assoc-string is a built-in function in `C source code'.

(assoc-string KEY LIST &optional CASE-FOLD)

Like `assoc' but specifically for strings (and symbols).

This returns the first element of LIST whose car matches the string or
symbol KEY, or nil if no match exists. When performing the
comparison, symbols are first converted to strings, and unibyte
strings to multibyte. If the optional arg CASE-FOLD is non-nil, case
is ignored.

Unlike `assoc', KEY can also match an entry in LIST consisting of a
single string, rather than a cons cell whose car is a string.