Disallow assigning to struct rvalue

[ntrel]

When assignment would call an operator overload.
Fixes #21507.

Note: This does not disallow calling e.g. opAssign on an rvalue, only actual assignment.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#21717"

[ibuclaw]

What other rvalues could appear here? Does it make sense to only restrict structs?

[ntrel]

The check in opOverloadBinaryAssign would get hit by:

    int[] a;
    a.length += 1;

It would also get hit for a class C (new C) += 4;. That should be prevented too, but I don't want to do that in this pull.

[kinke]

rvalues cannot be assigned to by definition, correct?

[ntrel]

I snipped the buildkite failures below. The libdparse one is fixed upstream. The other projects are failing due to one of these dependency failures.

    Building vibe-d:redis ~master: building configuration [vibe-d-redis-test-library]
redis/vibe/db/redis/idioms.d(74,8): Error: cannot assign to struct rvalue `this.opIndex(id)`
            this[id] = args;
                ^

phobos:
../dmd/generated/linux/release/64/dmd -conf= -I../dmd/druntime/import  -w -preview=dip1000 -preview=dtorfields -preview=fieldwise -m64 -fPIC -O -release -unittest -version=StdUnittest -c -ofgenerated/linux/release/64/unittest/std/container/array.o std/container/array.d
make[2]: *** [Makefile:407: generated/linux/release/64/unittest/std/container/array.o] Segmentation fault (core dumped)

ae:
net/ietf/message.d(347,7): Error: cannot assign to struct rvalue `list(this.author, this.authorEmail)`
        list(author, authorEmail) = decodeAuthor("From" in headers ? decodeRfc1522(headers["From"]) : null);
            ^
net/ietf/message.d(350,8): Error: cannot assign to struct rvalue `list(null, this.authorEmail)`
            list(null, authorEmail) = decodeAuthor(decodeRfc1522(headers["Reply-To"].findSplit(", ")[0]));
                ^
net/smtp/client.d(159,7): Error: cannot assign to struct rvalue `list(codeStr, null, line)`
        list(codeStr, null, line) = line.findSplit(" ");
            ^

Compiling parse tester... ../src/dparse/parser.d(8981): Error: cannot modify struct rvalue `checked(this.suppressMessages.back())`
            ++suppressMessages.back.checked;
                                   ^

../.dub/packages/mir-algorithm/3.22.4/mir-algorithm/source/mir/bignum/fixed.d(966,23): Error: cannot assign to struct rvalue `a.view()`
    size_t overflow = a.view *= b;
                      ^
../.dub/packages/mir-algorithm/3.22.4/mir-algorithm/source/mir/bignum/fixed.d(1063,61): Error: cannot assign to struct rvalue `view.topLeastSignificantPart(1LU)`
    view.coefficients[$ - 1] += view.topLeastSignificantPart(a.data.length) += c.view;
                                                            ^

[ntrel]

rvalues cannot be assigned to by definition, correct?

Currently dmd allows it for a struct overloading assignment.