Copying a small string into a larger string container copies data beyond the end of the small string

[sbillig]

What is wrong?

Copying a smaller string into a bigger one (that is, one with a larger maximum size) results in an out-of-bounds memory copy.
Example:

let s1: String<3> = "abc"
let foo: Foo = Foo(x: 123)
let s2: String<4> = s1 // 4 bytes of memory, containing "abc" and the value 123

struct Foo {
  x: u8
}

How can it be fixed

#777 introduces a StringSizeIncrease type adjustment. This could be used in MIR to insert a memcopy that copies the smaller string's number of bytes.

[Y-Nak]

Is this Issue about performance? I don't think this leads to a bug because a string object stores its actual length.