appear on left hand side of assignment, so string literal now
asignable.


literal on ref parameter as a side effect.

It seems to me that it is correct refusing string literal on ref parameter.
So this change breaks existing code.

(If D specification treats string literal specially, my think is mistaken.
But it might be not.)

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yebblies gmail.com
           Platform|x86                         |All
         OS/Version|Windows                     |All



A new patch that reapplies the old one and fixes the failing tests:
https://github.com/D-Programming-Language/dmd/pull/188

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The current patch for this issue is
https://github.com/D-Programming-Language/dmd/issues/164

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jens.k.mueller gmx.de



*** Issue 6882 has been marked as a duplicate of this issue. ***

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Denis <verylonglogin.reg gmail.com> changed:

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                 CC|                            |verylonglogin.reg gmail.com



MSK ---
*** Issue 7161 has been marked as a duplicate of this issue. ***

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---
Updated patch.
https://github.com/D-Programming-Language/dmd/pull/164

A string literal should be able to bind a reference to static array type.
In the context of ref binding, string literal should work as like static array
value.

Example:

void main()
{
    void foo1(ref string s){}         // ref slice
    void foo2(ref const char[10] s){} // difference of length
    void foo3(ref char[5] s){}        // mutable element

    void foo4(ref const char[5] s)
    {
        assert(s[0] == 'h');
        assert(s[4] == 'o');
    }
    void foo5(ref const ubyte[5] s)
    {
        assert(s[0] == 0xc3);
        assert(s[4] == 0x61);
    }

    static assert(!__traits(compiles, foo1("hello")));
    static assert(!__traits(compiles, foo2("hello")));
    static assert(!__traits(compiles, foo3("hello")));
    foo4("hello");
    foo5(cast(ubyte[5])x"c3fcd3d761");

    import std.conv;
    static assert(!__traits(compiles, parse!int("10") == 10));
}

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Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/7f33ed71d8897ec0d03828c41063213af0283d02


Retry to fix issue 4539 - Refuse assignment to string literal

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


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With the latest DMD2.058head this code:


void foo(immutable ref string) {}
void main() {
    foo("hello");
}


Gives:

test.d(3): Error: function test.foo (ref immutable(char[]) _param_0) is not
callable using argument types (string)

Is this expected?

(Also note the function signature is (ref immutable(char[]) _param_0) while I
have specified an immutable ref).

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---


 With the latest DMD2.058head this code:
 
 
 void foo(immutable ref string) {}
 void main() {
     foo("hello");
 }
 
 
 Gives:
 
 test.d(3): Error: function test.foo (ref immutable(char[]) _param_0) is not
 callable using argument types (string)
 
 Is this expected?
 
 (Also note the function signature is (ref immutable(char[]) _param_0) while I
 have specified an immutable ref).


Yes, it is expected behavior.
'ref' storage class requires lvalue slice in this case, but string literal
DOESN'T have slice, because it has only content.

Instead, you can get a reference to string literal by `ref immutable(char[5])`
or `ref const(char[5])` (5 == "hello".length), they bind just only content.

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20:51:39 PST ---


 void foo(immutable ref string) {}


I'm curious, what does `immutable ref string` buy you compared to just
immutable string? Can't the compiler pass by ref automatically if it's an
immutable type?

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 Yes, it is expected behavior.


I have just seen that some of my code that used to compile now gives a problem.
This is a reduced version:


void foo(ref string t) {}
void main() {
    immutable string s;
    foo(s);
}


DMD 2.058head gives:
test.d(4): Error: function test.foo (ref string t) is not callable using
argument types (immutable(char[]))


Is this correct?

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 Is this correct?


Yes, it's correct, sorry.

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