refactor(options): reduce `findoption()` usage

Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.

Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.

2 files changed, 8 insertions, 5 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index b4f0be85e5..c35e0b2ada 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -7239,7 +7239,7 @@ int do_searchpair(const char *spat, const char *mpat, const char *epat, int dir,
     // If it's still empty it was changed and restored, need to restore in
     // the complicated way.
     if (*p_cpo == NUL) {
-      set_option_value_give_err("cpo", CSTR_AS_OPTVAL(save_cpo), 0);
+      set_option_value_give_err(kOptCpoptions, CSTR_AS_OPTVAL(save_cpo), 0);
     }
     free_string_option(save_cpo);
   }
diff --git a/src/nvim/eval/vars.c b/src/nvim/eval/vars.c
index 8864edbf69..9b7a61f969 100644
--- a/src/nvim/eval/vars.c
+++ b/src/nvim/eval/vars.c
@@ -834,7 +834,7 @@ static char *ex_let_option(char *arg, typval_T *const tv, const bool is_const,
     }
   }
 
-  const char *err = set_option_value(arg, newval, scope);
+  const char *err = set_option_value_handle_tty(arg, opt_idx, newval, scope);
   arg_end = p;
   if (err != NULL) {
     emsg(_(err));
@@ -1945,15 +1945,18 @@ static void set_option_from_tv(const char *varname, typval_T *varp)
     semsg(_(e_unknown_option2), varname);
     return;
   }
-  uint32_t opt_p_flags = get_option(opt_idx)->flags;
 
   bool error = false;
+  uint32_t opt_p_flags = get_option_flags(opt_idx);
   OptVal value = tv_to_optval(varp, varname, opt_p_flags, &error);
 
   if (!error) {
-    set_option_value_give_err(varname, value, OPT_LOCAL);
-  }
+    const char *errmsg = set_option_value_handle_tty(varname, opt_idx, value, OPT_LOCAL);
 
+    if (errmsg) {
+      emsg(errmsg);
+    }
+  }
   optval_free(value);
 }