String concatenation misses the last character sometimes

[ghost]

Some combinations of string variables and literals are not concatenated properly using '+'. It seems to be if there's a variable between two literals. Example

fn main() {
  let str ff = "abc";
  log ff + "ABC" + ff;
  log "ABC" + ff + "ABC";
}

Produces
rt: ---
rt: e6c6:main:main: rust: abcABCabc
rt: e6c6:main:main: rust: ABCabABC
Note the missing 'c' in the second line.

[froystig]

When translating vec-append, delay destination string's null-byte-accounting length decrement until the destination string has already been resized. Closed by 3c3421e.