String literals are mutable in the interpreter

[HertzDevil]

Examples:

icr:1:0> a = "abc"
=> "abc"
icr:2:0> b = a
=> "abc"
icr:3:0> a.to_unsafe.value = 33_u8
=> 33
icr:4:0> a
=> "!bc"
icr:5:0> b
=> "!bc"

icr:1:0> 1.class.name.to_unsafe.value = 33_u8
=> 33
icr:2:0> 1.class.name
=> Error while calling inspect on value: instantiating 'String#inspect()'
"!nt32"
icr:3:0> 1.class.name
=> Error while calling inspect on value: 
"!nt32"
icr:4:0> 1.class.name
=> Invalid memory access (signal 11) at address 0x0
[0x55bcc21fff56] *Exception::CallStack::print_backtrace:Nil +118 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc21d6496] ~procProc(Int32, Pointer(LibC::SiginfoT), Pointer(Void), Nil) +310 in /home/quinton/crystal/crystal/.build/crystal
[0x7f81ba83daf0] ?? +140195156712176 in /lib/x86_64-linux-gnu/libc.so.6
[0x55bcc30711aa] *Crystal::Repl::Interpreter#interpret<Crystal::ASTNode+, Crystal::Type+>:Crystal::Repl::Value +2330 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc306fe7c] *Crystal::Repl::Interpreter#interpret<Crystal::ASTNode+, Hash(String, Crystal::MetaVar)>:Crystal::Repl::Value +2284 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc3404b23] *Crystal::Repl::Value#to_s<IO::FileDescriptor>:(IO::FileDescriptor | Nil) +931 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc237d743] *IO::FileDescriptor +67 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc237d6ed] *IO::FileDescriptor +77 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc21d13c9] *puts<Crystal::Repl::Value>:Nil +137 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc33f9601] *Crystal::Repl::Prompt#display<Crystal::Repl::Value>:Nil +81 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc363a661] *Crystal::Repl#run:Nil +673 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc357cb0a] *Crystal::Command#repl:(Crystal::Repl::Value | Nil) +1226 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc3577401] *Crystal::Command#run:(Bool | Crystal::Repl::Value | Nil) +1409 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc3576cfb] *Crystal::Command::run<Array(String)>:(Bool | Crystal::Repl::Value | Nil) +43 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc3576c6e] *Crystal::Command::run:(Bool | Crystal::Repl::Value | Nil) +46 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc21b5987] __crystal_main +2583 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc23a1783] *Crystal::main_user_code<Int32, Pointer(Pointer(UInt8))>:Nil +19 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc23a16ea] *Crystal::main<Int32, Pointer(Pointer(UInt8))>:Int32 +58 in /home/quinton/crystal/crystal/.build/crystal
[0x55bcc21c2f36] main +6 in /home/quinton/crystal/crystal/.build/crystal
[0x7f81ba82920a] ?? +140195156627978 in /lib/x86_64-linux-gnu/libc.so.6
[0x7f81ba8292bc] __libc_start_main +124 in /lib/x86_64-linux-gnu/libc.so.6
[0x55bcc21b4ea1] _start +33 in /home/quinton/crystal/crystal/.build/crystal
[0x0] ???
Segmentation fault

It seems there is some kind of stack corruption for the second example too.

To fix this we should probably simulate read-only memory in the interpreter; even crashing on the pointer write, like in compiled mode, is better than introducing undefined behavior that only gets picked up later. This might also be necessary if we want to be able to form pointers to constants (e.g. #12396).