Document that destructors may not allocate memory

[dlangBugzillaToGithub]

FeepingCreature (@FeepingCreature) reported this on 2020-03-09T10:13:39Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20651

Description

Consider the code from issue 20650:

class Foo { ~this() { new int; } }
void main() { new Foo; }

This runs into core.exception.InvalidMemoryOperationError@src/core/exception.d(647): Invalid memory operation .

The reason is that the GC may not be invoked during finalizers, ie. when destructors are run. There is almost never a reason to do this; however, this should still be documented in the language spec at https://dlang.org/spec/class.html#destructors .